Journal of Environment and Health Science | Volume 2 | Issue 1 | 2016-05-11

Organophosphorus Flame Retardants (OPFR): Neurotoxicity

Mohamed AbouDonia

Abstract

Organophosphorus flame retardants (OPFRs) are used as additives in plasticizers, foams, hydraulic fluids, anti-foam agents, and coatings for electronic components/devices to inhibit flames. These chemicals were developed and used as flame retardants because of environmental and health concerns of previously used brominated and chlorinated flame retardants (FRs). OPCFRs are divided into five main groups: organophosphates, organophosphonates, organophosphinates, organoposphine oxide, and organophosphites. Most of OPFRs are organophosphate esters that are further classified into the following five groups: 1. Aliphatic, 2. Brominated aliphatic, 3. Chlorinated aliphatic, 4. Aromatic-aliphatic, and 5. Aromatic phosphates. These OPFRs have the following neurotoxic actions: 1. Cholinergic Neurotoxicity, 2. Organophosphate-Induced Delayed Neurotoxicity (OPIDN), and 3. Organophosphate-Induced Chronic Neurotoxicity (OPICN) in addition to being endocrine disruptors. OPFRs have very low cholinergic neurotoxicity and this effect does not pose significant health hazards to adults or children. On the other hand, some OPFRs have shown to cause OPIDN that is a delayed central-peripheral axonopathy, characterized by neuronal cell death of the lower brain regions, spinal cord and peripheral nervous systems, leading to long-term neuronal injury. OPICN is characterized by neuronal cell death in the cortex, hippocampus campus and cerebellum and spinal cord. Finally, OPCFRs act as endocrine disrupters, that affect many functions of the body such thyroid glands and reproductive functions, and may be involved in the development of diabetes and cancer. Residues of these OPCFRs are widespread in the environment, home and workplaces. These chemicals adversely affect human health, especially for vulnerable population such as the elderly, pregnant women, fetuses, and children. Because some OPFRs cause neuronal cell death in the brain and spinal cord that do not repair as well as act as endocrine disrupters they may lead to permanent functional deficits such obesity, memory impairment, decreased motor skill and even more serious diseases such as diabetes and cancer. Because recent reports have accredited FRs for significant decrease in building fires, it is important to balance the risk and benefits of FRs and to use only the safest available FRs including OPFRs.

Keywords: OPFRs, Neurotoxicity, Flame retardants

Full text

There are over 80,000 chemicals in use today including flame retardants. Flame retardants (FRs) are chemicals added to materials, for the purpose of inhibiting flames. FRs can be classified according to their binding into two types: reactive or additive OPFRs are classified (WHO 1997, Fisk et al., 2003) into two types: The fi rst type is reactive OCPFRs, which are built into a polymer molecule and has limited loss due to their chemical binding. The second type is the additive PFRs, which are mixed into the polymer and continually lost during the lifetime of the treated product resulting in decrease in the fl ame retardant properties (WHO, 1997; Van der Veen I., De Boer J. 2012). (WHO, 1997 The reactive FRs, are reactive components chemically built into a polymer molecule). This type has a limited loss because of their chemical binding (Fisk et al., 2003). The additive FRs is mixed into the polymer (WHO, 1997). In the additive FRs type there is continuous decrease in concentration during the lifetime of the treated product and subsequent decrease in the fl ame retardant properties (Fisk et al., 2003). Mineral flame retardants are typically additive while organohalogen flame retardants and organophosphorus flame retardants (OPFRs) can be either reactive or additive. Additive FRs are applied to a wide range of materials, to slow combustion process (Ellis et al., 2007). Chemically, plastics are materials which consist of synthetic or naturally based macromolecular organic compounds. Additives are added to plastics to improve material properties according to the targeted use. These additives may influence mechanical properties (e.g., plasticizers), or added for safety aspects (e.g., fire protection) (Wensing et al., 2005). Phosphorus Compounds Phosphorus, P , atomic number 15, atomic weight 30.97, belongs to Group V of the periodic table of the elements. Phosphorus is not found free in nature and almost occurs in the fully oxidized state as phosphate (Table 1). The electronic structure of the phosphorus atom is 1s 2 2s 2 2p 6 3p 2 3p 3 with three unpaired electrons in the outer 3 orbitals that are available for bonding. Phosphorus atom can be formally trivalent or pentavalent, using only three or all five electrons in the outer M shell, respectively. Pentavalent organophosphorus compounds, such as phosphorus oxychloride (POCl 3 ) has tetrahedral configuration. Whereas tri-substituted phosphorus acid esters, such as phosphorus trichloride (PCl 3 ) has a pyramidal configuration. Most synthetic organophosphorus compounds are pentavalent. Organophosphorus compounds are used as insecticides, acaricides, nematocides, insect chemosterilants, fungicides, herbicides, nematocides, insect repellants, nerve agents, pharmaceuticals, and as flame retardants (Abou-Donia, 1992). Trivalent compounds include phosphites that are used as antioxidants, defoliants, and flame retardants. Figure 1: PHOSPHORUS ATOM AND ORGANOPHOSPHOROUS COMPOUNDS. CAS Number: 7723-14-0 1S 2 K 2S 2 2P 6 L 3S 2 3P 3 M Number ofElectrons: 15 Number of Protons: 16 Number ofNeutrons: 15 Pentavalent Compounds Chemical Formula Trivalent Compounds Chemical Formula Phosphoric Acid (PO 4 H 3 ) Phosphite (PO 3 ) Triphenyl Phsphites Phosphonic (PO 3 H 3 ) Phosphine (PH 3 ) Triphenyl Phosphine Phosphinic Acid (PO 2 H 3 ) Diethyl methylphosphonite Phosphine Oxide(POR 3 ) Phosphorus, symbol P, atomic number 15, atomic weight 30.97, belongs to Group V of the periodic table of the elements. Phosphorus is not found free in nature and is almost always occurs in the fully oxidized state as phosphate. The electronic structure of the phosphorus atom is 1S 2 2S 2 2P 6 3S 2 3P 3 with three unpaired electrons in the outer 3 orbitals that are available for bonding. Organophosphorus Compounds. Phosphorus atom can be formally trivalent or pentavalent, using only three or all five electrons in the outer M shell, respectively. Pentavalent organophosphorus compounds, such as phosphorus oxychloride (POCl 3 ) has tetrahedral configuration. Whereas tri-substituted phosphorus acid esters, such as phosphorus trichloride (PCl 3 ) has a pyramidal configuration. Organophosphorus flame retardants (OPFRs) Because of environmental and health concerns about Brominated Flame Retardants (BFRs), such as Polybrominated Diphenyl Ethers (PBDEs), organophosphorus flame retardants (OPFRs) were developed. Most of these chemicals are organophosphate esters. These compounds are used as flame retardant additives in plasticizers, foams, hydraulic fluids, anti-foam agents, coatings for electronic components/devices, furniture, textile, electronics, construction, vehicle and petroleum industries (Marklund et al., 2003). Similar to brominated flame retardant compounds, the increased use of OPFRs has caused concerns regarding their adverse effects on the environment as well as animal and human health. Furthermore, studies have shown that indoor air and dust had concentrations of OPFRs higher than that of PBDEs (Stapleton et al., 2009). In particular, major concern was the potential carcinogenic, neurological effects and endocrine disruption of these compound (Betts, 2007; Waaijers et al., 2013). OPs are widely utilized as fl ame retardants, plasticizers and anti-foaming agents in a variety of industries, including plastics. Because OPFRs are frequently present as additives rather than chemically bonded to materials, they are easily released to different environmental compartments via volatilization, leaching and/or abrasion (Carlsson et al., 2000; der Veen and de Boer, 2012). Meanwhile, due to the need for rapid BFR substitution, many of these OPs are already being marketed, although their environmental behavior and toxicological effects were not properly assessed (Dishaw et al., 2011). Classification of OPFRs OPFRs are divided into five main groups: organophosphates (Tables 1.1 - 1.5), organophosphonates (Table 2), organophosphinates (Table 3), organoposphine oxide (Table 4) organophosphites (Table 5). 