Has the rise in central air conditioning led to an increase in asthma in the United States or is this just a coincidence?
A recent article claims that the increased use of central air conditioning systems in homes over the past 30 years has led to an increase in asthma. But is this really the case or is it that both factors have increased and the association between the two is just a coincidence? There is good reason to believe these two factors are not related.
The results of the analysis supported 5 trends: 1) housing age (quality) and amenities trend with lead poisoning over time; 2) changes in heating and air conditioning systems and prevalence of broken windows and bars on windows trend with the prevalence of asthma; 3) housing air conditioning trends with obesity; 4) cardiovascular health trends with changes in proximity to open space, commercial and industrial facilities, noise and neighborhood air quality; and 5) general health status by race/ethnicity has remained much the same and follows trends in housing over time.
Of particular interest to the HVAC/indoor air communities is the possible connection between the changes in heating and air conditioning systems and the increase in asthma. The specific change in the HVAC system is the increase in central air conditioning. To rephrase their “finding” – more central air conditioning leads to more asthma. This is contrary to other studies and to NIH Guidelines on the management of asthma. It is very possible that the “relationship” is the result of coincidence rather than causality – ie. both central air conditioning and asthma went up during this 30-year period.
The article goes on to point out that central HVAC systems in houses do not normally introduce fresh air but rely on building leakage for fresh air supply. They go on to state: “reduced fresh air introduction can be expected to increase exposure to allergens, oxides of nitrogen and other airborne asthma triggers because they will not be diluted.” Again, this statement is not supported by other studies of indoor air or medical outcomes. It appears to be purely conjecture on the part of the authors. While fresh air ventilation is an important consideration, it is not clear that the average leakage of the average US home is not sufficient. What is clear is that numerous studies showing the negative effects of outdoor contaminants such as ozone, allergens, PM2.5, diesel engine pollution, oxides of nitrogen and other pollutants on those with asthma. That is why NIH Guidelines for those with asthma emphasize the importance of staying indoors in an air conditioned space in periods of high outdoor allergen levels, high ozone levels or high particle pollution levels.
The authors also state that the decreased outdoor air ventilation could result in more exposure to the byproducts of combustion from cooking stoves and other sources and that it may increase exposure to phthalates from flooring. These are interesting speculations but hardly supported by the data since neither factor was recorded in the housing surveys.