Development of California Public Health Goals (PHGs) for chemicals in drinking water.

As part of a program for evaluation of environmental contaminants in drinking water, risk assessments are being conducted to develop Public Health Goals (PHGs) for chemicals in drinking water, based solely on public health considerations. California's Safe Drinking Water Act of 1996 mandated the development of PHGs for over 80 chemicals by 31 December 1999. The law allowed these levels to be set higher or lower than federal maximum contaminant levels (MCLs), including a level of zero if data are insufficient to determine a specific level. The estimated safe levels and toxicological rationale for the first 26 of these chemicals are described here. The chemicals include alachlor, antimony, benzo[a]pyrene, chlordane, copper, cyanide, dalapon, 1,2-dichlorobenzene, 1,4-dichlorobenzene, 2,4-D, diethylhexylphthalate, dinoseb, endothall, ethylbenzene, fluoride, glyphosate, lead, nitrate, nitrite, oxamyl, pentachlorophenol, picloram, trichlorofluoromethane, trichlorotrifluoroethane, uranium and xylene(s). These risk assessments are to be considered by the State of California in revising and developing state MCLs for chemicals in drinking water (which must not exceed federal MCLs). The estimates are also notable for incorporation or consideration of newer guidelines and principles for risk assessment extrapolations.

Irritative and systemic symptoms following exposure to Microban disinfectant through a school ventilation system.

Microban, a pesticide not registered in California, was sprayed into an operating heating/ventilation/air conditioning (HVAC) unit at an elementary school in San Francisco, California. This incident occurred on Monday, September 28, 1992, while 396 students and 67 staff members were in the school. The Microban formulation used contains ortho-phenylphenol (0.21%), a quaternary ammonium complex (di-isobutylphenoxy-ethoxy-ethyldimethylbenzyl-ammonium chloride, 0.69%), and bromine (0.04%). This study of the health effects of Microban mist exposure on the school staff was conducted as a result of legal and toxicological concerns. California registration for this formulation had been denied because of inadequate data and because there were concerns about inhalation toxicity in test animals. Predicted health effects from short-duration exposure to Microban are primarily skin and mucous membrane irritation. A self-administered health symptom questionnaire that covered the work week following the evacuation was used to determine a pattern of higher symptom risks for those who were at work on Monday and who reported that they felt they were exposed to a chemical. Symptoms, which were generally consistent with exposure to an irritating chemical, were elevated on Monday and Tuesday; the symptoms normalized by the end of the work week. No additional health effects were detected following application of chlorpyrifos to cracks and crevices for ant control 2 d following the Microban incident. Strict supervision and coordination of pesticide use in public schools are recommended to prevent adverse health effects and emotional trauma in students and staff.

Dose-response assessment of airborne methyl isothiocyanate (MITC) following a metam sodium spill.

A tank car derailment in northern California in 1991 spilled metam sodium into the Sacramento River, and released its breakdown product, methyl isothiocyanate (MITC), into the air. This paper describes the risk evaluation process used. Over 240 individuals reported symptoms such as eye and throat irritation, dizziness, and shortness of breath. Reference exposure levels (RELs) for 1 hr were developed for MITC and compared to exposure concentrations. Ocular irritation in cats was the most sensitive endpoint reported. The no observed adverse effect level (NOAEL), divided by an uncertainty factor (UF) of 100, produced an REL of 0.5 ppb of MITC in air to prevent discomfort. An REL to prevent disability was estimated to be 40 ppb. An REL to prevent life-threatening injury was estimated to be 150 ppb. Measured MITC levels ranged from 0.2-37 ppb and estimated peak levels ranged from 140-1600 ppb. The usefulness of RELs for emergency planning is discussed.

Community exposure to a paraquat drift.

