Over the past 50 years, a new man-made ecosystem has developed--the controlled indoor environment within the sealed exterior shells of modern non-industrial buildings. Emitted toxic volatile compounds from building materials, furnishings, and equipment, and inappropriate ventilation (resulting from the need to reduce expenses) contribute to reduce indoor air quality (IAQ), which has considerable potential to affect public health. Consequently, health problems related to this ecosystem have emerged. "Building-related illnesses" (BRI) refers to a group of illnesses with a fairly homogeneous clinical picture, objective abnormalities on clinical or laboratory evaluation, and one or more identifiable sources or agents known to cause infectious, immunologic, or allergic diseases. The term "sick building syndrome" (SBS) is used to refer to a heterogeneous group of work-related symptoms--including irritation of the skin and mucous membranes of the eyes, nose, and throat, headache, fatigue, and difficulty concentrating. These are considered illnesses because of the occurrence of symptoms, even though affected workers do not have objective clinical or laboratory abnormalities and causative agents cannot be found. The clinical symptoms of SBS, although not life-threatening are disruptive: they reduce productivity and increase absenteeism from work. Noteworthy, the association of symptoms with psychosocial factors does not mean that "the problem is all in the workers' heads". The results of psychological testing of symptomatic and asymptomatic office workers are similar. To improve IAQ and reduce symptoms of SBS adequate ventilation and fresh air, which will reduce volatile compounds, maintaining thermal comfort (with humidity not exceeding 60%), and adequate lighting should be ensured.