The Toxicity of Commercial Jet Oils Chris Winder1 and Jean-Christophe Balouet2 Contact: Assoc Prof Chris Winder, School of Safety Science, University of New South Wales, Sydney NSW 2052. Jean-Christophe Balouet, Managing Director, Environment International, 31 Rue du General Chanzy, 94130 Nogent sur Marne, France Keywords: Jet Oils, Tricresyl phosphate, N-phenyl-1-naphthylamine, synthetic oils, oil exposure. Abstract Jet oils are specialised synthetic oils used in high performance jet engines. The have an appreciable hazard based on toxic ingredients, but are safe in use provided that maintenance personnel follow appropriate safety precautions, and the oil stays in the engine. Aircraft engines that leak oil may expose others to the oils through uncontrolled exposure. Airplanes that use engines as a source of bleed air for cabin pressurisation may have this source contaminated by the oil if an engine leaks. Examination of the ingredients of the oil indicates that at least two ingredients are hazardous: N-phenyl-1-naphthylamine (a skin sensitiser) and Tricresyl phosphate (a neurotoxicant, if ortho-cresyl isomers are present). Publicly available information such as labels and MSDS understates the hazards of such ingredients, and in the case of ortho-cresyl phosphates, by several orders of magnitude. Introduction Some commercial jet oils have been in use as engine oils in aviation for decades. For example, Mobil USA note that one of their products “Mobil Jet Oil II has been essentially unchanged since its development in the early 1960s” and “most changes have involved slight revisions of the ester base stock due to changes in raw material availability”.[1] A complex approval process exists for ensuring that materials used in aviation are manufactured to relevant standards, and the jet engine oil specification of the US Navy MIL-PRF-23699 is used for jet oils. This process of approval and re-approval for new product formulations has meant that there is some resistance to modifying formulations (for example, for health and safety reasons). Consequently, changing approved formulations is not conducted without significant justification. In the case of the additive tricresyl phosphate (TCP), manufacturers have been reluctant to modify product formulations by substituting toxic TCP additives that perform well in critical applications. This has meant that potentially toxic products have continued to be available long after their toxicity was recognised.[2] It is not known if an approved formulation containing, for example 3% tricresyl phosphate, is considered a change in formulation if the proportion of individual isomers in the TCP mixture is altered, but the 3% remains unchanged. However, as Mobil indicate, only the base stock esters have been modified over the past thirty or so years, suggesting that the mixture of isomers in TCP stock has not been changed.[3] Mobil USA notes that one of their jet oil products (Mobil Jet Oil II) has a market share of 49%. With such a large market share, and the potential for significant exposure, it would be appropriate to investigate this material in some detail. Mobil Jet Oil II Mobil Jet Oil II is a synthetic oil product imported into Australia. All product worldwide is manufactured by one manufacturing facility in the USA. The product is not labeled in accordance with Australian requirements under the Hazardous Substances Regulation, but is assumed to comply by default.[4] This product is normally marketed in 0.946 L (1 US Quart) cans. Ingredients Various sources, such as the supplier's label on the cardboard box the cans are shipped in, the product Material Safety Data Bulletin (MSDB), and information from Mobil USA, lists the following ingredients: m                   synthetic esters based in a mixture of 95% C5-C10 fatty acid esters of pentaerythritol and dipentaerythritol; m                   3% tricresyl phosphate (Phosphoric acid, tris(methylphenyl) ester, CAS No 1330-78-5); m                   1% phenyl-alpha-naphthylamine (PAN) (1-Naphthalenamine, N-phenyl, CAS No 90-30-2); m                   Benzamine, 4-Octyl-N-(4-Octylphenyl), (CAS No 101-67-7); m                   a last entry "ingredients partially unknown" is also noted on some documentation. In Australia, classification of materials as being hazardous substances under the Hazardous Substances Regulation use a list of hazardous substances[5] and approved criteria,[6] with reference to the list being the primary step. Of the ingredients in Mobil Jet Oil II, the most