In air pollution control, "Background level" refers to the concentration of air pollutants in a definite area during a fixed period of time prior to the starting up, or the stoppage, of a source of emission under control. In toxic substances monitoring, it is the average presence in the environment, originally referring to naturally-occurring phenomena.
Source: National Wildfire Coordinating Group. 2012. Glossary of Wildland Fire Terminology. PMS 205. Accessed on 9 September 2013. Available at http://www.nwcg.gov