1. Organophosphates 1.1 Aliphatic organophosphate flame retardants: listed in Table 1.1, e.g., tris(2-butoxyethyl) phosphate (TBOEP), dimethyl phosphate, trimethylphosphate, diethyl phosphate (mono & di-), t riethyl phosphate, triisopropyl phosphate, tripropyl phosphate, tri-n-butyl phosphate (TnBP), triisobutyl phosphate, tripentyl phosphate, TPP), tris(2-butoxyethyl) phosphate, tris(2-ethylhexyl) phosphate Table 1.1: ALIPHATIC ORGANOPHOSPHATE FLAME RETARDANTS # CAS ABBREVIATION CHEMICAL NOMENCLATURE CHEMICAL FORMULA 1 813-78-5 DMP Dimethyl phosphate 2 512-56-1 TMP Trimethylphosphate 3 598-02-7 DEP Diethyl phosphate (mono & di-) 4 78-40-0 TEP Triethyl phosphate 5 513-08-6 TPP TRIPROPYL PHOSPHATE 6 513-02-0 TiPP, TiPrP Triisopropyl phosphate 7 126-73-8 TNBP Tri-n-butyl phosphate 8 126-71-6 TIPP Triisobutyl phosphate 9 2528-38-3 TPeP Tripentyl phosphate ( 10 78-51-3 TBOEP Tris(2-butoxyethyl) phosphate 11 78-42-2 TEHP Tris(2-ethylhexyl) phosphate 12 4090-51-1 BDMTD Bis(5,5-dimethyl-2-thiono-1,3,2-doxaphosphorin amyl) oxide 1.2 Brominated aliphatic phosphate flame retardant: listed in Table 1.2 include tris(tribromooneopentyl)phosphate and tris(2,3-dibromopropyl. Table 1.2: BROMO ALIPHATIC PHOSPHATE FLAME RETARDNTS. # CAS ABBREVIATION CHEMICAL NOMENCLATURE CHEMICAL FORMULA 1 19186-97-1 TTBNPP TRIS (TRI BROMONEOPENTYL) PHOSPHATE 2 126-72-7 TDB99 Tris(2,3-dibromopropyl) phosphate 1.3 Chlorinated aliphatic phosphate retardants: listed in Table 1.3 tris(2-chloroethyl)phosphate, tris(2-dichloropropyl)phosphate; tris(1-chloro-2-propyl)phosphate; 2,2-bis(chloromethyl)-1,3-propanediol bis[bis(2-chloroethyl)phosphate], tris(1-chloro-2-propyl) phosphate, tris(1-Chloro-2-propyl) phosphate (TCPP, TCiPP); Tris(2,3-dichloropropyl)Phosphate;tris(1,3-dichloro-2-propyl)phosphate, 2,2-bis(chloromethyl-)trimethylene bis(bis(2-chloroethyl) phosphate, tetrakis(2-chloroethyl). dichlo-isopentyl diphosphate; tris(1,3-dichloroisopropyl) phosphate (TDCPP); tris(2-chloroethyl) phosphate (TCEP);and tris(chloropropyl)phosphate (TCPP). TDCPP and these other chlorinated organophosphate flame retardants are all sometimes referred to as chlorinated tris . To comply with U.S. Flammable Fabrics Act of 1953, TDCPP was used as a flame retardant in children s pajamas until the late 1970s. TDCPP, is primary used as a flame retardants in flexible polyurethane foam for automobiles, upholstered furniture, and some baby products (Marklund et al.2005; Thorvad, 2005; Andersen et al., 2004; Gold et al., 1978). It is also used in rigid polyurethane foam boards used for building insulation (Carignan et al., 2013). In 2011 it was reported that TDCPP was found in about a third of tested baby products (Stapleton et al., 2009). Table 1.3: CHLORO ALIPHATIC PHOSPHATES FLAME RETARDANTS. # CAS ABBREVIATION CHEMICAL NOMENCLATURE CHEMICAL FORMULA 1 115-96-8 TCEP Tris(2-chloroethyl)phosphate 2 78-43-3 TDCPP Tris(2,3-dichloropropyl) phosphate 3 38051-10-4 BCMP-BCEP 2,2-Bis(chloromethyl)-1,3-propanediol bis[bis(2-chloroethyl)phosphate]T 4 13674-84-5 TCPP Tris(1-chloro-2-propyl) phosphate 5 78-43--3 TDCPP Tris(2,3-DICHLOROPROPYL)Phosphate 6 13674-87-8 TDCTPP Tris(1,3-dichloro-2-propyl) phosphate 7 38051-10-4 TCDDP Tetrakis(2-Chloroethyl) dichlo-isopentyl diphosphate (V6) 1.4 Aromatic-aliphatic phosphate flame retardants: e.g., isopropyl phenyl phosphate, 2-ethylhexyl diphenyl phosphate, isodecyl diphenyl phosphate, and cresyl phosphare isomers. Table 1.4: AROMATIC ALIPHATIC PHOSPHATES FLAME RETARDANTS. # CAS ABBREVIATION CHEMICAL NOMENCLATURE CHEMICAL FORMULA 1 1241-94-7 EDP, DPEHP 2-Ethylhexyl diphenyl phosphate 2 29761-21-5 IDP Isodecyl diphenyl phosphate 3 46355-07-1 PPHP Phenyl Propan-2-yl hydrogen phosphate 1.5 Aromatic phosphate retardants: e.g., triphenyl phosphate (TPP), 9, 10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide, triphenyl phosphate, tricresyl phosphate, bisphenol A bis (diphenylphosphate), cresyl diphenyl phosphate, tri-o-cresyl phosphate, tri-p-cresyl phosphate, polychlorobiphenyl Table 1.5: AROMATIC ORGANOPHOSPHATES FLAME RETARDANTS. # CAS ABBREVIATION CHEMICAL NOMENCLATURE CHEMICAL FORMULA 1 35948-25-5 DOPPO 9,10-Dihydro-9-oxa-10-phospha-phenanthrene-10-oxide 2 115-86-6 TPHP Triphenyl Phosphate 3 1330-78-5 TMPP Tricresyl phosphate 4 2502-15-0 TIPPP Triisopropylpara-phosphate 5 46355-07-1 IPPP Isopropyl phenyl phosphate 6 5945-33-5 BPA-BDPP bisphenol A bis (diphenylphosphate) 7 26444-49-5 (CDP) Cresyl diphenyl phosphate 8 78-30-8 o-TCP, TOCP, TOTP Tri-o-cresyl phosphate 9 78-32-0 p-TCP, TPCP, TPTP Tri-p-cresyl phosphate 10 2050-68-2 PCP Polychlorobiphenyl 11 139186-30-3 TIPPP tiPrPhP Phosphoric acid, P,P'-1,3-phenylene P,P,P',P'-tetrakis(2,6-dimethylphenyl) ester 12 139189-30-3 PPTPE Phosphoric acid, P,P'-1,3- phenylene-P,P,P',P'-tetraphenyl ester 13 5945-33-5 PBDPP RebDiPhP Phosphoric acid, P,P'-[(1-methylethylidene)di-4,1-phenylene] P,P,P',P'-tetraphenyl ester 14 26967-76-0 TIPPP Phenol, 4-(1-methylethyl)-phosphate (3:1) 15 78-32-0 TMPP Phosphoric acid, tris(methylphenyl) ester 16 63562-41-4 DOPO 6H-Dibenz[c,e][1,2]oxaphosphorin, 6-oxide 2. Organophosphonates e.g., Dimethyl Phosphonate, Dimethyl Phosphonate, Dimethyl propyl phosphonate, Diethyl ethyl phosphonate, 3-(Dimethylphosphono) propionic acid methylamide and Phosphonic acid, (1-(((2-loroethoxy)(2-chloroethyl)phosphinyl)-oxo)-ethyl Table 2: Organophosphonate Flame Retardants. # CAS Number Abbreviation Chemical Nomenclature 1 868-85-9 DMHP, DMP Dimethyl Phosphonate 2 756-79-6 DMMP Dimethyl methyl phosphonate 3 18755-43-6 DMPP Dimethyl propyl phosphonate 4 78-38-6 DEEP Diethyl ethyl phosphonate 5 20120-33-6 DMPPM 3-(Dimethylphosphono)propionic acid methylamide 6 4351-70-6 PPA Phosphonic acid, (1-(((2-loroethoxy)(2-chloroethyl)phosphinyl)-oxo)-ethyl 3. Organophosphintes e.g., Hydroxymethylphenyl phosphinic acid, Phenyl Phosphinic acid, p-Methoxyphenyl Phosphinic acid, Carboxyethyl- phenylphosphinic acid and Diethyl phosphin Table 3: Organophosphinite Flame Retardants. # CAS Number Abbreviations Nomenclature 1 61451-78-3 HMPP Hydroxymethylphenylphosphinic acid 2 1779 48-2 PPA Phenyl Phosphinic acid 3 53534-65-9 PMPP p-MethoxyphenylPhosphinic acid 4 14657-64-8 CEPPA Carboxyethyl-phenylphosphinic acid 5 813-76-3 Diethylphosphinic 4. Organophosphine oxides e.g., Tris(hydroxymethyl) phosphineoxide Bis(4-carboxyphenyl) phenylphosphine oxide Triphenyl phosphineoxide Table 4: Organophosphineoxide Flame Retardants. # CAS Number Abbreviations Nomenclature 1 1067-12-5 HMPPA Tris(hydroxymethyl) phosphineoxide 2 803-19-0 BCPPO Bis(4-carboxyphenyl) phenylphosphine oxide 3 791-8-6 TPPO Triphenylphosphineoxide 5. Organohosphites e.g., triisopropyl Phosphite and tri-phenyl Phosphite. Table 5: Organophosphite Compounds Flame Retardants. # CAS # Abbrev-iation Chemical Nomenclature Chemical Formula 1 116-17-6 TIPPi Triisopropyl Phosphite 2 101-02-0 TPPi Tri-phenyl Phosphite Triphenyl phosphite (TPPi): TPPi is used as a flame retardant to enhance the thermal stability of the electrolyte (Jia et al, 