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Cumulative Risk from Airborne Chemicals

Questions and answers about assessing and addressing risk from cumulative exposure to airborne chemicals.

Normally, people are exposed to many chemicals from many sources every day—both human-made and naturally occurring—in food, water, soil, and air. For example:

◦naturally occurring chemicals and food additives
◦emissions from cars, dry cleaners, and industrial sources
◦natural minerals such as arsenic and by-products from disinfection in drinking water
◦chemicals from soil that volatilize (become vapors)
◦emissions from vegetation and wildfires

Exposure to chemicals is associated with some level of risk of adverse effects on health (defined as damage to normal functions of the body, which can include such problems as irritation and lung disease, as well as cancer). The body can generally remove harmful chemicals, but sometimes it may not be able to get rid of a chemical before it can cause harm. As the amount of chemical a person is exposed to increases, so does the likelihood of harm. Exposure of a person to one chemical from multiple sources is called aggregate exposure. Exposure from multiple sources is called cumulative exposure.

The TCEQ’s extensive air-monitoring network helps verify that its permitting process has been effective for multiple emission sources even in the most industrialized areas of Texas. The TCEQ benefits from the largest stationary monitoring network in the country (Table 1). Many of these monitors are placed in areas with densely packed sources, such as industrial areas, which represent a worst-case scenario of aggregate exposure—giving the agency high confidence that policies and practices that work in those areas will work equally well in less-industrial areas.
This table contains a list of states that monitor for VOCs and a count of their respective monitoring sites.
Monitors provide reliable data on aggregate and cumulative exposure as they measure the air concentrations due to emissions from all sources (such as industrial sites, mobile sources such as cars, and area sources such as gas stations). The vast majority of the monitors in the state showed annual average concentrations under the 1-in-100,000 screening level for carcinogenic chemicals like benzene. Because actual monitoring data verify acceptable exposure levels and the ESLs are inherently conservative and health protective, the TCEQ is confident that the potential of additional impacts from individual sources in an area are minimal.
The TCEQ also uses cumulative risk assessments from other organizations, such as the EPA’s NATA, to identify areas with computer-modeled concentrations above a level of concern. Although NATA is based on a theoretical model of reported emissions rather than actual monitored concentrations, the assessment helps the TCEQ identify other potential issues.
Finally, in the limited areas (about 0.08 percent of the state) where actual monitored concentrations of chemicals indicate a potential concern, the TCEQ uses the Air Pollutant Watch List to reduce ambient (outdoor air) levels. The list considers all the possible sources of the chemical of concern.

For more detailed information on risk assessment at the TCEQ, please contact the Toxicology Division, by phone—toll-free: 877-992-8370, local: 512-239-3900—or e-mail: