Primary cultures of rabbit corneal epithelial cells as an experimental model to evaluate ocular toxicity and explore modes of action of toxic injury.

In vivo and in vitro animal models are used to investigate the toxicological modes of action of a substance as surrogates for humans since it is not ethical to conduct certain experiments in humans. The toxic actions of many compounds are manifested in specific organs, known as target organs of toxicity. Thus, in vitro systems that use cells derived from that target organ are best used to understand toxicological mechanisms. This article reviews the development of primary cultures of rabbit corneal epithelial cells which retain tissue specific and functional properties of in vivo cells as an experimental in vitro model to study ocular toxicity. This model system was used to evaluate initial ocular toxicity and mode of action of cocaine, tetracaine, and proparacaine, widely used local anesthetics. Initial toxicity and toxicity rankings of eight surfactants were determined using four different cytotoxicity endpoints. In addition, modes of action of delayed and prolonged cell injury after CE cells were treated with benzalkonium chloride, a cationic surfactant, and sodium dodecyl sulfate, an anionic surfactant, were investigated at multiple postexposure times after a 1-h treatment. The two surfactants produced distinctly different prolonged effects on cultured corneal epithelial cells, which may suggest these surfactants differentially affect cellular recovery.

Assessment of chronic inhalation non-cancer toxicity for diethylamine.

A non-cancer inhalation chronic toxicity assessment for diethylamine (DEA, CAS number 109-89-7) was conducted by the Texas Commission on Environmental Quality. A chronic Reference Value (ReV) was determined based on a high-quality study conducted in mice and rats by the National Toxicology Program. Chronic inhalation ReVs are health-based exposure concentrations used in assessing health risks of long-term (i.e. lifetime) chemical exposure. DEA is used industrially as an organic intermediate to produce corrosion inhibitors, and is widely used in rubber, pharmaceuticals, resins, pesticides, insect repellants, dye processing and as a polymerization inhibitor. Although systemic effects have been noted at higher concentrations, DEA acts primarily as a respiratory irritant with effects occurring in the upper respiratory tract. Rats were exposed to 0, 31, 62.5 and 125 ppm DEA and mice to 0, 16, 31 and 62.5 ppm DEA for 6 h/day, 5 days/week for 105 weeks. Mice were slightly more sensitive than rats. The critical effect identified in mice was hyperostosis in the turbinates although DEA caused a number of other non-neoplatic lesions. Dose-response data were suitable to benchmark concentration (BMC) modeling. The human equivalent point of departure (PODHEC) was calculated from the 95% lower limit of the BMC(10) using default duration and animal-to-human dosimetric adjustments. Total uncertainty factors of 90 were applied to the PODHEC to account for variation in sensitivity within the human population, toxicodynamic differences between mice and humans, and database uncertainty. The chronic ReV for DEA is 11 ppb (33 µg/m(3)).

Health- and vegetative-based effect screening values for ethylene.

Ethylene (ET) is ubiquitous in the environment and is produced both naturally and due to anthropogenic sources. Interestingly, the majority of ambient ET contribution is from natural sources and anthropogenic sources contribute only a minor portion. While microbes and plants naturally produce a large amount of ET, mammals are reported to produce only a small amount of ET endogenously. Anthropogenic sources of ET include the combustion of gas, fuel, coal and biomass. ET is also widely used as an intermediate to make other chemicals and products and is also used for controlled ripening of fruits and vegetables. Although, a review of human and laboratory animal studies indicate ET to be relatively non-toxic, there is concern about the potential toxicity of ET because ET is metabolically converted to ethylene oxide (EtO). EtO has been classified to be carcinogenic to human by the inhalation route by the International Agency for Research on Cancer (IARC) cancer. ET, however, has been classified as a Group 3 chemical which indicates it is not classified as a human carcinogen by IARC. Several studies have reported ET to cause adverse effects to plant species (vegetation effects) at concentrations that are not adverse to humans. Therefore, the Texas Commission of Environmental Quality (TCEQ) conducted detailed health and welfare (odor and vegetation) based assessments of ET to develop both health and vegetative based toxicity factors in 2008 in accordance with TCEQ guidelines. The health assessment based on well-conducted animal toxicity studies resulted in identification of higher points of departures and subsequently higher effect screening levels (ESLs) that were more than a magnitude higher than the threshold adverse effect level for vegetative effects for ET. Further, based on a weight-of-evidence evaluation of potential mutagenic and carcinogenic mode-of-actions for ET it appears the metabolic conversion of ET to EtO is of insufficient magnitude to cause concern of potential cancer risk. Therefore, the short-term ESL for air permit reviews and air monitoring evaluations is the vegetation-based ESL of 1200 ppb as it is more than a magnitude lower than the health-based acute ESL of 150,000 ppb. Similar to the acute derivation, the chronic evaluation resulted in the derivation of a chronic vegetation based ESL of 30 ppb that was much lower than the chronic ESL of 1600 ppb. In summary, the TCEQ's acute and chronic ESLs for vegetation will protect the general public from short-term and long-term adverse health and welfare effects. The general public includes children, the elderly, pregnant women, and people with pre-existing health conditions.

