There are a variety of chemical contaminants from a variety of sources. These could be of natural origin or resulting from human activities. The primary source of human-made compounds is the chemical industry (e.g., industrial solvents, paints, adhesives, oil and petrochemical industries, household products, pesticides and fertilizers, fabric materials & furnishings). Regarding chemicals of natural origin, a good example are heavy metals that are present in the environment (e.g., arsenic, mercury, lead) apart from being the product of human processes.