Some government agencies have developed standards and screening levels for toxic chemicals in the air based on health information about the chemicals. These agencies are listed below, with a brief description of the methods used in establishing their levels. Click on the agency name to go to the source of the levels.

EPA Region 6 Screening Levels

These levels are based on existing studies of chemical health effects. They levels are calculated for residential (as opposed to workplace) exposures. They reflect the risks of exposure to a certain level of the chemical. The levels listed as screening levels correspond to pre-determined levels of risk from exposure: either 1 in a million cancer risk or a "hazard quotient" of 1 for non-cancer effects, whichever corresponds to a lower concentration. These screening levels are not legally enforceable.

Texas Effects Screening Levels

These levels are based on existing studies of chemical health effects. Below these levels, no adverse health effects are thought likely to occur. They reflect the experimentally-determined levels at which the chemicals caused adverse effects in study populations (NOAELs or LOAELs) of people or animals, combined with safety factors to account for the differences among human populations and between humans and animals.

"Short-term" refers to an exposure duration of one hour. "Long-term" refers in most cases to a duration of one year; for benzene and ethylene dichloride, it indicates a 24-hour period.

These levels reflect both cancer and non-cancer effects. They are not legally enforceable.

Louisiana Ambient Air Quality Standards

These levels are legally enforceable standards in Louisiana, developed through Louisiana's regulatory process. They are found in Table 51.2 of Title 33, Part III.

They are based on health effects information about the chemicals: the eight-hour standard modifies occupational exposure levels to be appropriate for residential exposures; the annual standard is based on EPA procedures for calculating cancer risks.

North Carolina Ambient Air Standards

These levels are legally enforceable standards in North Carolina, developed through North Carolina's regulatory process.

They are based on health effects information about the chemicals.

ATSDR (American Toxic Substance and Disease Registry) Minimal Risk Levels

These levels are based on existing studies of chemical health effects. Exposure at these levels is said to pose "minimal risks" of adverse health effects. The levels were derived from the experimentally-determined levels at which the chemicals caused adverse effects in study populations (NOAELs or LOAELs) of people or animals, combined with uncertainty factors to account for the differences among human populations and between humans and animals.

"Acute" exposure is defined as up to two weeks, "Intermediate" as between two weeks and one year, and "Chronic" as longer than a year.

These levels reflect only non-cancer health effects. They are not legally enforceable.