AQ Monitoring Results:
Crockett: Benzene at John Swett

This page last reviewed September 27, 2010

Background

One of the important contributors to health risk from air pollution, benzene is a clear, colorless liquid with a sickly, sweet odor. Volatile organic compounds are readily released from this liquid into the air. Benzene can cause central nervous system depression and increased incidences of leukemia. California has determined under Assembly Bill 1807 and Proposition 65 that benzene is a cancer-causing compound. The ARB has taken regulatory actions to reduce benzene emissions.

The predominant sources of benzene emissions in the atmosphere are gasoline evaporation and gasoline motor vehicle exhaust. Mobile sources contribute 85% and industry-related stationary sources contribute 15% of the statewide emissions. Approximately 70% of mobile source benzene emissions are from on-road motor vehicles, with the remainder from non-road mobile sources, such as industrial processes to make resin and synthetic fibers. Indoor benzene sources include tobacco smoke, heating and cooking systems, evaporation from various products used in the home or work area, and drift from outdoor automobile exhaust. Benzene is also emitted naturally from volcanoes and forest fires, and occurs as a natural constituent of crude oil and plant volatiles.

The Air Resources Board (ARB) adopted a benzene control measure in 1987, which requires many gasoline stations to install vapor recovery systems. In 1990s, the ambient benzene levels showed a steady downward trend, based on the statewide annual average. The 2000 statewide ambient benzene concentration was approximately 72% lower than that in 1990.

Ambient Monitoring Results

Ambient levels of benzene are routinely monitored at approximately twenty sites in the California air toxics monitoring network. The statewide average concentration of benzene during 1998-2000 was 0.8 ppb (parts per billion), based on values ranging from 0.1 ppb to 9.9 ppb. The current routine monitor closest to Crockett is in Fremont. Relative to the statewide average, the Fremont region was 25% lower, with an average concentration of 0.6 ppb for the same time period.

The ambient monitoring results at John Swett are provided here:

  • A graph comparing the monthly summaries of benzene in the community with historical statewide and regional levels
  • A table of summary statistics
  • Raw data in Excel format

Cancer Risk

Cancer risk is the number of excess cancer cases among a million people if the people are exposed to levels of a toxic air pollutant over 70 years. Benzene represents approximately 20% of the potential cancer risk of the nine measured compounds, excluding diesel particulate matter. Benzene represents approximately 4% of the potential cancer risk of the nine measured compounds and the estimated diesel particulate matter.


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