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. Continue reading Air Conditioning and asthma