A mixture of paraquat and water was applied, by helicopter, to agricultural fields near a residential community and near an associated commercial complex. Drift from the application passed directly over the community, which resulted in resident complaints to the local county agricultural department. A community survey was undertaken to determine what health consequences, if any, resulted from the drift. A comparison of 2-wk self-reported symptom rates between the exposed community and three historical control communities indicated that 10 symptoms were elevated significantly at p < .05: cough, diarrhea, eye irritation, headache, nausea, rhinitis, throat irritation, trouble breathing, unusual tiredness, and wheezing. An internal comparison, which predicted symptom rates by an index of paraquat exposure (smelling an unusual odor in the prior 2-wk period), indicated fever (relative risk [RR] = 11.97) and nausea (RR = 3.75) to have elevated relative risks. Odor perception also predicted the report of a greater than the average number of symptoms. Based upon these findings, it was concluded that these residents probably did experience an increase in health symptoms from the drift. It is recommended that paraquat not be sprayed near residential communities.

Estimating dermal uptake of nonionic organic chemicals from water and soil: I. Unified fugacity-based models for risk assessments.

Contamination of water and soil that might eventually contact human skin makes it imperative to include the dermal uptake route in efforts to assess potential environmental health risks. Direct measurements of dermal uptake from either water or soil are only available for a small number of the thousands of chemicals likely to be found in the environment. We propose here a mass-transfer model for estimating skin permeability and dermal uptake for organic chemicals that contaminate soil and water. Statistical relationships between measured permeabilities and chemical properties reveal that permeability varies primarily with the octanol-water partition coefficient (Kow) and secondarily with the molecular weight. From these results, we derive a fugacity-based model for skin permeability that addresses the inherent permeability of the skin, the interaction of the skin with the environmental medium on skin (water or soil), and retains a relatively simple algebraic form. Model predictions are compared to measured human skin permeabilities for some 50 compounds in water and four compounds in soil. The model is adjusted to account for dermal uptake during both short-term (10-20 min) and long-term (several hour) exposures. This model is recommended for compounds with molecular weight less than or equal to 280 g.

Quaternary salts of 2-[(hydroxyimino)methyl]imidazole. 2. Preparation and in vitro and in vivo evaluation of 1-(alkoxymethyl)-2-[(hydroxyimino)methyl]-3-methylimida zolium halides for reactivation of organophosphorus-inhibited acetylcholinesterases.

A series of structurally related mono- and bis-1,3-disubstituted 2-[(hydroxyimino)methyl]imidazolium halides were evaluated in vitro for their ability to reactivate electric eel, bovine, and human erythrocyte (RBC) acetylcholinesterases (AChE) inhibited by ethyl p-nitrophenyl methylphosphonate (EPMP) and 3,3-dimethyl-2-butyl methyl-phosphonofluoridate (soman, GD). All new compounds were characterized for (hydroxyimino)methyl acid dissociation constant, nucleophilicity, octanol-buffer partition coefficient, reversible AChE inhibition, and kinetics of reactivation of EPMP-inhibited AChEs. For GD-inhibited AChEs, maximal reactivation was used to compare compounds since rapid phosphonyl enzyme dealkylation "aging" complicated interpretation of kinetic constants. For comparison, we also evaluated three known pyridinium therapeutics, 2-PAM, HI-6, and toxogonin. In vivo evaluation in mice revealed that when selected imidazolium compounds were coadministered with atropine sulfate, they were effective in providing lifesaving protection against both GD and EPMP challenges. This was a major accomplishment in the search for effective anticholinesterase therapeutics--the synthesis and preliminary evaluation of the first new monoquaternary soman antidotes with potencies superior to 2-PAM. Significantly, there was an apparent inverse relationship between in vitro and in vivo results; the most potent in vivo compounds proved to be the poorest in vitro reactivators. These results suggested that an alternative and possibly novel antidotal mechanism of protective action may be applicable for the imidazolium aldoximes. Selected compounds were also evaluated for their inhibition of AChE phosphorylation by GD and antimuscarinic and antinicotinic receptor blocking effects.

