David W. Layton

Deposition, resuspension, and penetration of particles within a residence

Aerosol concentrations and particle size distributions were measured indoors and outdoors at a two-storey residence in California during the summer months. A single central sampling point in the downstairs living area was used for all indoor samples. The deposition rate for supermicron particles was measured by raising the particle concentration indoors and simultaneously measuring air infiltration rates and particle concentration decay rates. For particles between 1 and 5 μm diameter, the deposition velocity closely matched the calculated settling velocity. For particles larger than 5 μm the deposition velocity was less than the calculated settling velocity, probably due to the nonspherical nature of these particles. The penetration factor for supermicron particles, a measure of the amount of filtration achieved by the building shell, was calculated using the experimentally determined deposition velocities and indoor/outdoor particle ratios when no resuspension or generation activities were present. A penetration factor of one was found, indicating that the building shell was not effective at removing infiltrating particles. Resuspension was measured under several different conditions and was found to have a significant impact on indoor particle concentrations. Just walking into a room can increase the particle concentration by 100% for some supermicron particle sizes. For light activity with four people in the residence, a resuspension rate between 1.8 × 10−5 and 3.8 × 10−4 h−1 was found for supermicron particles assuming a particle density of 1 gm−3. These calculated rates may be lower than the actual rates due to assumptions made about the particle size distribution of the floor dust.

Comparison of multi-media transport and transformation models: Regional fugacity model vs. CalTOX

Two multimedia environmental transport and transformation computer models are summarized and compared. The regional fugacity model published by Mackay and Paterson (1991), termed Fug3ONT, is a four compartment steady-state model designed to simulate the relative distribution of nonionic organic chemicals in a multimedia system. CalTOX is a seven compartment multimedia total exposure model for hazardous waste sites. Both models are based on the principles of fugacity. CalTOX, however, separates the soil into three layers (surface, root, and vadose) and uses a different approach to estimate the diffusive mass transfer rate in soil. These differences result in lower estimates of the steady-state contaminant concentrations of six environmentally relevant chemicals in the root soil of CalTOX as compared to the bulk soil of Fug3ONT. The difference is greatest for compounds with low mobility in soil such as 2,3,7,8-Tetrachlorodibenzo-p-dioxin and Benzo(a)pyrene where estimates from CalTOX and Fug3ONT differ by more than 3 orders of magnitude. Otherwise, the models provide similar estimates for the distribution of the six chemicals among the air, water, sediment and surface soil.

Factors determining dietary intakes of heterocyclic amines in cooked foods.

The identification of heterocyclic amines (HCAs) in cooked foods has focused attention on the potential health effects from their consumption in the diet. Recent studies have estimated daily dietary intakes of HCAs that vary 10-fold and implicated different cooked meats as the prime source of HCAs in the diet. These varied estimates can be attributed to the different dietary assessment methods used in these studies, as well as the different levels of HCAs ascribed to the most commonly consumed cooked meats. Epidemiological studies utilizing information on dietary practice and food intake have found higher risks for several cancers among individuals consuming the highest levels of HCAs. These studies have highlighted the importance of using information on cooking methods in addition to food intake to accurately estimate dietary exposure to HCAs.

Deriving allowable daily intakes for systemic toxicants lacking chronic toxicity data

The lack of human toxicological data for most chemical compounds makes it difficult to quickly assess health risks associated with exposure to contaminants at hazardous waste sites. It would therefore be advantageous to have a technique for estimating acceptable daily intakes (ADIs) of potentially toxic substances based on more widely available animal toxicity data. This article focuses on the use of LD50 data to derive provisional ADIs, and it suggests multiplying oral LD50 values (expressed in mg/kg of body wt) by a factor in the range of 5 X 10(-6) to 1 X 10(-5) day-1 to convert them to such ADIs. It is emphasized that these interim ADI values are no substitute for toxicity testing, but that such testing would most likely result in higher ADI estimates.

Screening the potential risks of toxic substances using a multimedia compartment model: estimation of human exposure

Managing environmental health risks requires the assessment of environmental fate, exposure, and health risk of an ever-increasing list of contaminants. The magnitude of this list precludes an experimental evaluation of each contaminant. For this reason, computer models are being used more frequently to simulate the transport and transformation of chemicals based on physical and chemical properties. This paper describes a multimedia compartment model that we have developed for screening toxic substances. This model, referred to as GEOTOX, uses a combination of physical, chemical, and landscape properties to establish the partitioning, reaction, and interphase-transport characteristics of a chemical. These properties are used to estimate concentrations in the air, soil, water, and food of a representative or generic environment. We use these concentrations in exposure-pathway models to calculate the quantities absorbed by humans; then dose-response data are used to estimate health risks. The capability of GEOTOX as a screening tool is illustrated in a sample ranking of three chemicals (i.e., 2,4,6-trinitrotoluene, hexahydro-1,3,5-trinitro-1,3,5-triazine, and benzene) being continuously added to the upper-soil compartment. We find that ranking based on both toxic potency and environmental fate can enhance the risk-management process when compared to ranking based on toxic potency alone.

