Brownfields are abandoned or underused industrial properties that pose, or potentially pose, a risk to humans and the environment. Brownfields may be contaminated with dangerous industrial waste products, making them impossible to redevelop. The U.S. Environmental Protection Agency estimates that there are close to half a million brownfields in the United States. The EPA Brownfields Program encourages the revitalization of contaminated land so that it can be reused without endangering human or environmental health.

Brownfields have been created by a range of different industries, so the types of pollutants vary among sites. Waste from fertilizer factories is rich in nitrogen, calcium, sodium and bicarbonate. Petroleum and pesticides contain dangerous hydrocarbons, while the waste from other types of manufacturing can contain a variety of metals, including lead, iron, mercury, arsenic, copper and cadmium. Heavy metals and hydrocarbons are of most concern to authorities because they are highly toxic and more pervasive in the environment, relative to other pollutants. Pollutants also include abandoned construction materials, which can be physically dangerous to both humans and wildlife and are unsightly.

Plants, wildlife and humans can come into contact with brownfield pollutants in several different ways. Plants growing in contaminated soil directly take up metals and other pollutants. Metal-tolerant plants allow the accumulation of heavy metals in their tissues. Plants are eaten by herbivores, which are in turn eaten by birds and mammals. Metals are passed up the food chain, accumulating at each level and increasing the risk that organisms will be exposed to a harmful dose. Many brownfield contaminants are soluble in water and can rapidly drain into groundwater. This poses a risk to humans and animals that use aquifers as a source of drinking water. Contaminated soil can be inhaled in the form of dust or contaminants can be absorbed through the skin.

The susceptibility of animals to brownfield pollutants varies among species and is also dependent on the degree of exposure. Research on the effects of lead accumulation in pied flycatchers, published in the May 2010 issue of "Environmental Pollution," found that contaminated birds laid fewer eggs, experienced higher egg and fledgling mortality, and were generally in poor health. The authors note that physical deformities and abnormal behavioral have been observed by similar studies. A study published in the same issue of "Environmental Pollution," however, found no such effects on wrens, despite detecting accumulation of several metals in study birds. The U.S. Environmental Protection Agency reports that a number of common brownfield contaminants are toxic to a wide variety of living organisms. Diagnosed cases of brownfield poisoning of humans is rare, but it is difficult to know whether this means that it occurs infrequently or if symptoms are attributed to other factors. Long-term exposure to metals and hydrocarbons has been linked to organ failure, cancer, nervous system damage, reduced fertility and respiratory disease in adults. Children are known to be more sensitive to lead, hydrocarbon and nitrate poisoning.

The EPA implemented a brownfield revitalization program in 1995. This program provides grants to communities and private businesses to help with the financial cost of cleaning up brownfield sites. The program has generated successful revitalization projects throughout the country. Sites can be cleaned by washing or heat treating contaminated soil. This can be done onsite, or soil can be removed and treated in a safer environment. It is cheaper to manage contaminated soils rather than try and remove contaminants at the outset. Management techniques include growing plants that break down, rather than accumulate toxins, and chemical transformation of pollutants by increasing soil pH or adding phosphates. Chemical transformation converts pollutants into compounds that are less available in the environment. The revitalization program also encourages companies to recover and recycle building materials from brownfield sites.