An updated inhalation unit risk factor for arsenic and inorganic arsenic compounds based on a combined analysis of epidemiology studies.

The United States Environmental Protection Agency (USEPA) developed an inhalation unit risk factor (URF) of 4.3E-03 per µg/m(3) for arsenic in 1984 for excess lung cancer mortality based on epidemiological studies of workers at two smelters: the Asarco smelter in Tacoma, Washington and the Anaconda smelter in Montana. Since the USEPA assessment, new studies have been published and exposure estimates were updated at the Asarco and Anaconda smelters and additional years of follow-up evaluated. The Texas Commission on Environmental Quality (TCEQ) has developed an inhalation URF for lung cancer mortality from exposures to arsenic and inorganic arsenic compounds based on a newer epidemiology study of Swedish workers and the updates of the Asarco and Anaconda epidemiology studies. Using a combined analysis approach, the TCEQ weighted the individual URFs from these three epidemiology cohort studies, to calculate a final inhalation URF of 1.5E-04 per µg/m(3). In addition, the TCEQ also conducted a sensitivity analysis, in which they calculated a URF based on a type of meta-analysis, and these results compared well with the results of the combined analysis. The no significant concentration level (i.e., air concentration at 1 in 100,000 excess lung cancer mortality) is 0.067µg/m(3). This value will be used to evaluate ambient air monitoring data so the general public in Texas is protected against adverse health effects from chronic exposure to arsenic.

Development of a unit risk factor for nickel and inorganic nickel compounds based on an updated carcinogenic toxicity assessment.

The TCEQ has developed a URF for nickel based on excess lung cancer in two epidemiological studies of nickel refinery workers with nickel species exposure profiles most similar to emissions expected in Texas (i.e., low in sulfidic nickel). One of the studies (Enterline and Marsh, 1982) was used in the 1986 USEPA assessment, while the other (Grimsrud et al., 2003) is an update to an earlier study (Magnus et al., 1982) used by USEPA. The linear multiplicative relative risk model with Poisson regression modeling was used to obtain maximum likelihood estimates and asymptotic variances for cancer potency factors (ß) using cumulative nickel exposure levels versus observed and expected lung cancer mortality (Enterline and Marsh, 1982) or lung cancer incidence cases (Grimsrud et al., 2003). Life-table analyses were then used to develop URFs from these two studies, which were combined using weighting factors relevant to confidence to derive the final URF for nickel of 1.7E-04 per µg/m³. The de minimis air concentration corresponding to a 1 in 100,000 extra lung cancer risk level is 0.059 µg/m³. The TCEQ will use this conservative value to protect the general public in Texas against the potential carcinogenic effects from chronic exposure to nickel.

A chronic reference value for 1,3-butadiene based on an updated noncancer toxicity assessment.

A chronic noncancer toxicity assessment for 1,3-butadiene (BD) has been conducted by the Texas Commission on Environmental Quality (TCEQ) using information not available to the U.S. Environmental Protection Agency (U.S. EPA) in 2002. The TCEQ developed a chronic reference value (ReV) of 33 microg/m3 (15 ppb). The chronic ReV is based on the same animal study and critical endpoint used by U.S. EPA for ovarian atrophy in B6C3F1 mice, but uses mode of action (MOA) information that indicates the diepoxide metabolite is responsible for ovarian atrophy. In addition, diepoxide-specific hemoglobin adduct data in mice, rats, and humans and other experimental data that became available after 2002 were used to support a conservative data-derived toxicokinetic animal-to-human uncertainty factor (UFA) of 0.3. The default toxicodynamic UFA of 3 was used, together with the data-derived toxicokinetic UFA of 0.3, resulting in a total UFA of 1. The necessary experimental data were not available to calculate a chemical-specific adjustment factor, although supporting data suggest the toxicokinetic UFA may range from 0.01 to 0.2. The chronic ReV value, along with a unit risk factor developed by the TCEQ, will be used to evaluate ambient air monitoring data so that the general public is protected against adverse health effects from chronic exposure to BD.

