If we consider these routes of entry, it is possible to think of a number of ways in which contaminants escaping into the environment from their source may reach a living plant or animal, or receptor. Each specific route a chemical might travel from a source to a receptor is called an exposure pathway.

The pathway may be either direct or indirect. If an open toxic waste dump were near you, you could inhale the vapors from the toxic material, or your skin could contact toxic contaminants if you walked through the substance. These are direct means of exposure. The substance can also reach you by indirect pathways. For example, toxic vapors or particles from a site at which hazardous waste has been illegally discarded could be carried in the air to a cornfield and deposited on the crop as it rains. You ingest some of the toxic chemical as you eat the corn; or perhaps a farm animal eats the corn and you later eat meat from that animal.

Another pathway might be through drinking water. When rain falls and passes through polluted soil, it carries chemicals deeper into the earth as well as horizontally across the surface of the soil. If they are able to move far enough—which depends on the geology of that particular area—they could contaminate the groundwater. The contaminants could also be carried along the land by means of surface runoff, water that moves along the top of the soil, until they reach a recreational pond where children swim. Now there would be another opportunity for dermal contact.