2014). Furthermore, TPPi improved the safety of chargeable lithium batteries with sulfur (Li-S) composite electrode and enhanced the electro-performance of the Li-S batteries. Neurotoxicity of OPFRs Organophosphorus compounds have three neurotoxic actions: cholinergic neurotoxicity (Abou-Donia, 2003), organophosphate- induced del compounds ayed neurotoxicity (OPIDN, Abou-Donia, 1981. Abou-Donia and Lapadula, 1990), and organophosphate-induced chronic neurotoxicity (OPICN, Abou-Donia 2003).The last action includes organophosphates-induced endocrine disruption. Cholinergic Neurotoxicity The primary action of organophosphates is irreversible inhibition of acetylcholinesterase (AChE), resulting in the accumulation of acetylcholine and subsequent over stimulation of the nicotinic and muscarinic acetylcholine receptors leading to cholinergic effects (Abou-Donia, 2003). Acute exposures to an organophosphate are potentially fatal. Sub-acute levels are not necessarily fatal and may not even rise to the level of producing any initial observable or measurable symptoms in the exposed individual. Acute exposure to organophosphates at toxic dose levels in mammals causes some or all of the symptoms of organophosphorus ester cholinergic toxicity which include: constriction of pupils, difficulty with involuntary muscles (e.g., gastrointestinal tract muscles resulting in involuntary defecation and urination), sweating, wetness of the mouth, excessive salivation, excessive clear mucus from the nose, tearing of the eyes, fluid in the lungs, shortness of breath, slowing of the heart, tremors of the voluntary muscles (e.g., those muscles used to perform ordinary tasks such as picking up an object), nausea and vomiting, impaired gait and fatigue. In assessing the long-term adverse health effects of OPFRs, it very important to differentiate between toxicity induced following acute, high-level exposure and that after sub-acute, low-level exposure. Although the oral acute toxicity of a chemical, as indicated by the LD50 is high, it does not follow that this particular chemical is safe after long-term, low-level, sub-acute exposures. For example, cigarette smoke is not acutely toxic, and does not cause immediate death for those who smoke. On the other hand, available human incidents and experimental results indicate that long-term effects of cigarette smoking cause lung cancer and kill thousands of smokers every year. Clinical Manifestations resulting from cholinergic toxicity: Organophosphate-induced cholinergic neurotoxicity may be caused by small level resulting in the following symptoms: Flu-like effects, fatigue, headache, inability to concentrate, weakness, dizziness, tremors, sweating, and anxiety (Abou-Donia, 1994, 1995). Mild toxicity: Fatigue, dizziness, sweating, anorexia, headache, and malaise, tongue and eyelids tremors, miosis, visual impairment, and chest tightness (increased secretions and bronchospasm). Moderate toxicity: Symptoms of the mild toxicity plus increased secretions in the form of salivation and lacrimation; involvement of the cardiovascular system: decreased pulse, bradycardia, and fall of blood pressure. Severe toxicity: Increased the intensity of all the mentioned manifestations; chest tightness may reach respiratory failure leading to asphyxia because of the bronchospasm and laryngospasm in addition to increased secretions; cardiovascular manifestations: tachycardia and arrhythmia may develop up to heart block; advanced stage: convulsions and coma and may lead to death. Cholinergic Toxicity Induced by OPFRs Human exposure to OPFRs results in cholinergic neurotoxicity in a dose-dependent manner. As an example of the cholinergic toxicity induced by tri-ortho-cresyl phosphate (TOCP), an incident of poisoning of 58 cases was reported in Bombay, India in 1960 (Vora et al. 1962). The manifestations of those cases were similar to the typical picture of cholinergic toxicity stated above consistent with the inhibition of erythrocyte acetyl cholinesterase was reduced by 50 % within a short time of the onset of clinical symptoms and it recovered in 3 months. Also, it was reported that workers exposed to triphenyl phosphate (TBP) were presented with nausea and headache (ACGIH, 2001). Oral administration of large doses of TOCP to chickens produced acute cholinergic effect including leg weakness, diarrhea, and salivation (Carrington et al. 1988, Carrington and Abou-Donia, 1988a). Also, administering of large doses of TBEB to Wiser rats caused irregularity in respiration, mainly depressed respiratory rate, with increased salivation and tremors (Hoechst 1989 cited in IPCS 2000). Organophosphate-Induced Delayed Neurotoxicity (OPIDN) OPIDN is a neurodegenerative disorder characterized by central-peripheral axonopathy and the presence of a latent period (usually ranges between 6 and 14 days from the exposure) or a delayed onset of neurological manifestations, mainly locomotors ataxia followed by paralysis after a single or repeated exposure to organophosphorus esters (Abou-Donia 1981, Abou-Donia & Lapadula 1990). The earliest cases of OPIDN resulted from consuming of tri-o-cresyl phosphate (TOCP)-containing creosote oil as a treatment for pulmonary tuberculosis in the nineteenth century, mainly in 1899 in France. Then in the early 1930s, tens of thousands of patients in the US developed paralysis, known as Ginger-Jake paralysis, after consumption of a TOCP- adulterated ginger extract called Jamica Ginger (Smith et al., 1930; Aring 1942). Since then, incidence of OPIDN in Europe, Africa and all over the world was recorded (Abou-Donia and Lapadula 1990). Developing OPIDN depend on: a) the OP compound, b) dosage, c) frequency and duration of exposure, d) route of administration, e) metabolism, and toxicokinetics (which depends on the species-selectivity of the OP compounds). The compounds causing OPIDN have the some common features: a) they are OP compounds, b) they are direct or indirect inhibitors of esterases, c) presence of a latent period between exposure and appearance of clinical manifestations, d) they affect central (excluding higher brain) and peripheral nervous systems, e) producing well-defined neuropathological lesions related to the clinical neurological manifestations, and f) species-selectivity character (Abou-Donia , 1980; Abou-Donia & Lapadula 1990).OPIDN characteristics are categorized into three main categories: biochemical, histopathological, and molecular alterations. Biochemical changes: Early studies (Aldridge 1954) excluded both acetylcholine esterase AChE (Bloch and Hottinger 1943) and BChE (Earl & Thompson 1952 a,b) as primary targets of delayed neurotoxicity (Aldridge & Barnes 1966). Later, an enzymatic activity that was inhibited by OPs capable of producing OPIDN and was named Neurotoxicity Target Esterase (NTE) was proposed as the putative target for OPIDN (NTE) (Johnson 1969). This theory has not proven to be true and further evidence demonstrated that NTE-knockout mice show sensitivity to OPIDN, confirming that NTE isn t involved in the mechanism of the delayed toxicity (O Callaghan et al., 2003). Histopathological changes: OPIDN is characterized by primary axonal degeneration followed by secondary demyelination of specific regions in the central and peripheral nervous systems. Central Nervous System The site of degenerations of the myelin and axons is the spinocerebellar tracts and the posterior columns and cortico spinal tracts which degenerate below the cervical levels far away from their cell bodies (Abou-Donia 1981). The degenerative hallmarks include neurofilaments, vesicular and mitochondrial degeneration, aggregation and disintegration (Cavanagh 1964, Prineas 1969, Illis 1966). However, the higher centers aren t affected (Abou-Donia et al. 1981). Peripheral