Development of a unit risk factor for 1,3-butadiene based on an updated carcinogenic toxicity assessment.

The Texas Commission on Environmental Quality (TCEQ) has developed an inhalation unit risk factor (URF) for 1,3-butadiene based on leukemia mortality in an updated epidemiological study on styrene-butadiene rubber production workers conducted by researchers at the University of Alabama at Birmingham. Exposure estimates were updated and an exposure estimate validation study as well as dose-response modeling were conducted by these researchers. This information was not available to the U.S. Environmental Protection Agency when it prepared its health assessment of 1,3-butadiene in 2002. An extensive analysis conducted by TCEQ discusses dose-response modeling, estimating risk for the general population from occupational workers, estimating risk for potentially sensitive subpopulations, effect of occupational exposure estimation error, and use of mortality rates to predict incidence. The URF is 5.0 x 10(-7) per microg/m(3) or 1.1 x 10(-6) per ppb and is based on a Cox regression dose-response model using restricted continuous data with age as a covariate, and a linear low-dose extrapolation default approach using the 95% lower confidence limit as the point of departure. Age-dependent adjustment factors were applied to account for possible increased susceptibility for early life exposure. The air concentration at 1 in 100,000 excess leukemia mortality, the no-significant-risk level, is 20 microg/m(3) (9.1 ppb), which is slightly lower than the TCEQ chronic reference value of 33 microg/m(3) (15 ppb) protective of ovarian atrophy. These values will be used to evaluate ambient air monitoring data so the general public is protected against adverse health effects from chronic exposure to 1,3-butadiene.

Spatial and temporal trend evaluation of ambient concentrations of 1,3-butadiene and chloroprene in Texas.

This paper provides information on 1,3-butadiene (BD) and chloroprene as atmospheric pollutants in Texas and reviews available emission estimates and monitoring data. Ambient BD concentrations in most areas of Texas are predominantly influenced by on-road and off-road vehicular emissions or biomass burning, since BD is a product of combustion. However, large industrial point sources of BD emissions in Texas locally influence ambient concentrations. Total industrial BD emissions to the atmosphere in Texas for 2003 were estimated at 695 tonnes per year (TPY), approximately 70% of the total reported national industrial BD air emissions. Since 1998, there have not been any large industrial sources of chloroprene emissions in Texas, and total industrial chloroprene emissions for 2003 was estimated at only 0.09 TPY. Chloroprene was never detected at air monitoring sites. In 2003, the Texas Commission on Environmental Quality (TCEQ) monitored BD ambient air concentrations at 57 sites, some of which have been operational since 1992. These air monitors provide information on ambient BD concentrations in Texas and allow spatial and temporal trend evaluation. In 2003, annual average concentrations at monitoring sites in Texas ranged from less than the reporting limit of 0.01 to 3.2 parts per billion by volume (ppbv) with an overall average of 0.2 ppbv. This overall average is reduced to 0.1 ppbv if BD data from monitoring sites in Port Neches and Milby Park in Houston, which are located downwind of significant point sources of BD, are excluded. Ambient air monitoring has been conducted in Port Neches and in Milby Park in Houston since 1996 and 1999, respectively. At the Port Neches monitor, trend evaluation indicates that ambient concentrations of BD have declined since 1996 due to cooperative agreements with industries emitting BD. Annual average BD concentrations at the Port Neches monitor decreased from 8.3ppbv in 1996 to 1.3 ppbv in 2003, giving an 8-year average of 3.8 ppbv. Annual average BD concentrations at the Milby Park monitor varied between 2.1 and 4.4 ppbv from 1999 through 2003, giving a 5-year average of 3.1 ppbv. The results of cancer cluster studies based on Cancer Registry 1995-2001 incidence data and 1993-2002 mortality data conducted by the Texas Department of State Health Services for zip codes 77017/77012 (Houston) and 77651 (Port Neches) will be presented.