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The variety of hazardous substances used in our everyday lives brings with them an equally great variety of well-documented health effects. In some cases, these substances may irritate the skin or eyes, make it difficult to breathe, cause headaches and nausea, or result in other types of illness. Some hazardous substances may cause far more severe health effects, including behavioral abnormalities, cancer, genetic mutations, physiological malfunctions (e.g., reproductive impairment, kidney failure, etc.), physical deformations, and birth defects.

Impacts on the environment can be just as devastating: hazardous substances can kill living organisms in a lake or river, destroy wildlife and vegetation in a contaminated area, cause major reproductive complications in wildlife, or otherwise limit the ability of an ecosystem to survive. Certain hazardous substances also have the potential to explode or cause a fire, threatening both wildlife and human populations.

Some hazardous substances produce toxic effects in humans or the environment after a single, episodic release. These toxic effects are referred to as the acute toxicity of a hazardous substance. Other hazardous substances produce toxic effects in humans or the environment after prolonged exposure to the substance, which is called chronic toxicity.

EPA uses the acute and chronic toxicity of hazardous substances to guide different aspects of the emergency response program. For example, when first investigating sites of a hazardous substance relase, EPA will determine the seriousness of the threat, and the need for immediate response, based on the toxicity of the substances present and their possible health effects. EPA also uses the acute and chronic toxicity of a hazardous substance to establish its Superfund reportable quantities (RQs). If the substance is released to the environment in an amount equal to or greater than the RQ, the release must be reported to the federal government. In this way, EPA can respond to the release to protect human health and the environment from the adverse health effects of that hazardous substance.