toxicologically significant ingredients are: m                   N-phenyl-alpha-naphthylamine, which can contains a number of contaminants in trace amounts, including N-phenyl-beta-naphthylamine (135-88-6), 1-Naphthylamine (CAS No 134-32-7) and 2-Naphthylamine (CAS No 91-59-8); and m                   Tricresyl phosphate, a blend of ten tricresyl phosphate isomer molecules (including tri-ortho-cresyl phosphate), plus other structurally similar compounds, including phenolic and xylenolic compounds. There are a number of issues relevant to these ingredients, outlined below. N-Phenyl-1-naphthalenamine Chemistry N-Phenyl-1-naphthylamine, (CAS No 90-30-2), also known as Phenyl-alpha-naphthylamine (PAN), is a lipophilic solid used as an antioxidant used in lubrication oils and as a protective agent in rubber products. In these products, the chemical acts as a radical scavenger in the auto-oxidation of polymers or lubricants. It is usually used in these products at a concentration of about 1%. The commercial product has a typical purity of about 99%. Named impurities are: N-Phenyl-2-naphthylamine (CAS No 135-88-6, 500 to below 5000 ppm), 1-Naphthylamine (below 100-500 ppm) and 2-Naphthylamine (below 3 to 50 ppm), aniline (below 100 to 2500 ppm), 1-naphthol (below 5000 ppm), 1,1-dinaphthylamine (below 1000 ppm) (see Figure below).[7] Figure 1: Possible Contaminants in N-Phenyl-1-naphthylamine 2-Naphthylamine (CAS No 91-59-8) is also known as the established carcinogen b-Naphthylamine.[8] Similarly 1-Naphthylamine is also known as a-Naphthylamine. The formulation concentration of N-Phenyl-1-naphthalenamine in Mobil Jet Oil II is about 1%. As ingredients such as the naphthylamines have been deleted from product documentation such as the MSDB, the level of contamination of naphthylamines is presumed to be below the concentration cut off values for disclosure of Category 1 carcinogens specified in the Approved Criteria for Classifying a Hazardous Substance of 0.1% (1000 ppm).6 Indeed, information from Mobil Australia notes that the level of contamination of some of the contaminants in this material is partially known (50 ppm for N-Phenyl-2-naphthylamine; 0.5 ppm for 2-Naphthylamine), and that they stopped listing such ingredients in about 1992 “solely to a reassessment of what was considered meaningful information from a hazard communication perspective”.[9] 2-Naphthlyamine is not listed on the 1992 Australian inventory of Chemical Substances (AICS),[10] and dependent on the amount present in the formulated product (0.2%), could technically breach the requirements of the Commonwealth Industrial Chemicals (Notification and Assessment) Act 1989. However, the probable concentration of this contaminant in Mobil Jet Oil II is too low to exceed requirements of this legislation. Further, this chemical is listed as a prohibited substance under the Australian Hazardous Substances Regulation. Toxicology PAN is readily absorbed by mammalian systems and rapidly converted to metabolites.[11] Both urine and feces appear to be the main routes of excretion.[12] By single dosing, PAN does not seem particularly toxic, with LD50s above 1 g/kg. The chemical has a similar mechanism of toxicity of many aromatic amines, of methaemoglobin production. PAN is not irritating in primary skin and eye irritation studies. However, in a guinea pig maximisation test, PAN was shown to be a strong skin sensitiser.[13] This result is supported by case studies in exposed workers.[14],[15] At the concentration used (1%), Mobil Jet Oil II is classified as a hazardous substance in Australia for its sensitisation properties.6 Most genotoxicity studies report negative results, suggested little genotoxicity potential.12 Most repeated dose toxicological studies focus on its potential carcinogenicity. An experimental study, using both PAN and the related compound N-phenyl-2-naphthalenamine administered subcutaneously to mice found a heightened incidence of lung and kidney cancers.[16] While the methodology used in this study makes evaluation of the results problematic (use of one gender, small sample sizes, limited number of dose groups, subcutaneous administration as an inappropriate route of exposure, and so on). A high incidence of various forms of cancer was also found among workers exposed to antirust oil containing 0.5% PAN.