Nerves (Motor & Sensory) TOMPP-treated cats and chickens demonstrated alterations in the axoplasm including accumulation of neurofilaments and neurotubules without affecting mitochondria in the early stages of developing ataxia. Also, those cats showed damage to the sensory system in both the interosseous and flexor digitorum brevis muscles of the foot (Cavanagh 1964, Prineas 1969, LeVay et al. 1971). Electron microscopic studies showed granules and degeneration of mitochondria and synaptic vesicles without affecting pre- and postsynaptic membranes (Prineas 1969). Regarding the effect on muscles, it was shown that victims of Ginger-Jake paralysis showed loss of muscle fibers and replacing those fibers with connective fat and tissue due to poisoning with TOCP (Aring, 1942). Mechanisms of OPIDN Protein kinases have been hypothesized as targets of OPIDN (Patton et al, 1983; Abou-Donia et al., 1984), based on three observations: a) organophosphorus compounds are phosphorylating agents, so they will interfere with the normal kinase-dependent phosphorylation of target proteins; b) early histopathological events in OPIDN are changes in the cytoskeletal structure (including microtubules and neurofilaments; c) the structure and function of cytoskeletal proteins are dependent on kinases-mediated phosphorylation. Follow-up studies established calcium/calmodulin kinase II as the major target for OPIDN. Thus, it was found that di-isopropyl phosphorofluoridate (DFP)-treated hens caused increase in the autophosphorylation and hence the enzymatic activity of CaM kinase II, in addition to amplification of its expression on both of the protein and mRNA levels (Schulman 1988, Patton et al. 11983, 986, Suwita et al. 1986, Abou-Donia 1993, Gupta and Abou-Donia 1998). As a result, the phosphorylation of cytoskeletal proteins as: tubulin and tau proteins were increased (Gupta and Abou-Donia 1994, Gupta & Abou-Donia 1998). Microtubules (MTs) are formed of polymers of alpha- and beta- tubulin proteins (Janke and Kneussel 2010). One of the main functions of microtubules in neurons is preserving the stability and participating in producing neuronal plasticity via organization of intracellular compartments and trafficking of the cargo molecules to the pre- and postsynaptic domains (Lim et al. 2014, Janke and Kneussel 2010). As a member of the microtubule-associated proteins (MAPs) family, tau protein is found mainly in the neural tissue in addition to being found in trace amounts in non-neuronal cells (Weingarten et al. 1975, Buee et al. 2000). Binding of tau proteins to microtubules through binding domain give the microtubules the chance to be connected to both cytoskeletal structures (as neurofilaments) and other organelles (as mitochondria) as well (Leterrier et al. 1982, Miyata et al. 1986, Rendon et al. 1990, Buee et. 2000). Furthermore, phosphorylation state of tau protein is a regulatory key of microtubule assembly (Biernat et al. 1993, Clevland et al. 1977 a, b). The homeostasis of tau protein in the neurons is controlled by a balance between phosphorylation and dephosphorylation. Although the phosphorylated tau is more effective than the dephosphorylated state in the process of microtubule assembly or polymerization, it was demonstrated that the hyper-phosphorylation (increased phosphorylation) disrupts tau s homeostasis and hence the association between the microtubule and tau (Lindwall and Cole 1984, Lim et al. 2014). Moreover, it was found that neuron degenerations (neurodegenerative diseases) are associated with hyper-phosphorylation and aggregation of tau protein (Reddy 2011, Brunden et al. 2008, Iqbal et al. 2009, Timm et al. 2008). In OPIDN, there is marked inhibition of fast axoplasmic flow (Pleasure et al. 1969, James & Austin 1970, Reichert & Abou-Donia 1980, Gupta et al., 2001). This leads to the assumption that tau protein is one of the leading causes of neurodegeneration through its association with microtubules; and hence affecting the stability of neurons. We postulate that protein CaMkinaseII can be the main target of OPIDN leading to necrosis and apoptotic cell death. Clinical manifestation of OPIDN OPIDN has four distinct clinical phases: 1). Delayed phase before onset of clinical signs of one to four weeks with an average of 2 weeks (Abou-Donia and Lapadula 1990), 2) Progressive Phase (3 - 6 months); exhibiting early symptoms of peripheral neuropathy as: cramping, burning and pain in the calves of the legs. Concurrently or within a short time after that tingling and numbness sensation in feet and legs start. Glove and Stock hypothesia is the first characteristic sensory manifestation to take place (mainly in feet). These symptoms are followed by decreased sensation of touch, pain, temperature in the same manner of hypothesia. 3) Stationary phase (3 - 12 months) characterized by two properties; persistence of paralysis which may be paraplegia or quadriplegia, and disappearance of sensory manifestations within 2 - 9 weeks after the progressive stage. 4) Improvement phase that takes place 6-18 months after the onset of manifestations. Recovery takes place in a reversed manner when compared to incidence of the lesion, where hands recover first then feet and legs. Furthermore, the extensors recover first. Progress and prognosis: It depends on the severity of the lesion in the peripheral nervous system; and whether the CNS is affected. If CNS is affected, the characteristic picture of the upper motor neuron lesion will take place with no recovery and with no significance attributed to the severity of the lesion in contrast with what happens in case of peripheral nervous system affection. Organophosphate-Induced Chronic Neurotoxicity (OPICN) The third neurotoxic action induced by organophosphates is a long-term neurological and neurobehavioral deficits accompanied by brain neuronal cell death that has been characterized recently as organophosphorus ester-induced chronic neurotoxicity (OPICN; Abou-Donia., 2003). This action can occur following a single acute organophosphate exposure at a toxic dose level or to small sub-acute exposures at sub-lethal dose levels, and results in long-term cognitive deficits and sensorimotor dysfunctions, even in the absence of clinical signs of acute cholinergic toxicity. Individuals with OPICN develop a chronic neurotoxicity that persists for years after exposure and is distinct from both the acute cholinergic action and OPIDN actions. OPICN is a nervous system disorder induced by organophosphorus compounds which involves neuronal degeneration and subsequent structural, functional, physiological, and neurological and neurobehavioral abnormalities and neuropsychological consequences and is largely characterized by chronic neurobehavioral alterations. Within the brain, neuropathological lesions are seen in various regions which are characterized by neuronal cell death, resulting from early necrosis or delayed apoptosis and may be diagnosed as central and/or cerebral atrophy of the brain. The regions of the brain that exhibit neuronal degeneration include the hippocampus, resulting in cognition and memory deficits as well as problems with emotions. Such effects may result from exposure to high doses that cause acute cholinergic neurotoxicity, as well as by low-level, sub-clinical doses of organophosphorus compounds. OPICN results from death of brain cells in various regions in the brain such as the cerebral cortex, hippocampus and cerebellum. Damage to these areas results in the following neurological deficits: Cortex damage: Results in dysfunctions in fine motor control movements, e.g., fingers, speech (laryngeal muscles), it also leads to muscle weakness and getting tired easily. Hippocampus damage: causes cognition and memory deficits as well