The exposure pathway of a hazardous substance is an important consideration in determining emergency response actions. An exposure pathway refers to the way a person can come into contact with a hazardous substance. There are three basic exposure pathways: inhalation, ingestion, or direct contact. The degree or extent of exposure is determined by measuring the amount of the hazardous substance at the point of contact. Health and ecological hazards can result from such exposures. Common ways in which people can become exposed to hazardous substances include:

• Groundwater and Surface Water: Exposure will occur if people drink contaminated groundwater or surface water, accidentally ingest it while swimming, or if it comes into contact with their skin (e.g., in the shower, while swimming, etc.).

• Soil, Sediment, Dust: People will be exposed to hazardous substances in soil, sediment, or dust if they accidentally ingest it (e.g., the contaminants land on their food), if they breathe it in (especially dust), or if their skin comes into direct contact. Because of their play habits, children are highly susceptible to exposure through these pathways.

• Air: When the hazardous substance takes the form of vapors or is absorbed by particulate matter, the simple act of breathing can expose people. In some cases, a person's skin can absorb a hazardous substance in vapor form, although inhalation is considered the greater threat.

• Food: Eating food that has been contaminated is another common exposure route. In some cases, food found on people's plates may be contaminated as a result of direct exposure to the hazardous substance. In other cases, food contamination can occur further down the food chain. For example, hazardous substances can collect in the fatty tissues of animals that ingest contaminated plants. The contamination can then be transferred to the animals' natural predators, and eventually, to people.