In a 1998 State of the Debate Report, the National Round Table on the Environment and the Economy defined brownfields as follows:
Brownfield sites are abandoned or under-used properties where past actions have caused real or suspected environmental contamination. Although they are classified as a subset of contaminated sites, these sites exhibit good potential for other uses and usually provide economically viable business opportunities. They are mainly located in established urban areas, where existing municipal services are readily available, or along transportation corridors. They may include, but are not limited to: decommissioned refineries, railway yards, dilapidated warehouses, abandoned gas stations, former dry cleaners, and other commercial properties where toxic substances may have been stored or used.26
Brownfields are not just an issue for old industrial areas. New contamination is still being created, for example, by leaking petroleum product storage tanks, domestic uses of solvents, paint strippers and other chemicals, and farm disposal of fertilizers, lubricants, and pesticides. Conventional uses of agricultural chemicals such as pesticides, spreading of biosolids such as paper waste, and spray irrigation of domestic or industrial sewage can all exceed Ministry of the Environment criteria. Roads also often have measurable impacts on nearby lands.
Current damaging, but legal, activities, such as soil conditioning and the use of pesticides, may create future brownfields as the detection limits for contaminants improve and evidence of harm increases. As an example, the developing science of endocrine disruptors could force regulatory tolerances for pesticides to be set at unprecedentedly low levels. Future regulatory criteria may also be set for other widely used chemicals that are not currently regulated in soil or groundwater, such as pharmaceuticals, plasticizers, and phthalates (a component of plastic).
Even supposed "greenfields" may contain undiscovered contamination from past or present deposition of air contaminants, from farm activities or from waste disposal. There are many cases in which contaminated fill, chemical agricultural wastes, and industrial wastes have been dumped on or buried in innocuous-looking rural lands.