As climate change increases outdoor temperatures, air conditioning will more often be used to maintain comfortable indoor conditions. Climate change is expected to stimulate installation of air conditioning in some buildings and locations that would otherwise not need air conditioning. In the 2011 American Housing Survey , 64% of housing units had central air conditioning and 21% of housing units had one or more room air conditioning units, so at least 15% of existing US houses had no air conditioning. Also, where air conditioning is already present, operation times will increase. The increased use of air conditioning driven by climate change is expected to affect health both positively and negatively. In some locations and buildings, existing air conditioning systems may become unable to maintain comfortable temperatures as the climate warms .
Air conditioning can attenuate high indoor air temperatures during heat waves. Increased availability of air conditioning, stimulated by climate change, would be expected to diminish the increases in adverse health effects resulting from the increases in frequency and duration of heat waves. Ostro et al.  found that the increase in respiratory hospitalizations with high outdoor temperature was approximately halved among people who reported ownership or use of air conditioning, after controlling for income. The associated long-term health benefit will depend on extent of increased air conditioner availability in homes, which is not known.
When air conditioning is employed in naturally-ventilated buildings such as most homes, windows are more often maintained closed; consequently, rates of building ventilation with outdoor air are reduced. By keeping windows closed in naturally-ventilated buildings, indoor air concentrations of some outdoor air pollutants, particularly particles and ozone, from outdoor air are diminished. At the same time, indoor air concentrations of pollutants emitted from indoor sources will increase in naturally-ventilated buildings when air conditioning is employed due to the closing of windows and reduction in outdoor air ventilation. Some new and remodeled homes and most commercial and institutional buildings provide outdoor air ventilation mechanically using fans. In these buildings, changes in use of air conditioning will have a smaller impact on rates of outdoor air ventilation.
The filters in many air conditioning systems can reduce indoor particle concentrations. Also, air conditioning systems may become sources of microbial contaminants. The net effects for health are not well understood, but some relevant findings are available.
- Increased use of air conditioning and associated reductions in air exchange rate are expected to reduce the health effects of indoor ozone exposures. Outdoor air is the only or dominant source of ozone in most buildings. Because of indoor ozone removal by chemical reactions, when windows are closed and ventilation rates are low, indoor ozone concentrations may be only 10% to 30% of the outdoor air concentration . In one experimental study in a U.S. home, the ratio of indoor-to-outdoor ozone concentration was 0.28 when windows were closed and the air conditioner operated, 0.26 with windows closed and no air conditioning, and 0.59 with windows open . In a study over time of three mechanically-ventilated office spaces, indoor-to-outdoor ratios of ozone ranged from 0.2 to 0.8 and increased with ventilation rate . Mortality rates appear to increase less with increased outdoor air ozone in cities with a high fraction of homes employing central air conditioning . Dutton et al.  modeled the effect of converting 10% of California's offices to natural ventilation. The increased indoor ozone levels were projected to cause a few premature deaths per year and from approximately 10 to 30 annual cases of asthma attacks, restricted activity days, and respiratory hospital admissions. Thus, modest improvements in ozone-related health effects are anticipated from the increased use of air conditioning stimulated by climate change.
- Ozone is removed from indoor air through chemical reactions with other air pollutants and indoor materials. These reactions reduce the indoor ozone concentration but often create other pollutants that may pose health risks [69, 73, 113]. The decreases in outdoor air ventilation in naturally-ventilated buildings as more air conditioning is used may increase indoor air concentrations of the pollutants produced from chemical reactions of ozone with air pollutants and indoor materials .
- Increased use of air conditioning, and associated reductions in natural ventilation rates from closing windows, will have a less consistent impact on indoor particle concentrations. Indoor concentrations of particles from outdoor air will be decreased while indoor concentrations of particles emitted from indoor sources will increase. Depending on the outdoor air particle concentration and the strength of indoor particle sources, total indoor air particle concentrations may decrease or increase. A recent review of available data  indicates that concentrations of particles less than 2.5 micrometers in diameter, which are the particles most clearly associated with health, tend to be higher indoors in North American and European homes than outdoors , indicating that decreases in ventilation will, on average, increase indoor particle exposures. However, these studies include homes with and without tobacco smoking and do not compare homes with no air conditioning and open windows to homes with air conditioning and closed windows. Analyses of data from U.S. homes by Wallace  found that indoor concentrations of particles from outdoor air were only 30% of the outdoor air concentrations in homes with air conditioning and 70% of outdoor air concentrations in homes without air conditioning, but these percentages reflect only indoor particles from outdoor air. He also reported that in homes with tobacco smoking, indoor particle concentrations were about twice as high with central air conditioning compared to without central air conditioning. At present, data are insufficient to project the overall average impact of increased air conditioning on indoor air particle concentrations. Adding to the uncertainty, much is known about the health risks of increased exposures to particles from outdoor air while relatively little is known about the health risks of indoor-generated particles. The unknown relative potency of particles from these two sources in causing health effects will influence whether increased use of air conditioning reduces of increases the adverse health effects of exposures to particles.
- For reasons that are not well understood, air conditioning is associated with increases in acute health symptoms often called sick building syndrome (SBS) symptoms, and also with asthma symptoms. SBS symptoms include irritation of eye, nose, and throat, headache, and fatigue, and sometimes other effects. SBS symptoms are not clearly linked to a specific disease or specific pollutant exposures. A review of data available from office buildings indicates that the prevalence rates of SBS symptoms are 30% to 200% greater among occupants of air conditioned offices . A more recent study reported similar size and statistically significant increases in SBS symptoms among office workers in a tropical climate . A large study of U.S. homes reported an increase in asthma symptoms by about 10% in air conditioned homes, but the increase was not quite statistically significant . A larger multi-center study in Europe  found 30% to 40% increases in wheezing and breathlessness and current asthma in homes with air conditioning, and the increases were statistically significant. A study of 104 child care centers in Singapore found that air conditioning, relative to natural ventilation, was associated with statistically significant 20% to 40% increases in cough and lower respiratory illness defined as bronchiolitis, bronchitis, pneumonia, or croup . The results of an experimental study in three office buildings  suggest that the increases in health symptoms in air conditioned buildings may be a consequence, at least in part, of microbial contamination on the frequently wet cooling coils and drain pans of air conditioning systems. Irradiation of the coils and drain pans with ultraviolet lights dramatically reduced the microbial contamination on irradiated surfaces and decreased mucosal and respiratory symptoms by 30% to 40%.
At present, one cannot predict with confidence the net influence for health of increases in air conditioning stimulated by climate change. It appears that increased air conditioning will reduce the health effects of heat stress and ozone exposures, but increase SBS and asthma symptoms.