Quaternary salts of 2-[(hydroxyimino)methyl]imidazole. 3. Synthesis and evaluation of (alkenyloxy)-, (alkynyloxy)-, and (aralkyloxy)methyl quaternarized 2-[(hydroxyimino)methyl]-1-alkylimidazolium halides as reactivators and therapy for soman intoxication.

A series of structurally related monosubstituted 1-[(alkenyloxy)methyl]-, 1-[(alkynyloxy)methyl]-, and 1-[(aralkyloxy)methyl]-2-[(hydroxyimino)methyl]-3-methyli midazolium halides were prepared and evaluated. All new compounds were characterized with respect to (hydroxyimino)methyl acid dissociation constant, nucleophilicity, and octanol-buffer partition coefficient. The alkynyloxy-substituted compounds were also evaluated in vitro with respect to reversible inhibition of human erythrocyte (RBC) acetylcholinesterase (AChE) and kinetics of reactivation of human AChE inhibited by ethyl p-nitrophenyl methylphosphonate (EPMP). In vivo evaluation in mice revealed that coadministration of alkynyloxy-substituted imidazolium compounds with atropine sulfate provided significant protection against a 2 x LD50 challenge of GD. For the alkynyloxy-substituted imidazolium drugs there is a direct relationship between in vitro and in vivo activity: the most potent in vivo compounds against GD proved to be potent in vitro reactivators against EPMP-inhibited human AChE. These results differ from the observations made on the sterically hindered imidazolium compounds (see previous article) and suggest that several antidotal mechanisms of protective action may be applicable for the imidazolium aldoxime family of therapeutics. The ability of the alkynyloxy substituents to provide life-saving protection against GD intoxication was not transferable to the pyridinium or triazolium heteroaromatic ring systems.

Hearing loss in rats caused by inhalation of mixed xylenes and styrene.

We have reported that inhalation exposure of rats to toluene causes permanent hearing loss, e.g. Pryor et al. Neurobehav. Toxicol. Teratol. 5, 53-62 (1983). Therefore, it was of considerable interest to examine the ototoxic potential of two structurally related solvents--mixed xylenes and styrene--compared with that of toluene. Male, weanling Fischer-344 rats were exposed to clean air or solvents in four identical 62.5 l Plexiglas chambers. Exposures to 800, 1000, and 1200 ppm were daily for 14 hours/day and lasted 6 weeks for mixed xylenes, 3 weeks for styrene. An additional experiment with xylenes examined the effect of exposure for only 4 (1700 ppm) or 8 (1450 ppm) hours or for 8 (1450 ppm) hours on three consecutive days to compare the results with those obtained with toluene under comparable exposure schedules. Both xylenes and styrene caused marked hearing loss as assessed by behavioral (conditioned avoidance) and electrophysiologic (brainstem auditory-evoked response) methods. Moreover, both solvents appeared to be more potent ototoxicants than toluene, as indicated by effective concentration, effective durations of exposure, and the range of frequencies affected.

Nonquaternary cholinesterase reactivators. 3. 3(5)-Substituted 1,2,4-oxadiazol-5(3)-aldoximes and 1,2,4-oxadiazole-5(3)-thiocarbohydroximates as reactivators of organophosphonate-inhibited eel and human acetylcholinesterase in vitro.

