A recent article claims that the increased use of central air conditioning systems in homes over the past 30 years has lead 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 coincidental? 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 is written by David E. Jacobs of the National Center for Healthy Housing, J. Wilson, S. Dixon, J. Smith and A. Evans. It appeared in Enviornmental Health Perspectives 117:597-604 (2009). It was based on an analysis of the data from two large nationally representative surveys of U.S. housing – the National Health and Nutrition Examination Survey (NHANES) and the American Housing Survey (AHS). The data was used to identify housing trends from 1970 to 2000. These housing trends were then compared with health trends of individuals living in the houses for the same period. The two sets of data were 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 and asthma/allergy 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 – 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 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 or a mold infestation – the outdoor particle counts are higher than the indoor particle counts. To illustrate the point I have taken particle counts today in our offices and outside. Our offices are connected to a factory and we have continuous high traffic. We use MERV 7 filters in a central air conditioning system. The particle counts inside at 1 micrometer are 268,900/cubic foot and at 5 micrometers they are 1,600/cubic foot. The particle counts outside are 427,000/cubic foot at 1 micrometer and 5,400/cubic foot at 5 micrometers. This is typical. Everytime we open the door, 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 at all clear that the average leakage of the average home in the U.S. is not sufficient. What is clear is that there are numerous studies showing the negative effects of outdoor contaminants such as ozone, allergens, PM2.5, diesel engine pollution, oxides of nitrogen, etc. 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 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.
While the discussion of the rise in central ventilation trending with the increase in asthma is somewhat troubling, the explanation of the higher incidence of “broken windows and bars on windows” corresponding to higher rates of asthma borders on the humorous. It is well known that asthma rates in inner cities are higher than in the general population. Although, we are not sure why this is the case it is likely a combination of genetics, higher particulate pollution, higher ozone levels, higher levels of cockroaches and rodents, higher levels of dust mites in older beds, and higher levels of household smoking. Rather than using the “broken windows and bars on windows” data to confirm what is known about inner-city asthma rates, the authors state: “Broken windows and bars on windows are both measures of stress, which is thought to be related to asthma.” Such a statement shows a lack of understanding of the underlying causes and conditions of asthma. “Stress” is a trigger for an asthma episode. In and of itself, it does not “cause” the asthma episode. Inflammation is a necessary component of the episode. The inflammation is driven by the many other factors mentioned above. It is almost like saying that more firemen cause larger fires because the trend of the number of firemen at a fire trends with the size of the fire.
It is true that the authors point out the trends do not indicate 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 dangers 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 that there is a connection. The increase in asthma in the 30 year period from 1970 to 2000 would also correlate 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.