[17] While these animal and human results offer only limited information, they are at least supportive of a mild carcinogenic effect. This must be contrasted with the results of long term carcinogenicity bioassays in rats and mice conducted by the US National Toxicology Program with the structurally related N-phenyl-2-naphthylamine (studies were not carried out on PAN), which have not reported any carcinogenic potential for this chemical.[18] Regulatory Classification PAN is not listed on the NOHSC Designated List of Hazardous Substances. However, the NOHSC Approved Criteria for Classifying Hazardous Substances6 note that mixtures containing sensitisers should be classified as an “Irritant” hazardous substance if included in the product at a concentration at or greater than 1%. Further, a product containing a skin sensitiser at or above this value should carry risk statement R43 - May cause skin sensitisation by skin contact. The data on carcinogenicity of PAN is too limited to make a determination sufficient to allow classification for regulatory purposes. Nevertheless, based on established sensitisation properties and possible carcinogenic properties, exposure to materials containing N-phenyl-1-naphthylamine should be avoided. Tricresyl phosphate Phosphoric acid, tris(methylphenyl) ester (CAS No 1330-78-5) is better known as Tricresyl phosphate (TCP) or Tri-tolyl phosphate. Chemistry of the Cresols and Tricresyl phosphate Industrial manufacture TCP is a molecule comprised of three cresyl (methylphenyl) groups linked to a phosphate group. Cresol is an aryl structure comprising a hydroxyl (-OH) and methyl (CH3) group attached to a benzene molecule. Industrial cresol is a mixture of three isomers, ortho- para- and meta-cresol molecules in varying concentrations. The ortho-, meta- or para- prefixes denote how far apart the hydroxyl and methyl groups are on the cresol molecule (see Figure below). Figure 2: Structure of Tricresyl Phosphate TCP molecule showing designation of o, m and p cresyl groups o-Cresol m-Cresol p-Cresol Phosphate Tricresyl phosphate Industrially, the chemical is made by reaction of phosphorus oxychloride (POCl3) with industrial cresol. Commercial grade TCP is a complex mixture of structurally related compounds, some of which are known to have neurotoxic properties. These are produced from the ortho-alkyl substituted phenols or xylenol present in the manufacturing process. ortho-methyl phenols (cresol) or ortho-ethyl phenols lead to toxic components, whereas ortho-substituted xylenols do not.[19] Initially, TCP contained high levels of all isomers. The neurotoxic potential of the ortho-cresyl isomers, most notably tri-ortho-cresyl phosphate (TOCP), was recognised quite early.[20] Indeed much research has been carried out on the toxicity of TOCP, presumably on the basis that as it had three cresyl groups, it must be more toxic than molecules with less. There have been substantial modifications of TCP containing materials. Earlier TCP products, such as “torpedo oil” used in World War II, were highly toxic, containing perhaps 25-40% ortho-cresol. Notably, this product was more toxic than TOCP itself.[21] This is a critical finding, because it meant that the conventional view that the toxicity of TCPs was correlated to their tri-ortho-cresyl content was incorrect. The presence of other ortho-cresyl containing molecules (not just TOCP) needs consideration in evaluating the overall toxicity of TCP. Manufacturers reduced the levels of ortho-cresyl and ortho-ethylphenyl isomers to reduce the potential for neurotoxicity. Changes to the phenolic mixture used to manufacture TCP, introduction of processing alternatives and improved purification methods all assisted in reducing ortho-cresol content. By the 1950s, commercially available TCP contained about 3% ortho-cresol isomers. Further refinements in the 1980s to 1990s have decreased the ortho-cresol content further. How much these refinements had removed the toxic impurities outlined above is not known. Indeed, toxicity was still being detected in commercially available products in 1988.2 It is difficult to obtain data on the amount of TOCP contamination in commercially available materials now being marketed world-wide containing TCP. However, conservative estimates of about 0.1-1% (100-1000 ppm) seem realistic. This suggests that a product containing 3% TCP would contain about 0.003-0.03% TOCP (3-30 ppm). The “new generation” materials are claimed to have an even lower TOCP content, although data on content is sparse. 