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Air Risk Assessment

In the context of IDEM’s Air Toxics Program, risk assessment refers to analyzing the sampling data and determining what, if any, risk is posed to public health.

There are three pieces to the risk assessment puzzle: concentration, exposure, and toxicity. Concentration refers to the amount of pollutant measured in the air. This data is acquired from monitoring or modeling. Exposure refers to how much contact a person has with the pollutant. For air pollutants, this usually takes the form of the following questions:

  • How many hours a day is a person exposed?
  • How many days a year is a person exposed?
  • How many years during a lifetime is a person exposed?

For air toxics, IDEM assumes continuous exposure. This means that we assume that a person is exposed 24 hours a day, 365 days a year, for 70 years. This is an extremely health protective assumption.

The last piece of the puzzle is toxicity. IDEM gets toxicity data from sites such as U.S. EPA’s Integrated Risk Information Service (IRIS) and the Agency for Toxic Substances and Disease Registry (ATSDR). These sites post data, often developed from animal studies, on how toxic a substance is and how likely it is to cause cancer.

With concentration, exposure, and toxicity information, IDEM calculates the estimated non-cancer hazard and/or the cancer risk posed by the pollutant. Hazards are an “all-or-nothing” effect. In other words, with hazards the concentrations are either high enough to cause a potential effect, or there is no effect expected. Cancer risk is different. In cancer calculations it is assumed that any exposure can raise the risk of developing cancer, so cancer risks are probabilities. For example, a measured concentration of benzene may result in a 7-in-a-million cancer risk estimate. This means that if a million people were exposed to that concentration of benzene continuously, for 70 years, we would expect, at most, 7 of them to develop cancer from their exposure to benzene. Threshold concentrations are the amount of air pollutant that would cause a cancer risk of 1-in-a-million or a hazard effect.

All pollutants are not equally toxic. For example, IDEM regularly monitors for both acetone and benzene. While acetone has a threshold at concentrations up to 31,000 µg/m3 (micrograms per cubic meter), benzene becomes a concern at concentrations as low 30 µg/m3. IDEM updates its toxicity data as it becomes aware of changes, and performs a complete toxicity data review every two to three years.

In addition to the more in-depth analysis mentioned above, IDEM performs a monthly screening of all air toxics data to look for unusually high hazards or risks.

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