as emotional problems Cerebellum damage: results in change in gate (ataxia) and body imbalance, disorders in the coordination of hand and finger muscle, and body tremors. Neurotoxicity of Specific Organophosphorus Flame Retardants Aliphatic Organophosphates Flame Retardants Tributyl Phosphate (TBP): The oral LD 50 tributyl phosphate (TBP) in rats and chickens is 400 and 1,500 mg/kg, respectively (ACGIH, 1986). The LC 50 in rats is 1,359 mg/m³ (125 ppm). Despite showing the signs of cholinergic toxicity using an LD 50 dose, test animals didn t show any clinical sign of OPIDN (Carrington et al. 1989, 1996; Healy et al. 1995; Johannsen et al. 1977). Also, TPP does not produce OPIDN in hens (Johannsen et al. 1977). Its potential to produce OPIDN following exposure to multiple small doses has not been reported. It is significant that although TBP is a weak acetylcholine inhibitor in rats, inhalation exposure to 15 mg/m³ TBP caused workers to complain of nausea and headache (ACGIH, 1986). Development of acute cholinergic neurotoxicity symptoms in humans exposed to TBP, indicate that it is readily absorbed primarily via inhalation, leading to its accumulation in the nervous system and resulting in acute poisoning. The Threshold Limit Value (TLV) for TBP is 2.2 mg/m³ (0.2 ppm) and its Permissible Exposure Level (PEL) is 5 mg/m³ (ACGIH, 1986). Tributoxyethyl phosphate (TBEP): In an 18-week study, Sprague Dawley male and female rats were treated with TBEP in two doses either 0.25 or 0.5 ml/kg bw for 5 days/week (Laham et al. 1984). Treated female rats showed ataxia and muscular weakness which disappeared 4 weeks later. Ataxia was confirmed by significant reduction of nerve conduction velocity at the end of the 18 weeks. This was consistent with histopathological, degeneration of the sciatic nerve was spotted by light and electron microscope examination. TBEP caused chronic neurotoxicity (OPICN) in male and female Sprague Dawley rats in a dose-dependent manner, where the high doses result in advanced degeneration. Degenerative lesion was supported by electrophysiological tests; where, they reported reduction in caudal nerve conduction velocity, in addition to the histopathological degenerative findings, consistent with OPICN (Laham et al. 1984, a and b;1985). Tris(2-ethylhexyl) phosphate (TEHP): No studies were reported regarding OPIDN of TEHP. This compound was studied in guinea pigs following 6 hours aerosol exposure daily for 5 daily for 5 days/week for 3 months. Neither the biochemical investigations (plasma and erythrocyte cholinesterase activity) nor the histopathological studies of the sciatic nerve and the spinal cord were affected. Bromo Aliphatic organophoshate Flame Retardants Tris(2,3-dibromopropyl) phosphate (TDBP): No enough information about its neurotoxicity is available. Chloro Aliphatic Organophosphates Flame Retardants Tris(1,3-dichloroisopropyl) phosphate (TDCPP): TDCPP is a poor inhibitor of acetylcholinesterase, and has low acute toxicity (Meeker et al., 2013). Animals that were treated with very high doses ( >1 g/kg/day) exhibited clinical signs of organophosphate poisoning, including muscle weakness, loss of coordination, hyperactivity, and death (Carignan et al., 2013). No studies have been reported for the ability of TDCPP to produce OPIDN. However, there are several in vivo and in vitro studies reporting its neurotoxicity (Kamata et al. 1989, IPCS 1998, Bullock & Kamienski 1972, Dishaw et al. 2011). Mice treated with a single oral dose of TDCPP-treated exhibited ataxia in addition to convulsions and hyperactivity 14 days after treatment. (IPCS 1998, Kamata et al. 1989). On the other hand, TDCPP-treated hens didn t exhibit signs of OPIDN (Bullock & Kamienski 1972 cited in IPCS 1998). In this study, TDCPP was given orally to white Leghorn hens in a dose of 420 mg/kg bw for 5 consecutive days and then observed for 21 days; however, no clinical signs of delayed neurotoxicity were detected. In another study (Wang et al. 2015), Zebrafish embryos were given TDCPP for six months until sexual maturation. The five days post fertilization larvae didn t show any biochemical or molecular changes in the expression of proteins related to CNS development as alpha-tubulin. However, downregulation of nervous system developmental genes was reported in adult fish. On the other hand, the result that alpha tubulin protein expression was affected, suggests that polymerization of microtubules may be affected leading to neurotoxicity. Furthermore, Wang et al. postulated that accumulation of TDCPP in brain tissue may be a possible mechanism for TDCPP neurotoxicity by directly targeting the brain. It was demonstrated that TDCPP causes neurotoxicity in vitro using PC12 cells (Dishaw et al. 2011, Ta et al. 2014). Dishaw et al. reported that TDCPP showed concentration-dependent neurotoxicity in PC12 cells (Dishaw et al. 2011,) Furthermore, treating PC12 cells with TDCPP resulted in different actions on expression of some genes; where, it caused the decrease in expression of GAP43, Nf-H and tubulin proteins as well (Ta et al., 2011). On the other hand, it led to the increase of the expression of CaM KII gene. These effects explain TDCPP-induced neurotoxicity via affecting neural network structures, growth of neural synapses, damaging nerve fibers and hence preventing information exchange between neurons (Wang et al., 2015). Tris(2-chloroethyl) phosphate (TCEP): Despite being widely used as a flame retardant, not many studies have been done to evaluate its ability to induce OPIDN. The neurotoxicity of TCEP has been reported more than once (Tilson et al. 1990, Umezu et al. 1998, Ta et al. 2014). A case of 5-year old girl exposed to TCEP in household timber (600 mg/kg wood) was reported to develop neurological deficit. The girl was exhibited symptoms characteristic of OPIDN such as weakness in the arms and abdominal muscles, in addition to reduction of nerve conduction velocity. Furthermore, 9 months later she was admitted to hospital with dystelectatic pneumonia; where the final diagnosis was spinal muscle atrophy with no further functional abnormalities detected at 2 years post exposure (NICNAS report No. 17, 2001) TDCPP was evaluated for OPIDN using white Leghorn hens in comparison with TOCP-treated hens as positive control (Bullock & Kamienski, 1972). No signs of OPIDN were shown in case of TCEP in contrast with TOCP-treated hens. In another study on White Leghorn hens, Sprague et al. 1981 evaluated the neurotoxicity of TCEP. The animals were given two doses of TCEP 3 weeks apart. In the group which survived, the histopathological examination of the brain, spinal cord and sciatic nerve showed no abnormality. Furthermore, biochemical evaluation of a separate group of hens (assessing NTE and plasma cholinesterase) was done after 24 hours of the first dose; where inhibition in cholinesterase activity was reported. However, no OPIDN was demonstrated (Sprague et al. 1981). In the same study on PC12 cells, Ta et al. showed that treating PC12 cells with TCEP lead to similar results to that of TDCPP. Moreover, TCEP led to decrease in the protein level of GAP43, and the tubulins; while it resulted in increasing in that of CaM KII and NF-H proteins. The same as TDCPP, affecting those proteins expression on the gene and protein level led to neurotoxicity via the effect mentioned earlier (Ta et al. 2014). Aromatic Aliphatic Organophosphates Flame Retardants Dibutyl Phenyl Phosphate (DBPP): The oral LD 50 of dibutyl phenyl phosphate (DBPP) in rats is 2,140 mg/kg. Its TLV is 3.5 mg/m³ (0.3 ppm) (ACGIH, 1986). In humans, DBPP causes eye injury and is an irritant to the skin and upper respiratory tract (ACGIH, 1986). Aerosolized or vaporized DBPP produced nose and throat irritation accompanied by