2 Importantly however, is that the focus of attention on the toxicity of TOCP has masked the study of the toxic potential of other orthocresyl isomers. Further, work by Henschler and colleagues in the 1950s (published, but published in German) was not reconsidered until the 1990s. Typically, jet turbine engine oils are formulated with about 3% TCP. This includes Mobil Jet Oil - 3% TCP is stated on MSDB, and is supported by data published in elemental analyses,[22] where a Mobil Jet Oil was shown to contain 0.29% Phosphorus, which extrapolates to about 3.5% organophosphate. Uses of TCP TCP has been a commercially useful material, and has been used as a plasticiser, lubricant, hydraulic fluid, paint additive, oil additive, dust suppressant and so on.[23],[24] Most commercial uses have now ceased. In jet oil, TCP is used in the formulation of lubricants as an anti-wear additive to enhance load bearing properties and improve tolerance to increasing speed of rotating or sliding motion. It also has flame retardant properties. While some other triaryl phosphates have similar properties and may also be used as oil additives, the anti-wear properties of TCP are considered unique. For example, pure tri-para-cresyl phosphate is considered to have poorer lubricating properties than commercial TCP.2 Isomers of TCP Generally, the chemical known as TCP comprises a mixture of unspecified ortho- para- and meta-cresol molecules (as cresyl groups, see above), which can be formed into a number of separate structures with similar chemical formulas (isomers). Technically, there are ten possible tri-cresyl phosphate structures (see below). Figure 3: Possible Isomers of Tricresyl phosphate o ortho-cresyl group m meta-cresyl group p para-cresyl group ortho-cresyl group containing molecules are highlighted in bold The structures of the ten different isomers are shown below. Figure 4: Possible Tricresyl phosphate Structures   ortho- ortho- Ortho- ortho- ortho- meta-         tri-ortho-cresyl phosphate di-ortho-meta-cresyl phosphate   ortho- para- ortho- ortho- para- meta- ortho- meta- meta- di-ortho-para-cresyl phosphate ortho-para-meta-cresyl phosphate ortho-di-meta-cresyl phosphate ortho- para- para- meta- para- para- para- para- para- ortho-di-para-cresyl phosphate meta-di-para-cresyl phosphate tri-para-cresyl phosphate   meta- meta- meta- meta- meta- para-         tri-meta-cresyl phosphate para-di-meta-cresyl phosphate   The different isomers of TCP have different properties, and indeed, different toxicities. Most notably, tri-orthocresyl phosphate (TOCP) is a well established neurotoxicant (see below). TCP Nomenclature Describing Tricresyl phosphate isomers chemically can be a complicated task. However, the Chemical Abstracts Service (CAS) has simplified this process by allocating four unique identifying CAS registry numbers to Tricresyl phosphate mixtures. These are listed on the Australian Inventory of Chemical substances:10 m                   CAS No 1330-78-5 Phosphoric acid, tris(methylphenyl) ester (C21H21O4P), which denotes Tricresyl phosphate (unspecified cresyl groups); m                   CAS No 78-30-8 Phosphoric acid, tris(2-methylphenyl) ester (C21H21O4P), which denotes Tricresyl phosphate (containing ortho-cresyl groups); m                   CAS No 563-04-2 Phosphoric acid, tris(3-methylphenyl) ester (C21H21O4P), which denotes Tricresyl phosphate (containing para-cresyl groups); m                   CAS No 78-32-0 Phosphoric acid, tris(4-methylphenyl) ester (C21H21O4P), which denotes Tricresyl phosphate (containing meta-cresyl groups). In the past, disclosure of tricresyl phosphate ingredients in products containing this chemical invariably used the nonspecific 1330-78-5 CAS number. Unfortunately, this provides no information about the various isomers in the mixture. In its classification systems for hazardous substances, the European Union (EU) has introduced modifications of two of the CAS descriptions for tricresyl phosphate chemicals, being: m                   CAS No 78-30-8 Tricresyl phosphate (containing o-o-o, o-o-m, o-o-p, o-m-m, o-m-p, o-p-p isomers); m                   CAS No 78-32-0 Tricresyl phosphate (containing m-m-m, m-m-p, m-p-p, p-p-p isomers). The reason for this change was to discourage use of the general TCP mixture CAS Number 1330-78-5 (which is proposed to be deleted), and encourage better disclosure of ortho-cresyl containing mixtures. Newer documentation by jet oil manufacturers suggests this has not yet happened, with the older 1330-78-5 CAS Number still in use on product information. It can be argued that the continued use of the older 1330-78-5 number by industry indicates that they are ignorant of the changes at