Excessive indoor air pollution can cause sick building syndrome (SBS), cases of which still occur in Japanese homes despite strict regulations on the value of indoor chemical substances established by the Ministry of Health Labour and Welfare. Idiopathic environmental intolerance (IEI), so-called multi-chemical sensitivity (MCS) has become another issue because it is caused by an extreme low concentration of chemical substances. These problems are discussed from the viewpoint of environmental hygiene in the present study. First, indoor air quality and its adverse effect on health is reviewed according to the history of sick buildings. Next, the factors concerning indoor air quality are indicated, and then measures to combat these problems are considered to improve the indoor environment. No ideal solution has been found: however, we summarize important knowledge on research to regain patient health as a result SBS and MCS.