Levels of chemical or physical agents that are normally found in the environment. Two types of background levels may exist for chemical substances or physical agents: (a) Naturally occurring levels: ambient concentrations of substances or agents present in the environment, without human influence; (b) Anthropogenic levels: Concentrations of substances or agents present in the environment due to human-made, non-site sources (e.g., automobiles, industries).

(Source: US EPA Glossary of IRIS Terms )