Biodegradation of potential diesel oxygenate additives: dibutyl maleate (DBM), and tripropylene glycol methyl ether (TGME)

The addition of oxygen-bearing compounds to diesel fuel considerably reduces particulate emissions. TGME and DBM have been identified as possible diesel additives based on their physicochemical characteristics and performance in engine tests. Although these compounds will reduce particulate emissions, their potential environmental impacts are unknown. As a means of characterizing their persistence in environmental media such as soil and groundwater, we conducted a series of biodegradation tests of DBM and TGME. Benzene and methyl tertiary butyl ether (MTBE) were also tested as reference compounds. Primary degradation of DBM fully occurred within 3 days, while TGME presented a lag phase of approximately 8 days and was not completely degraded by day 28. Benzene primary degradation occurred completely by day 3 and MTBE did not degrade at all. The total mineralized fractions of DBM and TGME achieved constant values as a function of time of approximately 65% and approximately 40%, respectively. Transport predictions show that, released to the environment, DBM and TGME would concentrate mostly in soils and waters with minimal impact to air. From an environmental standpoint, these results combined with the transport predictions indicate that DBM is a better choice than TGME as a diesel additive.

137Cs and 210Po in Pacific walrus and bearded seal from St. Lawrence Island, Alaska

The activity concentration of Cesium-137 ((137)Cs) and naturally-occurring Polonium-210 ((210)Po) were measured in the muscle tissue, kidney and liver of Pacific walrus (Odobenus rosmarus divergens) and bearded seal (Erignathus barbatus) collected by native hunters from the Bering Sea during May 1996. The mean (137)Cs concentrations in muscle, liver and kidney of Pacific walrus were 0.07, 0.09 and 0.07 Bq kg(-1) (n=5, wet weight), respectively, and 0.17, 0.10, and 0.17 Bq kg(-1) (n=2, wet weight), respectively, in bearded seal. In general, (137)Cs tissue concentrations are significantly lower than those previously reported for mammals from other regions. By comparison, (210)Po activity concentrations are more variable and appear to be higher level compared with mammal data from other regions. The mean (210)Po concentration in the muscle tissue, liver and kidney of Pacific walrus (n=5, wet weight) were 28.7, 189, and 174 Bq kg(-1), respectively. This compares with (210)Po concentration values (n=2, wet weight) of 27, 207 and 68 Bq kg(-1) measured in the muscle tissue, liver and kidney, of bearded seal, respectively. Estimated concentration factors--as defined by the radionuclide concentration ratio between the target tissue to that in sea water--were two to three orders of magnitude higher for (210)Po that those of (137)Cs. We conclude from radiological dose estimates that ingestion of (137)Cs in foods derived from walrus and seal will pose no threat to human health. This work has important implications for assessment of risks of Alaskan coastal communities concerned about the dumping of nuclear waste in the Russia Arctic.

Risk management for plausibly hormetic environmental carcinogens: The case of radon

Risk management typically involves efforts to reduce human exposures by establishing regulations that limit the concentration of the substance in environmental media. In cases where a substance is widely used in commerce or is naturally occurring in the environment, compliance costs can be substantial because of nationwide requirements to add expensive control technologies. Uncertainties in a dose-response function further impact risk management decisions because they may correspond to large differences in health benefit per unit exposure reduction. These problems are highlighted in the case of plausibly hormetic environmental carcinogens, for which a linear-no-threshold (LNT) dose-response model has been the traditional regulatory default assumption. In this case, model uncertainty is pivotal, and risk management is consequently inherently controversial. However, marginal cost functions that arise for plausibly hormetic carcinogens are expected to possess a common analytic feature that may be particularly useful for this type of risk management problem. Specifically, marginal cost functions in this context are expected to have roots reflecting contaminant concentration values above which regulatory goals may be optimally placed subject to cost constraints. Here we illustrate this heuristic feature in the case of residential radon, using both a LNT model and a biologically plausible hormetic model to predict associated risks of lung cancer mortality.

Comparative Environmental Performance of Two Diesel-Fuel Oxygenates: Dibutyl Maleate (DBM) and Tripropylene Glycol Monomethyl Ether (TGME)

Many studies have shown that the addition of oxygen bearing compounds to diesel fuel can significantly reduce particulate emissions. To assist in the evaluation of the environmental performance of diesel-fuel oxygenates, we have implemented a suite of diagnostic models for simulating the transport of compounds released to air, water, and soils/groundwater as well as regional landscapes. As a means of studying the comparative performance of DBM and TGME, we conducted a series of simulations for selected environmental media. Benzene and methyl tertiary butyl ether (MTBE) were also addressed because they represent benchmark fuel-related compounds that have been the subject of extensive environmental measurements and modeling. The simulations showed that DBM and TGME are less mobile in soil because of reduced vapor-phase transport and increased retention on soil particles. The key distinction between these two oxygenates is that DBM is predicted to have a greater potential than TGME for aerobic biodegradation, based on chemical structure.

Assessing and managing the risks of accidental releases of hazardous gas: A case study of natural gas wells contaminated with hydrogen sulfide

Abstract Natural gas wells contaminated with the toxic gas hydrogen sulfide (i.e., sour-gas wells) pose potential health risks to workers and to nearby residents. The health risks are a function of the dose-response relationship of hydrogen sulfide, the likelihood of accidental releases, gaseous emission rates, the nature of releases at the well head, dispersion of the emitted gas, and the characteristics of the population at risk. We discuss each of these factors and present a risk analysis of a hypothetical sour-gas well in the vicinity of Evanston, WY. We found that the greatest risks for life-threatening effects would occur in the northwest downwind sector after a horizontal release of gas at the well. Subacute effects (e.g., respiratory irritation) after a vertical release of gas would occur primarily to the northeast. Management of health risks involves the use of techniques for preventing inadvertent releases and methods for limiting population exposures.