Indoor concentrations of some outdoor air pollutants can increase with ventilation rate. Increases in indoor ozone and particle concentrations may be most significant. Higher outdoor air ozone concentrations are associated with adverse respiratory and irritation effects and several other health effects . Outdoor air polluted with ozone is normally the major source of indoor ozone. Because ozone is removed from indoor air through chemical reactions with indoor pollutants and materials, indoor ozone concentrations tend to be substantially lower than outdoor air ozone concentrations ; i.e., buildings tend to shield us from outdoor ozone. However, as ventilation rates increase, indoor ozone concentrations become closer to outdoor concentrations . Thus, increasing the ventilation rates will increase our exposures to ozone.
Increases in ventilation rates will also generally increase indoor concentrations of, and exposures to, outdoor air respirable particles, while simultaneously reducing our exposures to indoor-generated particles. Higher outdoor particle concentrations are associated with a broad range of adverse health effects . If the incoming outdoor air is filtered to remove most particles, the influence of ventilation rate on indoor particle concentrations can be small .
Ventilation rates, if stable over time, will not generally affect time-average indoor concentrations of non-reactive gaseous outdoor air pollutants such as carbon monoxide, but higher ventilation rates can increase peak indoor concentrations.
A higher indoor humidity, which, in turn, can lead to more indoor dust mites (an important allergen source) and to a greater risk of indoor mold growth, is another potential consequence of increased ventilation rate. Indoor humidity will increase with ventilation rate only when the outdoor air is more humid than the indoor air, e.g., during hot humid weather, and when the building mechanical systems also do not dehumidify sufficiently to counteract the effects of increased moisture entry. When outdoor air is less humid than indoor air, e.g., during cool winter weather, more ventilation decreases the indoor humidity.
From the discussion provided above, it is clear that more ventilation can increase our exposures to some pollutants, particularly where and when the outdoor air is highly polluted or warm and humid. At the same time, the increases in ventilation rate will diminish our exposures to a variety of indoor-generated air pollutants. On balance, the scientific literature points to improvements in health and performance with increased ventilation rate; however, at polluted locations where it may not be possible or practical to adequately remove pollutants from incoming ventilation air, it is possible that some moderate intermediate ventilation rate is better for health than higher ventilation rates. At hot-humid locations, mechanical cooling and dehumidification systems should be designed to accommodate the moisture loads associated with the prescribed ventilation rates. More research is needed to examine the tradeoffs associated with increases in building ventilation.