The use of these substances by humans, however, usually results in unnaturally large, localized concentrations. All synthetic chemicals are of concern, because wildlife and humans are not resistant or adapted to them. There are an estimated 70,000 chemicals in commerce in the US, with an additional 1,000 or so new chemicals produced each year. We still do not know the effects of most of these manufactured chemicals-because, among other reasons, of the sheer number of contaminants, because of the lack of information on biological effects of complex mixtures, because different chemicals affect different species in different ways, and because the nature and extent of negative effects varies according to the magnitude, timing and duration of exposure, and the susceptibility of the organism. These uncertainties have delayed or prevented regulatory action, particularly since the onus is on government and society to prove damage' rather than on the chemical producer, user or polluter to prove safety.
International treaties can initiate stronger efforts to reduce toxic pollution worldwide.
A new treaty banning the production and use of certain persistent organic pollutants is being negotiated as a result of an Intergovernmental Conference on the Prevention of Marine Pollution Caused by Activities on Land, held in Washington, D.C., in 1995. However, such treaties can only address a fraction of the total contaminants being produced. Another approach is through prevention by preventing or reducing waste discharge when it has a potential for causing pollution, rather than regulating it only after it has caused pollution.