This book presents WHO guidelines for the protection of public health from a number of chemicals commonly present in indoor air. The substances considered benzene, carbon monoxide, formaldehyde, naphthalene, nitrogen dioxide, polycyclic aromatic hydrocarbons (especially benzo[a]pyrene), radon, trichloroethylene and tetrachloroethylene have indoor sources, are known for their hazardousness to health and are often found indoors in concentrations of concern to health. For each substance, the chapter covers a general description, the sources and pathways of exposure, the indooroutdoor relationship, kinetics and metabolism, the health effects, a health risk evaluation, the guidelines, a summary box and references.These guidelines are targeted at public health professionals involved in preventing the health risks of environmental exposures, as well as at specialists and authorities involved in the design and use of buildings, indoor materials and products. They provide a scientific basis for legally enforceable standards.