coughing and wheezing. DBPP did not pr OPIDN in hens following oral administration (Hathaway 2004). Butyl Diphenyl Phosphate (BDPP): No information is available on the toxicity of BDPP. 2,6-Di-tert-butyl-p-cresol (butylated hydroxytolune, BHT): The oral LD 50 of BHT in rats is greater than 2,930 mg/kg. It functions as an antioxidant. BHT has a TLV of 2 mg/m³ (0.22 ppm) and a PEL of 10 mg/m³ (ACGIH, 1986). Ingestion of 4 and 80 g of BHT by two patients resulted in transient dizziness, brief loss of consciousness, unsteadiness of gait and slurred speech. Aromatic Organophosphates Flame Retardants Triphenyl Phosphate (TPP): No delayed neurotoxicity in TPP-treated hens (Johannesen et al. 1977). Nine hens were treated by 5000 mg TPP/Kg bw, and then observed daily till sacrificed on the 42 nd day of experiment. However, no pathological findings of OPIDN were observed. In accordance with Johannesen s study, Wills et al. demonstrated that TPP-treated cats didn t show OPIDN clinically or histologically (Wills, et al. 1979). Also, in vitro studies were done to evaluate TPP s ability to induce neurotoxicity. It was reported more than once that TPP can cause neurotoxicity (Gant et al. 1987, Flaskos et al. 1994, Hendricks et al. 2012). However, Hendricks et al. reported in 2014 that TPP doesn t induce neurotoxicity in Phenochromocytoma (PC12) cells or neuroblastoma (B35) cells (Hendricks et al. 2014). In a study evaluating workers in a TPP-producing plant, Sutton et al (1960) reported that the only difference between exposed and non-exposed groups was in the red blood cell cholinesterase activity (Sutton et al. 1960). They reported 18 % decrease in cholinesterase activity in the exposed group with no clinical manifestations, or unusual lab findings when compared to the non-exposed group. As a result, it was concluded that TPP doesn t induce neurotoxicity. TCP or trimethyl phenyl phosphate (TMPP ) is composed of several ortho (o), meta (m) , and para (p) isomers at the following ratios: 15 - 25% tri- meta -cresyl phosphate, 5 - 10% tri- para -cresyl phosphate, 60 - 75% mixed meta - and para -cresyl phosphate, and small amounts of ortho -cresyl phosphates (mainly in the mono- ortho -cresyl form), with low amounts of di- ortho -cresyl isomers and minute amounts of the tri- ortho -cresyl isomer, resulting in more than 10 cresyl isomers. Because jet oils contain up to 3% TCP as an anti-wear agent, inhalation exposure to the chemical constituents in this product is likely. Although the cholinergic neurotoxicity of TCP isomers is low, six members of this group of chemicals contain one or more of the ortho -cresyl moiety and are capable of causing OPIDN. There are 10 possible TCP isomers (Ramsden, 2013) listed in Table 6. Table 6: Isomers tri-cresyl phosphate (TCP). Ortho Content Isomers OPIDN Tri- ortho -MPP (TOMPP) o,o,o + Di- ortho -MPP (DOMPP) o,o,m, o,o,p + Mono- ortho -MPP (MOMPP) o,m,m o,m,p o,p,p + Non- ortho -TMPP m,m,m m,m,p m,p,p p,p,p - +, Produces OPIDN _, does not produce OPIDN The oral LD 50 of TOMPP in rats and chickens is more than 4,640 and 10,000 mg/kg, respectively (ACGIH, 2001). In contrast, a single oral dose of 250 mg/kg or 36 daily oral doses of 0.5 mg/kg (total dose of 18 mg/kg) TOCP produces OPIDN in adult hen, the animal of choice to study OPIDN, indicating that daily small doses were seven times more effective than a single oral dose in producing OPIDN (Abou-Donia, 2005). Further more, the dose of TMPP needed to cause OPIDN is more than 4,000 times the dose required to elicit acute cholinergic toxicity. Consistent with this is the finding that inhalation exposure to TMPP, in a manufacturing plant, produced toxic polyneuritis (Hunter et al., 1994). Furthermore, jet engine lubricating oils contain up to 3% TCP-including 0.1% TOCP, the potent OPIDN-producing isomer (Hewstone, 1994). Also, long-term inhalation exposure of chickens to concentrations of between 23 and 100 mg/m³ produced neurotoxic effects (Barrett and Oehme, 1985). It has been suggested that humans are 10 to 100 times as susceptible to developing OPIDN as chickens, the animal required by the U.S. EPA to test organophosphates for OPIDN (Barrett and Oehme, 1985. The available information suggests that the inhalation of contaminated aircraft cabin air may be related to OPICN. Aircrew members (including pilots and flight attendants) have consistently complained of neurological illnesses, such as headache, dizziness, cognitive dysfunction, difficulty concentrating, tremors, generalized weakness and lack of motor control, which are typical of OPICN. A study reported an unexpected high neurotoxic potency of aviation engine lubricants containing 3% TMPP levels and less than 0.02% of ortho -isomer (Freudenthal et al., 1993). In addition to the ortho -isomer, the presence of para -methyl phenyl phosphate (TPMPP) has been confirmed in two jet engine lubricating oils, Castrol 5000 and Exxon 2380 (Van Netten, and Leung 2000). Furthermore, preliminary results have shown that dermal exposure to each of the three isomers (that is, TOMPP, TMMPP, and TPMPP) caused sensorimotor deficits in rats and neuropathological lesions in the brain. Although most of the investigations of the health effects caused by contaminated cabin air have focused on OPIDN, TMPP and its constituent isomers, other components of the hydraulic fluids and engine lubricating oils should also be studied for their action in producing OPICN. These chemicals include: tributyl phosphate; tri-isobutyl phosphate; butyl diphenyl phosphate; dibutyl phenyl phosphateand tri-phenyl phosphate Endocrine Disruption by OPER Organophosphorus Compounds are endocrine disruptors that interfere with the natural hormones of the body. The endocrine system is composed of glands of an organism that secrete hormones directly into the circulatory system which are carried to a distant target organ. The major endocrine glands include: pineal gland, pituitary gland (O Connor et al., 2000), Pancreas, Ovaries, testes, thyroid gland (Dickhoff et al, 1983), parathyroid gland, gastrointestinal tract (Pederson et al., 2008), adrenal glands and hypo-thalamus (Silverman et al., 2005). In vertebrates, the hypothalamus is the neural control center for all endocrine systems. In contrast, the exocrine system, which secretes its hormones using ducts. The endocrine system s effects are slow to initiate, and prolonged in their response, lasting from a few hours up to weeks. Endocrine disruptors cause endocrine diseases that are characterized by disregulated hormone releases of the endocrine system Developmental Toxicity Infants and young children are believed to have a higher exposure to FRs when compared with adults because they spend more time indoors, in close proximity to contaminant sources, and engage in frequent hand-to-mouth contact (US EPA, 2008). A recent study found that toddlers had significantly higher exposure levels to TDCPP compared with their mothers (Butt et al., 2014). The higher levels in toddlers suggest that future research may be warranted to determine if exposure during early development is associated with adverse health outcomes in children. Studies in animals showed that prenatal exposure to low levels of OPs (e.g., chlorpyrifos) produced behavioral changes in adulthood affecting memory and learning (Eaton et al., 2008). These developmental neurotoxic effects may be related to OPs impact on different neurotransmitter systems (e.g. elevation of acetyl choline in nicotinic and muscarinic receptors (Abou-Donia, 1995). Neurotransmitters affects development of nervous system in different ways, including regulating cell migration, replication, growth, and differentiation (Behra et al., 2004; Lauder and Schambra, 1999). Moreover, OPs pesticides have epigenetic effects which regulate