the EU level and the implications of these changes for disclosure on labels and material safety data sheets. The new CAS numbers will assist in identifying those products that contain the toxic ortho-cresyl ingredients. At the moment, it may be presumed that from a marketing perspective, disclosure of the new CAS number that indicates the presence of ortho-cresyl containing TCP in commercial products is undesirable, and therefore companies are persisting with the older generic CAS number. From this, it may be assumed that the absence of the non-ortho-cresyl containing TCP CAS number indicates that ortho-cresyl groups are present in the mixture. This is further supported by the absence of positive statements about the absence of ortho-cresyl containing isomers in TCP products. The EU chemical names and numbers are listed in the Australian List of Designated Hazardous Substances, which forms a major part of the classification of hazardous substances under the hazardous substances regulations.5 Suppliers of tricresyl containing materials should be referring to the new CAS numbers and chemical descriptions as soon as practicable. Further, a requirement to “state on the label whether the substance is a specific isomer or a mixture of isomers” is included in the List. Toxicity of Tricresyl phosphates Toxicology of the Organophosphates Human toxicity to organophosphorus compound has been known at least since 1899, when neurotoxicity to phosphocreosole (then used in the treatment of tuberculosis) was reported.[25] The study of the toxicity is extensive, with two very well established mechanisms on esterases and on neurotoxic esterases (NTE). Poisoning with Organophosphates The organophosphorus compounds are generally characterised by a toxicity of inhibition of the esterase enzymes, most particularly cholinesterases[26] and neurotoxic esterases.[27] The mechanism of effect is phosphorylation.[28] The effect is a specific mechanism of organophosphate toxicity. An organophosphorus molecule can be represented by the general structure: Where P is the Phosphorus atom, O is an oxygen atom and R1-R3 represents organic structures that can give the molecule a wide range of properties. Because cholinesterases break down endogenous choline esters, inhibition of these enzymes produces an accumulation of levels of choline esters. Most critical of these esters is acetylcholine, a neurotransmitter molecule released throughout the cholinergic nervous system. Any organ or tissue that receives a cholinergic input will become more active or excited if cholinesterases are not available to catalyse the breakdown of acetylcholine. Indeed, cholinergic overstimulation produces most, if not all, of the symptoms of poisoning from single and short term exposure to organophosphates. Signs of low level intoxication include headache, vertigo, general weakness, drowsiness, lethargy, difficulty in concentration, slurred speech, confusion, emotional lability and hypothermia.[29] The reversibility of such effects has been questioned.[30] Signs of poisoning are usually foreshadowed by the development of early symptoms related to acetylcholine overflow and include salivation, lacrimation, conjunctivitis, visual impairment, nausea and vomiting, abdominal pains and cramps, diarrhoea, parasympathomimetic effects on heart and circulation, fasciculations and muscle twitches.[31] This is the basic site of inhibition for all OP molecules.[32],[33] Organophosphate Induced Delayed Neuropathy (OPDIN) There is a second reaction that leads to further neurotoxic and neuropathological changes. Inhibition of neurotoxic esterases (NTE) can lead to a neuropathological condition of progressive neuronal damage, called organophosphorus induced delayed neuropathy (OPIDN).[34],33 The mechanism of toxicity is now fairly well understood, as indeed are the organophosphorus structures which are predicted to cause OPIDN.[35] Basically, all OP molecules react with any -OH groups on the active site of the enzyme: Enzyme-OH + = The basic process is the initial phosphorylation of a group of esterases called the neurotoxic esterases (NTE). This is followed by a second reaction of enzyme “aging”, where the enzyme structure (or its microenvironment) was modified so that it can no longer function properly. The basic mechanism is a break in the P-O-R bond, resulting in a negatively charged P-O- group, and a free -R group. A determinant of toxicity is the extent of inhibition of these enzymes, in that marked toxicity occurs after inhibition of over 50%.