This book presents WHO guidelines for the protection of public health from risks due to a number of chemicals commonly present in indoor air. The substances considered in this review, i.e. benzene, carbon monoxide, formaldehyde, naphthalene, nitrogen dioxide, polycyclic aromatic hydrocarbons (especially benzo[a]pyrene), radon, trichloroethylene and tetrachloroethyl-ene, have indoor sources, are known in respect of their hazard-ousness to health and are often found indoors in concentrations of health concern. The guidelines are targeted at public health professionals involved in preventing health risks of environmental exposures, as well as specialists and authorities involved in the design and use of buildings, indoor materials and products. They provide a scientific basis for legally enforceable standards. Trichloroethylene (TCE) (CAS Registry Number, 79-01-6; C6 H3Cl 3; molecular weight 131.40 g/mol) is a widely used industrial solvent. It is a volatile, colourless liquid with a sweet ethereal (chloroform-like) smell. TCE is mainly used for the vapour degreasing and cold cleaning of manufactured metal parts (80–95% of consumption). Other applications include industrial dry cleaning, printing, the production of printing ink, extraction processes, paint production and textile printing