Assessing Toxicity

Air toxics are chemicals that pose a number of health risks and environmental effects. Toxicity refers to the degree to which a chemical is harmful to human health. Certain air toxics may cause cancer, others may cause serious non-cancer illnesses, and some chemicals pose both cancer risks and non-cancer hazards. The Air Toxics Program within IDEM’s Office of Air Quality utilizes toxicity data for air risk assessments.

Thresholds and Dose-Response Level

Scientists conduct research, including animal studies, to determine the levels at which toxic chemicals increase cancer risks and/or cause non-cancer health effects. Threshold is the term for the lowest dose of a chemical that may cause a measurable effect. The term dose refers to the concentration (amount) of a chemical and the length of exposure. Dose-response level is another term for threshold. Scientists identify concentrations that cause acute (immediate) effects or may cause problems from chronic (continuous, long-term) exposure. Because chemicals can build up in the body from continual exposure over time, chronic thresholds are usually lower than acute thresholds.

Cancer Risks and Non-Cancer Hazards

Chemicals that are carcinogenic (have the potential to cause cancer) affect the body differently than chemicals that cause other serious non-cancer hazards. Any level of exposure to a carcinogen poses an increased risk to health. 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.

For non-cancer hazards, exposure below a chemical’s threshold is not considered to pose a hazard. In other words, non-cancer hazards are an “all-or-nothing” effect, meaning the chemical concentration is high enough to cause a potential effect (equal to or above the threshold) or there is no effect expected (below the threshold).

IDEM provides complete findings of its air risk assessments and works to help citizens understand accuracy of estimates and put risks in perspective.

Dose-Response Table

The Air Toxics Program’s dose-response table [ZIP] contains data derived from the following sources.

• Integrated Risk Information System (IRIS)
• Agency for Toxic Substances and Disease Registry (ATSDR)
• Health Effects Assessment Summary Tables (HEAST)
• California Environmental Protection Agency Air Resource Board (CARB) [PDF]
• International Agency for Research on Cancer (IARC)
• Regional Screening Levels (RSL)
• U. S. EPA Office of Pesticide Programs (OPP-CAN) [PDF]
• U. S. EPA Hazardous Waste Identification Rule (HWIR)
• Michigan Air Toxics
• TRI Risk-Screening Environmental Indicators (TRI)
• Superfund Chemical Data Matrix (SCDM)
• American Conference of Governmental Industrial Hygienists (ACGIH)

As shown in the Air Toxics Program dose-response table, not all chemicals are equally toxic. For example, acetone and benzene are both air toxics; however, they have different degrees of toxicity. Benzene, which has a threshold of 30 micrograms per cubic meter (30 µg/m3), is more toxic than acetone, which has a threshold of 31,000 µg/m3. There are certain chemicals for which thresholds are not determined. IDEM updates its toxicity data as it becomes aware of changes and performs a complete toxicity data review every two to three years.

Scientists and air risk staff often use exponents and scientific notation for thresholds, because they are usually extremely small numbers. (It is generally easier to use scientific notation for calculations with very small numbers.) Following are examples of fractions and decimals expressed in exponents and scientific notation.