As an extension of an earlier investigation (J. Med. Chem. 1984, 27, 1431), we prepared a series of 3-substituted 5-[(hydroxyimino)methyl]-1,2,4-oxadiazoles and the corresponding 5-thiocarbohydroximic acid 2-(N,N-dialkylamino)ethyl S-esters. The compounds were evaluated in vitro as reactivators of phosphonylated electric eel and human erythrocyte (RBC) acetylcholinesterases (AChE). The compounds were characterized with respect to (hydroxyimino)methyl acid dissociation constant, nucleophilicity, octanol/buffer partition coefficient, reversible AChE inhibition, and kinetics of reactivating ethyl methylphosphonylated AChE. One compound was also tested for effectiveness in preventing AChE phosphonylation. All of the tested compounds significantly reactivate ethyl methylphosphonylated AChE: the 3-n-octyl- and 3-(1-naphthyl)-substituted aldoximes are as reactive (within a factor of 5-10) toward the inhibited enzymes as the benchmark pyridinium reactivators, 2-PAM and HI-6. All of the substituted thiocarbohydroximic acid S-esters are powerful reversible inhibitors of AChE's: the 3-n-octyl- and 3-(1-naphthyl)-substituted thiocarbohydroximates inhibit eel AChE to 50% initial activity at concentrations less than 5 microM. When added to an eel AChE solution at concentrations between 5 and 50 microM, the 3-phenyl-substituted thiocarbohydroximate effectively antagonizes AChE inhibition by ethyl p-nitrophenyl methylphosphonate (EPMP), suggesting the potential utility of this compound for preventing anti-AChE-agent poisoning.

Nonquaternary cholinesterase reactivators. 4. Dialkylaminoalkyl thioesters of alpha-keto thiohydroximic acids as reactivators of ethyl methylphosphonyl- and 1,2,2-trimethylpropyl methylphosphonyl-acetylcholinesterase in vitro.

In the search for improved lipophilic centrally active acetylcholinesterase (AChE) antidotes, a series of alpha-keto thiohydroximates were prepared and evaluated for their ability to reactivate AChEs inhibited by ethyl p-nitrophenyl methylphosphonate (EPMP) and soman (GD). The compounds conformed to the general structure 4-RC6H5C-(O)C(NOH)S(CH2)nN+R'R''.X- where R = H, CH3, F, Br, Cl, OCH3, CN;R' = CH3, C2H5, i-C3H7; R'' = H, CH3; X = Cl, I; and n = 2, 3. In this series, varying R substituents on the aryl ring produced compounds with oxime pKa values from 6.8 to 8.0, optimum for an AChE reactivator. Increasing lipophilicity of the amine segment correlated with reactivator potency, as did electron-withdrawing groups on the aryl moiety, presumably due to increased binding to hydrophobic sites surrounding the AChE active site. The in vitro reactivation potency of the alpha-keto thiohydroximates approaches and even surpasses that of 2-PAM and toxogonin for GD-inhibited AChE. These initial findings point to additional structure-activity relationships to assist in the design of improved antidotal compounds.

Toluene-induced ototoxicity by subcutaneous administration.

Inhalation exposure of rats to toluene causes irreversible hearing loss (e.g., Pryor et al.). To determine whether noise emanating from the inhalation system was a major contributing factor and whether exposure by a noninhalation route would cause a similar effect, weanling, male Fischer-344 rats were injected SC twice daily in a quiet environment with PEG-300 (control) or with 1.5 or 1.7 g/kg of toluene for 7 days. After being trained to perform a multisensory conditioned avoidance response (CAR) task, tone intensity-response functions were generated at 4, 8, 12, and 20 kHz, and behavioral auditory response thresholds were estimated. Toluene caused a dose-related hearing loss at frequencies of 8 kHz and above, with no effect on performance of the CAR in response to light, nonaversive footshock, or the 4-kHz tone. The similarity of this effect to that observed following inhalation exposure indicates that noise is not a major factor in the toluene-induced hearing loss, although possible interactions between noise and toluene remain to be investigated. These results also demonstrate that direct penetration of the toluene vapors through the external ear structure, as might occur during inhalation exposure, is not a necessary condition for inducing the hearing loss.

Interactions between toluene and alcohol.