[36] Several theories about the significance of these events in the development of OPIDN,[37] and a pathway of events have been proposed. [38] The likelihood of this reaction occurring is dependent on the molecular structure of the OP molecule. Where either or both of the R1 or R2 groups are linked to the phosphorus with a P-O-R bond (instead of a P-R bond), OPIDN can develop. These OP structures are: The main classes of organophosphorus molecules that have the potential to cause OPIDN are phosphates (two P-O-R bonds) and phosphonates (one P-O-R bond). A further group known to cause OPIDN are the phosphoroamidates, where the oxygen in the P-O-R bond is replaced by nitrogen (R-N-R). Where the OP molecule only contains P-R bonds, aging (and therefore delayed neuropathy) will not occur. The main classes of organophosphorus molecules that have these structures are the phosphinates.[39] Not all animal species are susceptible to developing OPIDN: for example, rodents are not particularly sensitive[40] (although neurological damage can be produced in the rat[41]). However, along with the cat[42] and chicken,[43],[44] humans are considered to be among the most sensitive species.[45] OPIDN is caused when the organophosphate molecule binds with NTE in the long processes of the nerves (the axons). The enzymes have functions related to transport of nutrients and energy molecules from the cell body to the end of the nerves. Phosphorylation of such proteins results in localised disruption of axoplasmic transport. If prolonged, these effects are followed by swelling of the axon, followed by degeneration from the site of the damage to the end of the axon. If exposure continues, this process can continue up the axon by the phosphorylation of more proteins. Lesions are characterised by degeneration of axons followed by degeneration of the cells that surround (and contribute to the insulation of the fibres) the myelin containing support cells.45 This effect can occur in sensory or motor nerves in either the central or peripheral nervous systems.[46] Initially, the condition arises as a distal symmetrical sensori-motor mixed peripheral neuropathy mainly affecting the lower limbs with tingling sensations, burning sensations, numbness and weakness. In severe cases paralysis may develop.[47] Longer nerves are affected more, probably because or their requirements for active nutrient supply (shorter nerves may continue to get supplied through passive mechanisms, such as diffusion). Regeneration is possible if exposure ceases and damage is not too extensive.[48],37 The Intermediate Syndrome OPIDN is severe. It is quite likely that such a severe condition would be presaged with a range of clinical and pre-clinical signs and symptoms. These have been reported extensively, and an “intermediate syndrome” was defined in 1987.[49] Symptoms of the intermediate syndrome include: proximal limb paralysis, weakness of neck muscles, inhibition of respiratory muscles and cranial nerve involvement. The mechanism of effect is different from poisoning or OPIDN effects, and is considered to be due to the effect of the organophosphate at the level of the neuromuscular synapses.[50] Chronic Organophosphate Neuropsychological Disorder (COPIND) More recently, chronic exposure to organophosphates has been associated with a range of neurological and neuropsychological effects.[51],[52],[53],[54],[55] Such symptoms (mainly neurological and neurobehavioural symptoms) may also be seen in exposed individuals who have been sufficiently fortunate in not having exposures that were excessive enough in intensity or duration to lead to clinical disease. A distinct condition - chronic organophosphate neuropsychological disorder (COPIND) has been described, of neurological and neuropsychological symptoms.[56] These include: m                   diffuse neuropsychological symptoms (headaches, mental fatigue, depression, anxiety, irritability); m                   reduced concentration and impaired vigilance; m                   reduced information processing and psychomotor speed; m                   memory deficit and linguistic disturbances; COPIND may be seen in exposed individuals either following single or short term exposures leading to signs of toxicity,52 or long term low level repeated exposure with (often) no apparent signs of exposure.54 The basic mechanism of effect is not known, although it is not believed to be related to the esterase inhibition properties of organophosphorus compounds. It is also not known if these symptoms are permanent. Toxicology of TCP and TOCP Much of the early study of OPIDN was investigated not just with organophosphorus compounds, but with the tricresyl phosphates[57],[58] following outbreaks of poisoning after accidental or criminal adulteration of food or beverages with TCP containing products. A large literature is now available on the toxicity of the tricresyl phosphates (most particularly, TOCP) and the basic mechanisms are well established.