This chapter describes the main sources responsible for the increased levels of inorganic and organic compounds in water resources and drinking water. Six inorganic compounds (arsenic, fluoride, lead, mercury, and nitrates and nitrites) and eight groups of organic compounds (hydrocarbons, chlorinated compounds, volatile organic compounds, solvents, trihalomethanes, pesticides, persistent organic pollutants and emerging chemicals) were chosen to demonstrate the diversity of risk sources (natural releases to aquatic environment, point sources and non-point discharges to surface waters, and releases from hazardous waste disposal sites and chemical spills). The role of the major routes of exposure to chemical compounds in urban populations and the identification of the main adverse health effects that each compound can cause are also described.

Chapter 4 also includes examples that illustrate the compounds most frequently reported in research published in scientific journals, to which population in cities from developed and developing countries are exposed. Some examples reflect national water quality status and the concern with monitoring, analysis and identification of compounds at risk level. Others focus on the hundreds and even thousands of people with some type of disease caused by the exposure to inorganic compounds naturally found in water resources. They also address the diversity of diseases a chemical agent may cause, the severity and endemic characteristics that some diseases have, and the poverty, the low sanitation level achieved, and the poor regulation conditions under which such exposures occur in some developing countries.

The last section includes some issues that should be taken into account in the implementation of a successful risk management strategy to prevent and reduce chemical risks in the urban water cycle. Factors such as low income, poor housing and public services, unplanned urbanization and environmental mismanagement increase the risk. Vulnerability to chemical risks is dynamic and influenced by a multitude of factors, including population growth and regional shifts in population, technology, urban government policies, land and water use trends, and increasing environmental awareness. Consequently, it is important that cities identify plans at all levels, including prevention, mitigation, response and public health advisories, through multidisciplinary and inter-sector interactions, according to local socio-economic and cultural circumstances.