Weanling male Fischer-344 rats were exposed by inhalation to air or 2000 ppm toluene for 8 hours each day for 2 weeks. Subgroups had access to water or 6% alcohol as their only fluid sources, respectively. Rats exposed to both toluene and alcohol subsequently showed a marked preference for 6% alcohol in two-bottle choice tests that persisted for up to 20 days for some rats. Rats exposed to toluene without access to alcohol and control rats (exposed to air and water) showed a marked aversion to the alcohol solution, and only 2 of 12 rats forced to drink alcohol without exposure to toluene preferred alcohol in the preference tests. Exposure to both toluene and alcohol also caused greater inhibition of weight gain than exposure to either substance alone, accompanied by greater signs of organ toxicity as indicated by clinical blood chemistries. Exposure to toluene caused marked hearing loss as assessed by a behavioral technique (conditioned avoidance), and there was a trend toward enhancement of this ototoxic effect by forced consumption of alcohol.

Structure-activity relationships for reactivators of organophosphorus-inhibited acetylcholinesterase: quaternary salts of 2-[(hydroxyimino)methyl]imidazole.

A series of 1,3-disubstituted-2-[(hydroxyimino)methyl]imidazolium halides were prepared and evaluated in vitro with respect to their ability to reactivate acetylcholinesterase inhibited by ethyl p-nitrophenyl methylphosphonate (EPMP) and 3,3-dimethyl-2-butyl methylphosphonofluoridate (GD). The compounds conform to the general formula N(CH3)C(CHNOH)N(CH2OR)CHCH+ X Cl-, where R = CH3, (CH2)3CH3, (CH2)7CH3, CH2C6H5, CH2C10H7, (CH2)3C6H5, CH(CH3)2, CH2C(CH3)3, and CH(CH3)C(CH3)3. For comparison we also evaluated three known pyridinium reactivators, 2-PAM, HI-6, and toxogonin. The imidazolium aldoximes exhibit oxime acid dissociation constants (pKa) in the range 7.9-8.1, bracketing the value of 8.0, believed to be optimal for acetylcholinesterase reactivation. With imidazolium compound in excess over inhibited enzyme, the kinetics of reactivation are well behaved for EPMP-inhibited AChE and depend on the nature of the alkyl ether group R. For GD-inhibited AChE, maximal reactivation was used to compare compounds because rapid phosphonyl enzyme dealkylation and enzyme reinhibition complicate interpretation of kinetic constants.

Nonquaternary cholinesterase reactivators. 2. Alpha-heteroaromatic aldoximes and thiohydroximates as reactivators of ethyl methylphosphonyl-acetylcholinesterase in vitro.

We prepared six pairs of alpha-heteroaromatic aldoximes, RC(= NOH)H, and thiohydroximates, RC(= NOH)S-(CH2)2N(C2H5)2, where R represents various oxadiazole and thiadiazole rings. Each compound was characterized with respect to the following: structure, (hydroxyimino)methyl acid dissociation constant, nucleophilicity toward trigonal carbon and tetrahedral phosphorus, octanol-buffer partition coefficient, reversible inhibition of eel acetylcholinesterase (AChE), and in vitro reactivation of AChE inhibited by ethyl p-nitrophenyl methylphosphonate. Eight of the twelve compounds significantly reactivate ethyl methylphosphonyl-AChE, but inherent reactivities are moderate to low: the most potent nonquaternary reactivator, 3-phenyl-5-[(hydroxyimino)methyl]-1,2,4-oxadiazole, is 17 times less reactive than the well-known reactivator 2-[(hydroxyimino)methyl]-1-methylpyridinium iodide (2-PAM). One of the nonquaternary compounds, 3-phenyl-1,2,4-oxadiazole-5-thiohydroximic acid 2-(diethylamino)ethyl S-ester, is a powerful reversible inhibitor of AChE (I50 = 7.5 microM). The observed relationships between nonquaternary compound structure, reactivation potency, and anti-AChE activity reveal important molecular requirements for high reactivity toward phosphonylated AChE.

Transient cognitive deficits and high-frequency hearing loss in weanling rats exposed to toluene.