[59] TCP produces acute poisoning based on cholinesterase inhibition, and a well defined syndrome of neurological degeneration (either from short term or long term repeated dose exposure). As well as affecting the nervous system, TCP also has toxic effects in the adrenal glands, ovaries and testes.[60] TCP is also known to be a skin irritant and to cause allergic dermatitis.59 Neurotoxicity has been reported in TCP manufacture.[61] The toxic effects of oils containing TCP have also been long recognised.[62] The toxic properties of tri-ortho-cresyl phosphate have been recognised for decades, and the presence of this isomer in products containing TCP presents a significant occupational health problem. Further, as noted above, there are five other orthocresyl phosphate isomers: m                   two di-ortho-cresyl phosphates (di-ortho-mono-meta-cresyl phosphate or o-o-m and di-ortho-mono-para-cresyl phosphate or o-o-p); and m                   three mono-ortho-cresyl phosphates that contain only one ortho-cresyl group but various combinations of meta-cresyl and para-cresyl groups (o-p-p, o-p-m, o-m-m). These mono- and di- ortho-tricresyl phosphates are reported to have measurable toxicities similar to the neurotoxicity produced by TOCP. Other ortho-cresyl containing ingredients Tricresyl phosphate will also contain mixed esters of orthophosphoric acid with different cresyl radicals, of the mono- and di-cresyl types. Other contaminants, such as ortho containing di-cresyl phosphates may also be toxic. Further, mono-ortho-cresyl-diphenyl phosphate (that is, an organophosphate molecule with one cresyl group only (see below) appears to be the most toxic molecule of all.21 Mono-ortho-di-phenyl phosphate Further, other ortho-containing molecules, such as 2,3-Tri-xylenyl phosphate and 2,4-Tri-xylenyl phosphate, are weakly neurotoxic (this is a cresyl molecule with an extra methyl group, the 2- indicates the ortho- position, see below).19 Possible Tri-xylenyl phosphate Structures 2,3-Tri-xylenyl phosphate 2,4-Tri-xylenyl phosphate Other trixenyl phosphates, such as 2,5, 2,6, 3,4 and 3,5 were not neurotoxic. Still other impurities, such as triphenyl phosphate, di-phenyl-mono-cresyl- phosphate, di-phenyl-mono-xylenyl phosphate and tri-xylenyl phosphate may also be neurotoxic. The presence of structures with methyl groups adjacent to the ester -O-P bond, needs consideration in evaluating the overall toxicity of TCP. Recent research has focused on identifying a dose response relationship for TOCP. Results of a short term repeated dose study in hens of aviation engine oil containing various amounts of commercial TCP suggest that oil containing 1% TCP (a TCP equivalent of 20 mg/kg/day) was considered a no observable effect level.[63] Similar findings were reported in a later study.[64] Finally, it is generally assumed that most exposure to TOCP is by the inhalational route (ingestion is unlikely for persons not directly handling this material). However, absorption through skin exposure should not be discarded, as significant exposure (maximally estimated at a transdermal flux rate of 0.01 mg/cm2/hr) through this route is possible.[65] Relative Toxicity of the ortho-Cresyl Containing Tricresyl phosphate Isomers The ten isomers that make up TCP are toxicologically different, and it is well established that the ortho containing isomers are the most toxic. Much research in the past has concentrated on the tri-orthocresyl phosphate isomer (TOCP), which has shown to be associated with organophosphate induced delayed neuropathy (OPIDN). TCP manufacturers have expended considerable energy in reducing levels of TOCP in commercial grades of TCP. However, what is less well known is that there are other ortho containing isomers in TCP, three mono-ortho (MOCP) isomers and two di-ortho (DOCP) isomers. These are not specified in mandated lists of hazardous chemicals, and this may be one reason why they are not disclosed on labels and MSDS. All these compounds are neurotoxic in the same way as TOCP - however they are