Emergency response efforts must consider the health and ecological hazards of a hazardous substance release. These hazards impact emergency responders and effected communities. In some cases, hazardous 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 can 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: killing organisms in a lake or river, destroying animals and plants in a contaminated area, causing major reproductive complications in animals, 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 animals 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. 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. The toxicity of a hazardous substance are also used to establish its Superfund reportable quantities (RQs). If the substance is released into the environment with an amount equal to or greater than the RQ, the release must be reported to the federal government. This helps EPA respond to the release to protect human health and the environment from the adverse effects of that hazardous substance.