Acute Exposure --

An acute exposure is defined as an exposure to a chemical for a duration of 14 days or less. Although the duration of exposure during an unscheduled chemical release may be brief, the chemical concentration may be high. The onset of health effects resulting from an acute exposure can be immediate or delayed.

Estimating the risk of adverse effects from an exposure is known as health risk assessment. Health risk depends on the intrinsic toxic potential of the chemical, its concentration, the duration of exposure, and the health status of the person exposed. Even very toxic chemicals may not pose a risk of illness if the duration is brief or the concentration of the substance is low. Exposures to high concentration of even mildly toxic substances, however, can be dangerous.

The toxic potential of a chemical can be gauged by measuring the dose that will kill half of a group of test animals and comparing that dose to similar values for other chemicals. When the dose is administered to test animals by ingestion or injection, it is referred to as the lethal dose (LD(50)). The LC(50) refers to the lethal air concentration that is fatal to 50% of the test animals when it is inhaled. The lower the LD(50) or LC(50), the more potent the chemical. Although widely used to describe relative toxicity of chemicals, these numbers have limited clinical relevance. The values apply only to animal species, not humans. Also, they provide no information about doses that might not kill but could cause serious injury to the lungs, kidneys, or other organs.

The lowest concentration of a chemical in air that has been reported to cause death in humans or animals is referred to as LC(Lo). The dose of chemical at which there are no statistically or biologically significant increases in frequency or severity of adverse effects between the exposed population and appropriate controls is the No-Observed-Adverse-Effect Level or NOAEL.

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