This book presents WHO guidelines for the protection of public health from a number of chemicals commonly present in indoor air. The substances consideredbenzene, carbon monoxide, formaldehyde, naphthalene, nitrogen dioxide, polycyclic aromatic hydrocarbons (especially benzo[a]pyrene), radon, trichloroethylene and tetrachloroethylenehave indoor sources, are known for their hazardousness to health and are often found indoors in concentrations of concern to health.