neurotrophic factors during development of CNS (Slotkin et al., 2008). In accordance with these postulations, the neurotoxicity of tris (1,3-dichloro-2-propyl) phosphate (TDCPP) comparing it to chlorpyrifos (CPF) was evaluated using PC12 cells system and different endpoints, e.g., changes in DNA synthesis , differentiation into different neurophenotypes, and neurite growth. TDCPP inhibited DNA synthesis, decreased cell number, and altered neuro differentiation (Dishaw et al., 2011) In this study, adverse effects in both the undifferentiated state and during neuro differentiation were observed. These findings indicate that developing nervous system is vulnerable to OPFRs effects from the earliest events of neural cell division till full differentiation. What was impressive in this work is that adverse effects of TDCPP were reported on using lower concentrations than CPF. Moreover, these results were not obtained through nonspecific cytotoxicity (as evidenced by absence of any effects on cell viability). It seems that the small degree of oxidative stress observed in the TDCPP studies may play a role in the observed outcomes. In PC12 cells, levels of oxidative stress that are insufficient to compromise cell viability are known to initiate neuro differentiation in the absence of NGF which was the landmark her for TDCPP induced effects (Katoh et al., 1997). In summary, profound effects on neural cell replication and neuro differentiation, processes that are critical to early neurodevelopment were observed (Dishaw et al., 2011). In the same context but on another system, adult zebrafish, chlorpyrifos has shown neurobehavioral deficits in zebra fish (Eddins et al., 2010; Sledge et al., 2011). These behavioral changes were associated with altered neurotransmitter levels. Hence, OPFRs with similar chemical structure to chlorpyrifos e.g. TDCPP, TCPP, and TCEP, TDBPP were tested for their neuro developmental toxicity in zebra fish when compared to CPF (Dishaw et al. 2014). Those four agents were shown to readily pass through the chorion, accumulating in early life stage fish tissues. Thus, they suggested that early life OPFR exposure may result in developmental effects similar to CPF. Their work showed that these chemicals affect the developing nervous system at exposure levels that are not overtly toxic. TDCPP and TDBPP and their respective metabolites were detected in embryos on 5 dpf. For all chemicals tested, develop mental exposures that were not overtly toxic significantly altered larval swimming activity, indicating that OPFRs adversely affect development of early life stage zebrafish. In this study, all chemicals induced behavioral effects in larvae at exposure levels that did not result in significant death or malformations that would affect swimming ability (e.g. stunted fin growth or spinal deformities). The authors suggested that altered swimming activity could be elicited by neurotoxicant effects of developmental OPFR exposure. Thus, OPFRs may have effects similar to organophosphate pesticides on neurotransmitter-mediated morphogenesis that are driving the behavioral changes observed in the present study (Dishaw et al., 2014). Using the same assessment system, an investigation into the effects of acute and long-term exposure to TDCPP in zebrafish was carried out (Wang et al., 2015). Zebra fi sh embryos (2 h post-fertilization) were exposed to TDCPP (0 100 g/L) for 6 months up until sexual maturation. Concentrations of TDCPP and its metabolic product (bis(1,3-dichloro-2-propyl) phosphate, BDCPP) were measured in the tissues of 5 day post-fertilization (dpf) larvae. Although they noticed no effects on several endpoints e.g. locomotion, acetyl cholinesterase activity, levels of the neurotransmitters dopamine and serotonin, and expression of mRNAs in any exposure group, adult fish showed reductions of dopamine and serotonin levels in the brains of females but not males. These results may be more logic as they found that TDCPP concentrations were higher in female brains. In this work, they observed high body burdens of TDCPP in fi sh, indicating bioavailability and bioaccumulation of this hydrophobic chemical under laboratory exposure conditions (log K = 3.76) (OEHHA, 2011). They also observed the primary metabolite of TDCPP (BDCPP) that is produced in mammals (Cooper et al., 2011; Carignan et al., 2013). Moreover, high TDCPP (and BDCPP) contents were detected in brain tissue, suggesting that it is able to cross the blood brain barrier and accumulates in brain suggesting that the brain may be a key target organ for TDCPP toxicity and this may have direct effects on nerve cells, leading to neurotoxicity. Interestingly, chemical analysis showed lower ratio of TDCPP: BDCPP in larval stages, indicating that larvae may have high biotransformation potential. On the other hand, higher body burdens of TDCPP and BDCPP in larvae could also be due to larval fi sh having a lower capacity to excrete these chemicals. This is in accordance to a previous study with PBDE bioaccumulation in developing mice denoting that young animals have a reduced ability to excrete PBDEs during development (Staskal et al., 2006). These data may underline the potential hazardous effects of these structures in early developmental stages. Mechanisms of OPFR-induced Developmental Toxicity Organophosphphorus flame retardants disrupt normal development. TDCPP have been reported to affect embryonic survival, behavior (righting reflex) and Purkinje cell development in white leghorn chicken (Bradley et al., 2015). Exposure of chicken embryo to 45 ug/g TDCPP resulted in shorter head-to-bill lengths, decreased body weight, and smaller gallbladders, while 7.64 ug/g lowered free thyroxine (T4) levels in the blood (Farhat et al., 2013). Similarly, zebrafish raised in water containing TDCPP died or developed severe malformations. This can be explained by altering mRNA expression of several genes that regulate thyroid function in zebrafish embryos and larvae (Dishaw et al., 2011; Farahat et al., 2013). Early life exposure also changed thyroid hormone levels in both zebrafish and chick embryos: triiodothyronine (T3) levels increased in exposed zebrafish while thyroxine (T4) levels decreased in both species (Liu et al., 2012). Another proposed mechanism for TDCPP developmental toxicity was also demonstrated by treating PC12 cells with TDCPP, where the cells showed decreased replication and growth, increased oxidative stress, and altered cellular differentiation (Farahat et al., 2013). In a developing organism, these effects could change the way the brain cells communicate and function, resulting in permanent changes in nervous system function (Mcgee et al., 2012). In another study, Na et al. (2014) investigated the effects of TDCPP and TCEP on the same cell system (PC12) with the same endpoints. In addition, they quantified mRNA and protein expression levels of CaMKII, GAP43, tubulin and NF-H. These two OPFRs led to decreased cell growth and increased apoptosis, moreover, there were significant changes in the gene and protein levels. TDCPP reduced levels of the six proteins plus GAP43, NF-H and the two tubulins, but it resulted in an increase in CaMKII gene levels. TCEP showed similar effects as of TDCPP beside an elevation in NF-H protein levels. In this study, the two major OPFRs, TDCPP and TCEP, were both shown to have the potential to elicit neurotoxicity. Furthermore, in their study, it was found that exposure to TDCPP or TCEP disrupts the developing nervous system, resulting in the loss of PC12 cells, an increase in cell apoptosis, damage to nerve fi bres, a reduction of nerve nodes and the failed formation of neural network structures. Although both agents caused a concentration-dependent cytotoxicity and neurotoxicity, TCEP