Testimony concerning indoor air quality presented by NIOSH to the United States Department of Labor was discussed. Numerous epidemiological studies were cited implicating environmental tobacco smoke in the development of heart disease and lung cancer. Findings were reported which indicated that prohibiting or restricting cigarette smoking to designated areas in the workplace was both feasible and necessary. The recent research concerning building related illness and sick building syndrome was also discussed. These illnesses were attributed to the poor design, maintenance, and operation of heating, ventilation, and air conditioning (HVAC) systems. In one study, one or more deficiencies, such as dirty air handler coils, dirty pans under coils, and poor pan drainage, were observed by NIOSH in 93 out of 104 office buildings. Work related symptoms determined among the workers of these buildings included headache, respiratory symptoms, and skin symptoms. Analysis of the NIOSH data also indicated an association between HVAC maintenance quality and symptom frequency. NIOSH agreed with OSHA in recommending that indoor air quality rules focus on nonindustrial environments only. NIOSH also supported OSHA in advising an inspection of ventilation systems if carbon-dioxide (124389) levels exceeded 800 parts per million. NIOSH also recommended a minimum air flow rate of 20 cubic feet per minute. In addition, NIOSH suggested a new risk assessment approach, which incorporated the use of naturally ventilated buildings for background level comparisons with air conditioned buildings. Symptoms were more frequent and severe among the workers of air conditioned buildings than among those of naturally ventilated buildings. The author concludes that, while many problems with indoor air quality are difficult to interpret, NIOSH has identified common deficiencies in building design, maintenance, and operation. Many of these deficiencies can be remedied, resulting in improved worker health.