In the Encyclopedia of Earth, Emily Monosson points out

Humans have been producing and releasing environmental contaminants ever since the discovery of fire. Contaminants include both naturally occurring chemicals released in unnaturally high concentrations, and chemicals that are novel to natural environments, produced by human activity and released either intentionally or accidentally.

The sources of potential harm to human health are as prevalent in the home (for example, the quality of indoor air) as they are in the outdoor environment.