Compared to many other states in the region, Vermont enjoys clean air. Few Vermonters, however, realize that atmospheric concentrations of several toxic chemicals exceed state ambient health standards, many of which were established in the 1980s to protect public health from the adverse impacts of toxic chemicals in the air we breathe. One contaminant of particular concern is a known human carcinogen: benzene.

Benzene is a volatile organic compound emitted from both natural and human-created sources. Natural sources in Vermont include forest fires and organic chemicals emitted by trees and other vegetation through their natural physiological processes. Of much greater concern, however, are the benzene sources linked to our modern mobile lifestyle, which frequently revolves around the internal combustion engine.

Automobiles, snowmobiles, outboard marine engines, and lawnmowers all release benzene into our atmosphere. According to the U.S. Environ-mental Protection Agency’s National Scale Assessment Project for 1996, approximately 48 percent of benzene emissions nationwide come from mobile sources. In Vermont, where automobiles and other forms of transportation contribute a greater percentage to total toxic emissions, this percentage is likely to be higher. Vermont ranked 11th in the nation in 1999 for per capita vehicle miles traveled; on average, each Vermonter drove 11,566 miles. Total vehicle miles traveled in the state has increased by approximately 450 percent since 1950 (Figure 1); this increase has offset a lot of the pollution savings from improved emissions technology in the past three decades.

Other sources of benzene in Vermont include the combustion of wood and oil for home heating and the industrial storage and distribution of gasoline. Indoor air can also contain high concentrations of benzene from common household cleaning products, glues, paints, and cigarette smoke.

Vermonters are regularly exposed to benzene air emissions through short-term acute exposures or long-term chronic exposures. The principal human-made source of benzene in outdoor air is the motor vehicle. Evaporative emissions (unburned fuel) at gasoline station pumps may expose people to acute, high levels of benzene, although vapor recovery systems, now featured at larger gasoline stations, have greatly reduced this source of exposure. Recent advances in on-board automobile vapor recovery have decreased the potential for gasoline vapor exposure even further. Breathing combusted gasoline and evaporative emissions from automobiles results in considerable exposure. In addition, multiple sources of ambient benzene create a background level in the atmosphere that consistently exceeds the state health standard, leading to chronic exposure.

Benzene can be found in the air, water, or soil, although it evaporates quickly from the soil and water surfaces. Once in the air, the pollutant reacts with other atmospheric chemicals and breaks down within a few days. Benzene in the air may be returned to the ground by rain or snow.

To better assess the concentrations of toxic air pollutants in Vermont, the Air Pollution Control Division initiated the Hazardous Air Contaminants Monitoring Program, a monitoring network that measures a wide range of chemicals over a broad portion of the state. Starting in 1993, monitoring of air toxics has consistently occurred at four sites, ranging from an urban location in downtown Burlington to a rural “background” site on the western slope of Mount Mansfield in Underhill. Monitoring is currently taking place at five sites in the state.

Pollutant concentrations are generally higher in more populated urban sites than rural locations (Figure 2); however, median concentrations of benzene exceeded the Vermont Hazardous Ambient Air Standards of 0.12 µg/m3 (micrograms per cubic meter) many times from 1993 through 2000, even at the rural Underhill site. (The 0.12 µg/m3 standard is the concentration equal to the risk level at which 1 person in 1 million is expected to develop cancer due to a continuous lifetime exposure to benzene in the air.) At urban sites, median concentrations of benzene were as much as 30 times the state health standard (Figure 3). Annual median concentrations in Brattleboro, Burlington, and Rutland consistently posed a cancer risk of 1 in 100,000, while at Underhill, benzene concentrations exceeded the 1 in 1 million risk level at least 84 percent of the time.

An analytical minimum detection limit is the level below which a pollutant’s concentration cannot be quantified accurately. The minimum detection limit during the six-year period for benzene was 0.38 µg/m3, a value 3.2 times higher than the standard itself. As the minimum detection limit is above the standard, it is possible that non-detect levels of benzene still exceed the health standard. Of the 619 samples collected at the four monitoring sites during the period, only 5 percent were below the minimum detection limit (0.38 µg/m3), and 80 percent of the non-detects occurred at Underhill, the rural site. The maximum concentration measured in Vermont (15.4 µg/m3), on the other hand, occurred in Rutland on February 19, 1994. This concentration exceeded the air standards by 128 times.

In addition to vapor recovery systems on cars and at gasoline stations, the Air Pollution Control Division is taking other measures to deal with benzene emissions in Vermont. The Division is in the process of developing toxic action plans, technical documents that will provide detailed analysis of available air toxics data and toxicological information for critical target pollutants. The action plans will serve as technical and policy guidelines to reduce target pollutant concentrations to levels below the Vermont Hazardous Ambient Air Standards.

Vermonters can also be exposed to benzene through drinking water. Information provided by the Vermont Health Department indicates that few of the private wells tested for benzene actually have detectable concentrations of this pollutant. However, rare instances of well water contamination exist due to leaking underground gasoline storage tanks or gasoline spills.

The Agency at this time cannot estimate the combined average of benzene exposure due to inhalation and ingestion, but the Agency believes that the majority of the health risk associated with this carcinogen comes from inhalation exposure.

As Vermont’s population continues to grow and the demand for transportation increases, it is likely that, unless extensive strides are made to counter toxic emissions, the percentage of emissions derived from combustion sources will continue to increase. This implies potentially greater environmental and health problems, including higher cancer risks for Vermonters.