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 study is entitled “The Relationship of Housing and Population Health: A 30-year Retrospective Analysis.” It was written by David E. Jacobs of the National Center for Healthy Housing, J. Wilson, S. Dixon, J. Smith and A. Evans. It appeared in Environmental Health Perspectives 117:597-604 (2009). It was based on an analysis of the data from two large nationally representative surveys of US housing – the National Health and Nutrition Examination Survey (NHANES) and the American Housing Survey (AHS). The data was used to identify trends from 1970 to 2000. These housing trends were then compared with health trends of the individuals living in the houses for the same period. The two sets of data were then analyzed to determine possible relationships.
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.
It is interesting to look at the discussion in the paper explaining why this relationship might be valid. Instead of a thoroughly researched analysis of the literature, it appears that the “reasons” are largely conjecture and speculation. For example, the paper states that: “Even though such furnaces are typically equipped with filtration systems, they can be expected to result in higher airborne particulate matter due to higher air velocities that cause resuspension of dust particles that otherwise would settle out of the air.” This statement is simply not true. Houses with central ventilation systems generally have fewer airborne particles at all size ranges than houses without central ventilation. In addition, even a moderately effective pleated filter can dramatically decrease indoor particle counts.
We have done many particle counts of outside air versus indoor air. In every case – except when there is an extraordinary reason like a smoker indoors, active indoor air chemistry or a mold infestation – the outdoor particle counts are higher than the indoor particle counts. To illustrate this point we have taken particle counts today (July – Dallas/Fort Worth) in our offices and outside. Our offices are connected to a factory and we have continuous high traffic indoors. We use MERV 7 filters in a central air conditioning system. The particle counts inside at 1 micron and above are 269,900/ cubic foot and at 5 microns and above they are 1,600 / cubic foot. The particle counts outside are 427,000 / cubic foot at 1 micron and above and 5,400 / cubic foot at 5 microns and above. This is typical. Everytime we open the door (and improve ventilation) the particle counts go up – not down.
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.
It is true that the authors point out that the trends do not prove causality, but this widely reported study could lead to some unfortunate outcomes. For example, parents of asthmatic children could read the published accounts of the study and conclude that the central HVAC system was bad for their children. Instead of practicing allergen and pollution avoidance, they could decide that open windows and “fresh” air would be the best approach. Such a decision would most likely be contrary to their physician’s instructions and accepted medical practice.
What this analysis and discussion illustrates is the danger of using large databases with limited information to draw conclusions. All too often the analyses suffer from the reasoning fallacy “cum hoc, ergo propter hoc” (with this, therefore because of this). Just because the data trends in the same way, it does not mean there is a connnection. The increase in asthma in the 30 year period from 1970 to 2000 would also trend with the increase in SUV’s or the increase in the value of the Euro (Mark). In our view it is the analysts responsibility to thoroughly review the literature before publishing conclusions that may very well be coincidence. What is particularly troubling is a discussion that is filled with conjecture to try to explain the coincidences. Such conjecture could be reported or interpreted as “fact” to the detriment of vulnerable populations.