In a previous experiment we found that weanling rats subchronically exposed to toluene by inhalation were deficient in learning a multisensory conditioned avoidance response (CAR) task and a tone-intensity discrimination task when trained several hours after each daily 14-hour exposure ended. The present experiment was done to determine whether this deficit represented a residual pharmacologic effect or more persisting nervous system damage. Independent groups of rats were trained on the CAR task either during the last week of a 5-week exposure to 1400 or to 1200 pm toluene (14 hr/day, 7 days/week) or during the first or third weeks after the exposures ended. None of the three groups of toluene-exposed rats were able to acquire the auditory CAR, whereas it had been 4 kHz in the previous experiment. Subsequent tests, in which the frequency and intensity of the tone were varied, revealed that hearing in these rats was unimpaired at 4 kHz, slightly impaired at 8 kHz, and markedly impaired at 12 kHz and above. Thus, although toluene has been shown to be relatively innocuous as a toxicant, this new finding suggests that this solvent should be examined further in terms of its potential hazard, especially with regard to its frequent abuse.

Neurobehavioral effects of subchronic exposure of weanling rats to toluene or hexane.

Using several behavioral and neurophysiologic tests, we examined the effects of subchronic inhalation exposure of rats to toluene and compared them with the effects of the known neurotoxicant hexane. The rats were exposed to toluene (900 and 1400 ppm) or hexane (2000 ppm) 14 hr/day, 7 days/week for 14 weeks. Both solvents inhibited weight gain. Hexane caused a neurotoxic syndrome characterized by reductions of grip strength (especially hindlimb), motor activity, and startle responses, and increased latencies of several evoked potential components. Initial acquisition of a conditioned avoidance response (CAR) was also impaired, but subsequent performance was intact. Toluene did not cause the peripheral motor symptoms associated with exposure to hexane. However, a component of the brainstem auditory-evoked response was depressed and CAR acquisition was impaired along with the acquisition of a tone-intensity discrimination task when tested within hours after the daily exposure ended.

Toluene-induced hearing loss in rats evidenced by the brainstem auditory-evoked response.

Behavioral results showing that toluene causes hearing loss in rats precipitated an electrophysiologic study of the auditory thresholds of these rats using the brainstem auditory-evoked response (BAER). Twenty-three-day-old male Fischer-344 rats had been exposed to 1400 or 1200 ppm toluene 14 hrs/day, for 4 or 5 weeks while a control group was exposed only to air. The rats were tested 2.5 mo after termination of the exposures. BAERs, recorded with 25-gauge needle electrodes placed over the nose and posterior skull, were evoked by 100-microseconds-duration clicks and 1-msec-duration tone pips at eight intensities. Thresholds for the appearance of BAERs in the toluene-exposed rats were elevated by 13 to 27 dB, and latency-intensity functions were consistent with the occurrence of sensorineural hearing loss. The amplitudes of the third and fifth components of the BAER were attenuated at high stimulus intensities in the toluene-exposed rats. These behavioral and electrophysiologic results are apparently the first to indicate the ototoxicity of toluene in experimental animals.

A comparison of the rates of development of functional hexane neuropathy in weanling and young adult rats.

Chronic inhalation exposure of adult rats to hexane causes neural toxicities to develop over a period of several weeks. Because the developing organism is in many cases more vulnerable to toxic insult than the adult, and children make up a substantial proportion of the population of solvent abusers, we compared the effects of exposing weanling and young adult rats to 1000 ppm of hexane for 24 hrs/day, 6 days/week, for 11 weeks. Within two weeks of exposure, significance decreases in body weight and grip strength were observed in rats of both ages. However, the subsequent effects of the treatment on these indices of toxicity were greater in the young adults than in the rats first exposed as weanlings. The older rats also exhibited earlier and more severe signs of hindlimb flaccid paralysis. In contrast, the effects of hexane on tail nerve conduction time and on the brainstem auditory-evoked response were about the same in rats of both ages, with latencies increasing compared to controls over the exposure period. The relative resistance of the weanling rats to hexane neuropathy may be due to shorter, smaller-diameter axons, or to a greater rate of growth and repair in their peripheral nerves compared to those of adults.