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Petroleum Hydrocarbons / Benzene


Petroleum hydrocarbons are the primary constituents in oil, gasoline, diesel, and a variety of solvents and penetrating oils. As such, petroleum hydrocarbons are a primary focus of many site and product risk assessments. Because petroleum hydrocarbon products are complex mixtures of chemicals, risk assessments for these products generally focus on specific toxic constituents. The petroleum constituents of primary interest to human health have been the aromatic hydrocarbons (i.e., benzene, ethylbenzene, toluene, and xylenes), polynuclear aromatic hydrocarbons (PAHs), gasoline additives (e.g., MTBE, TBA), and combustion emissions from fuels (e.g., carbon monoxide, benzene, acetaldehyde, formaldehyde, diesel particulates).

Aromatic Hydrocarbons

The aromatic hydrocarbons are identified as chemicals of interest for health risk assessments at most petroleum-contaminated sites  and for sites affected by petroleum solvent.  Benzene is the only aromatic hydrocarbon classified by the U.S. Environmental Protection Agency and other health and environmental agencies as a “known human carcinogen,” and therefore, is the primary focus of many petroleum hydrocarbon risk assessments. Exponent has conducted hundreds of site risk assessments where benzene was a chemical of concern. The potential exposure to benzene at a petroleum-contaminated site depends on the concentrations of benzene in the soil, water, and air, and the frequency and duration of expected human contact with the contaminated media. If the petroleum release was gasoline from an underground tank, benzene will likely be found in soils and possibly also in shallow groundwater and air. The volatilization of benzene in subsurface soils and shallow groundwater and its migration into buildings (vapor intrusion), have recently become a risk assessment focus that has led to the reopening of Records of Decision (RODs) at a number of sites. Benzene and the other aromatic hydrocarbons are taken up through the skin, lung, and digestive system, so all three routes of exposure (dermal uptake, inhalation, and incidental ingestion) need to be considered in assessing health risks.

Assessing the health risks associated with the use of petroleum solvents requires additional consideration beyond those of a site risk assessment. Benzene is not added to petroleum solvents but was historically found in solvents, at relatively low levels, occurring as a contaminant in the distillate fraction that composes the majority of the solvent. Little is known about the benzene concentrations in many of these solvents, except that levels were less than 0.1 weight percent (1000 ppm by weight), which is the OSHA reporting limit for material safety data sheets (MSDSs). Therefore, the initial challenge in assessing benzene exposures from solvents is characterizing the benzene content of the solvent, knowing that solvent composition has changed over time to meet the requirements of federal and state air quality regulations. Exponent has collected a substantial amount of data on benzene concentrations in various distillate fractions (e.g., mineral spirits, petroleum naphthas), including other aromatic hydrocarbons such as toluene, to assist clients in characterizing their historical products. Exponent has also conducted laboratory studies to assess the effects of petroleum solvent composition and benzene concentration on benzene volatilization from the liquid solvent, and simulation studies to directly measure airborne benzene concentrations associated with the use of specific solvents. Much of the focus on petroleum solvents has been on the reconstruction of potential exposures of workers using historical formulations of petroleum solvents. Exponent has conducted exposure reconstructions for workers who used a variety of petroleum solvents, including parts-washing solvents, printing solvents, and penetrating oils.

Polynuclear Aromatic Hydrocarbons (PAHs)

Polynuclear aromatic hydrocarbons (PAHs) are often important contaminants of concern because of their chemical and toxicological properties. Composed of multiple aromatic rings, PAHs tend to be immobile and highly persistent in the environment, with relatively high bioaccumulation rates and toxicity. While PAHs occur naturally in crude oil and smoke and ash from forest fires, they are most often found as products of incomplete combustion, especially from incinerators. PAHs are often found at facilities formerly involved in creosote, coking, and wood preservative production, and at former manufactured gas plants that use coal as a feedstock. Exponent has conducted numerous risk assessments at these types of sites. Because of their accumulation in the food chain, PAHs usually drive the indirect risks associated with exposures via ingestion of soil, produce, fish, and animal products affected by emissions from hazardous waste combustion facilities. In conducting numerous human and ecological risk assessments for these facilities, we have determined that the methods used by EPA often overpredict concentrations in various media, and in many cases, we were able to get the EPA to modify its approach accordingly, thus reducing the estimated risk levels for PAHs.

Gasoline Additives

The health risks posed by gasoline additives have received much attention in recent years, particularly since California began phasing out the use of methyl tert butyl ether (MTBE) in gasoline supplies in 1999. MTBE and other substances are added to reformulated gasoline to aid combustion and reduce emissions to meet requirements of the Clean Air Act. Although MTBE was effective in reducing emissions from automobile engines, it was viewed as a problem when released from leaking underground tanks to groundwater, or from two-stroke engines to surface water used for drinking supply, primarily because it imparts an unpleasant taste and odor to the water at relatively low concentrations. MTBE also has been identified as a potential carcinogen by some regulatory agencies. Exponent conducted extensive evaluations of MTBE in drinking-water sources in California and elsewhere in the U.S., as well as assessments of the health risks posed to consumers by MTBE in drinking water . In addition, Exponent has evaluated the risks and benefits of MTBE and ethanol as gasoline additives.

Combustion Emissions

Combustion emissions are sources of both direct risks due to inhalation and indirect risks from consumption of affected food products. These sources, such as vehicles, fires, and incineration facilities, emit contaminants in both the vapor and particle phases that are associated with incomplete combustion. Benzene, formaldehyde, and diesel particulates are the major components of vehicle exhaust that drive the estimated inhalation risks in urban areas, while PAHs and dioxins/furans often drive risk estimates for stationary sources, such as combustion facilities. Exponent personnel have conducted risk assessments for such diverse combustion sources as mining operations, hazardous waste combustion facilities, and motor vehicles, including diesel truck traffic. In addition, our personnel have served on technical advisory committees for regulatory agencies involved with these issues, ranging from the South Coast Air Quality Management District (SCAQMD) MATES II study of air toxics for the Los Angeles basin to the EPA’s hazardous waste incinerator guidance and national air toxics study.