showed side effects at higher concentrations compared with TDCPP. They attributed these effects to the different halogen atoms which appear to be an important factor in determining the effects of OPFRs on phenotypic fate (Dishaw et al., 2011; Na et al., 2014) In another system for assessing neurotoxicity, fertile domestic chicken eggs were injected with TCPP or TDCPP (maximum dose = 51,600 and 45,000 ng/g egg, respectively) to determine effects on different parameters including development, and TH levels following 20 22 days of incubation. TDCPP exposure resulted in significant decreases in head length, and reduced plasma free T levels at 7640 ng/g. Chemical analysis of egg contents at incubation days 0, 5, 11, 18, and 19 revealed that > 92% of the injected TCPP or TDCPP concentration was detectable up to day 5. The observed effects of TCPP and TDCPP exposure may be due to disruption of the TH hormone (Farhat et al., 2013, a,b). Reproductive Toxicity Men living in homes with high concentrations of TDCPP in house dust were more likely to have decreased sperm counts and increased serum prolactin levels (Freudenthal and Henrich 2000). Previous study has demonstrated 100 mg TOCP/kg/day to be an OPIDN-inducing dose with minimal mortality in roosters. This dose level was administered to adult leghorn roosters (p.o., n = 10) for 18 consecutive days. By days 7 - 10 of the study, TOCP-treated birds exhibited limb paralysis characteristic of OPIDN. Analysis at termination revealed significant inhibition of neurotoxic esterase activity (NTE) in both brain and testis. There was also a slight decrease in brain acetyl cholinesterase (AChE) activity. Sperm motility was shown to be greatly decreased. In addition, sections of formalin-fixed, methacrylate-embedded testes from TOCP-treated birds showed vacuolation and disorganization in the seminiferous epithelium. The marginal body weight decreases (17%) in treated animals were not considered to contribute to the testicular toxicity induced by TOCP. Parathion (O,O-diethyl-O-4-nitrophenyl phosphorothioate, 0.1 mg/kg/day, p.o., n = 3) was used as a positive control for AChE inhibition and a negative control for inducing OPIDN. Roosters treated continuously with parathion showed a decrease in brain AChE activity, but no changes in NTE, testicular histology, or limb function. These studies demonstrate the testicular toxicity of TOCP in roosters and suggest that this effect is not related to the chemical s anticholinergic action (Somkuti et al., 1987b). A subchronic 63-day study (reflecting the 49-day length of the rat seminiferous epithelium cycle plus the 14-day transit time of spermatids through the epididymis was initiated. Dose-dependent (10 to 100 mg TOCP/kg/day) decreases in cauda epididymal sperm motility and density, testicular enzyme activities, and alterations in sperm morphology were observed. Concurrent pairfed controls (matched to the highest dose group, 100 mg TOCP/kg/day) indicated that weight loss resulting from TOCP administration had minimal contributory effects to the testicular toxicity seen. Plasma alpha-tocopherol acetate (vitamin E) and testosterone concentrations were unaffected. Tri-p-cresyl phosphate (TPCP), the non neurotoxic structural analog of TOCP, produced no toxic effects, demonstrating the necessity of the ortho -cresol moiety for induction of damage. A minimum effective (threshold) dose for observable testicular toxicity was determined to be 10 - 25 mg TOCP/kg in this study. These data suggest that TOCP interferes with spermatogenic processes and sperm motility directly and not via an androgenic mechanism or decreased vitamin E availability (Somkuti et al., 1987a). The onset and development of testicular lesions following tri-o-cresyl phosphate (TOCP) dosing have been documented through light and electron microscopic morphological studies. Male Fischer 344 rats (190 - 210 g body weight) were administered 150 mg TOCP/kg/day in corn oil for 1, 3, 5, 7, 10, 14, and 21 days. Vehicle-treated rats served as the control group. Sections of formaldehyde- and glutaraldehyde-fixed, methacrylate-embedded testes showed, by Day 5, numerous spermatid heads apparently detached from tails lying at oblique angles near the basement membrane of the seminiferous tubules. Columnar and spherically shaped vacuoles of the epithelium, radiating from the basement membrane to the lumen of the tubules, were also observed. Electron micrographs revealed that these were localized in Sertoli cells. Widespread dilation of Sertoli cell smooth endoplasmic reticulum was also noted. By 7 days of treatment, residual body abnormalities were noted in stage VIII tubules, along with spermatocyte-derived multinucleated giant cells. The lesion progressed with increased vacuolation of the epithelium and numbers of abnormal residual bodies and giant cells, together with spermatid karyorrhexis (Days 10, 14, and 21). There was also an apparent decrease in sperm density/tubule with continued exposure: 90% of the seminiferous tubules were devoid of sperm by Day 14. These morphological results indicate an initial effect of TOCP on Sertoli cells. Spermatogenesis is affected as seen by the decrease in sperm density and increase in necrotic spermatids (Somkuti et al., 1991) Hepatotoxicity In another study, exposing chicken embryonic hepatocytes (CEH) and chicken embryonic neuronal (CEN) cells to TDCPP and TCPP and using real-time reverse transcription-PCR (real-time RT-PCR) assays to test gene expression changes in genes associated with the chicken xenobiotic-sensing orphan nuclear receptor (CXR), the TH pathway, lipid regulation, and growth Crump et al. (2012) In this study, TDCPP was toxic to hepatocytes (LC5 60.3 45.8 mM) and neuronal cells (LC 50 28.7 19.1 mM), whereas TCPP did not affect viability in either cell type up to the highest concentration administered, 300 mM. However, both had a significant and similar impact on the expression of genes associated with CXR activation, the TH pathway, lipid regulation, and growth. Neurotransmitters Finally, the similarity in chemical structure may point to possible effects similar to organophosphate pesticides on neurotransmitter-mediated morphogenesis. OPFRs are derivatives of phosphoric acid with different substitutes including alkyl chains (e.g., tri-n-butyl phosphate (TnBP)), partly halogenated alkyl chains (e.g., tri(2-chloroethyl) phosphate (TCEP)) as well as aromatic contents (e.g., triphenyl phosphate (TPhP). Some studies showed that, similar to OPs pesticides, OPEs target esterases, such as the neuropathy target receptors. Animal data showed that some of these chemicals including tributoxyethyl phosphate (TBEP), TCEP, TPhP and tri- m -cresyl phosphate (TCrP) decreased AChE activity in rats (Bollman et al., 2012; Moller et al., 2012). Carcinogenicity Several studies showed that TDCPP is a carcinogenic. Rodents that were fed TDCPP over two years showed increased tumor formation in the liver and brain (Eaton et al., 2008). Metabolites of TDCPP were also determined to be mutagenic in bacteria using Ames test (Gold 1978). In 2011, TDCPP was listed as a carcinogen under California Proposition 65, a law that identifies and regulates chemicals determined by the California Environmental Protection Agency to cause cancer, birth defects or other reproductive harm (Eldefrawi et al., 1977). The effects of the flame retardant tris(2,3-dibromopropyl) phosphate (Tris-BP) on growth, sister chromatid exchanges (SCE), and chromosome aberrations of Chinese hamster V79 cells cultured either in vitro or in diffusion chambers (DC) implanted into mice were studied. Tris-BP caused a dose- and time-dependent reduction of cell growth as measured by colony-forming activities (Furukawa et al., 1978). A significant dose-dependent...