Understanding Risk and Hazard

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Scott Openshaw
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When chemicals are discussed in the context of a “risk assessment,” it refers to a comprehensive evaluation of both the inherent hazard of a particular substance as well as the expected amount and frequency of exposure to that substance by a person or the environment. Risk is the likelihood of harm based on both hazard and exposure.

Risk is the possibility of a harm arising, while hazard refers to the inherent properties that make a substance able to cause a risk.  For example, the sun could be considered a hazard. But, a person’s risk of getting burned depends on how strong the rays are, whether or not they are inside or outside, and whether or not they are wearing protection, such as sunscreen.  To better evaluate this situation, we would also need to understand how much sun the person is being exposed to, and over what period of time.

Converted to the language of scientific risk assessment:

  1. Hazards are identified.
  2. Potential exposures to the hazard are assessed.
  3. A determination is made about the amount of risk a person or the environment faces in a particular situation.

Hazard refers to the inherent properties of a substance that make it capable of causing harm to human health or the environment.

Exposure describes both the amount of, and the frequency with which, a chemical substance reaches a person, group of people or the environment.

Risk is the possibility of a harmful event arising from exposure to a chemical or physical agent, for example, under specific conditions.

There are hazards all around us. But a hazard doesn't become a risk unless you are exposed to it. And, if you are exposed, the exposure has to be at a level that might do harm; it makes a difference if the sun is shining or if it’s a rainy day.

Science can help put risks and hazards into context. From studies of human data, or environmental data, or from laboratory studies on animals, scientists can calculate the point at which a hazard might become a risk. For chemicals, regulatory agencies set safe exposure limits, using scientific data to determine a level where no effects have been observed and then often dividing by ten, a hundred, a thousand, or more, to build in a margin of safety.


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