Is there a link between central air conditioning and chronic disease? A new study hints at one

Think air conditioning, and you're likely to connect this modern amenity with the comfort it provides on scorching summer days. "The Relationship of Housing and Population Health: A 30-Year Retrospective Analysis," in the April 2009 issue of Environmental Health Perspectives, makes a different connection, suggesting a link between central air conditioning and chronic disease. The study connects the dots between the National Health and Nutrition Examination Survey (NHANES) and the Census Bureau's American Housing Survey (AHS).

(Note: Acting Surgeon General Steven K. Galson, M.D., M.P.H., on June 9 released "The Surgeon General's Call to Action to Promote Healthy Homes." Download a PDF of the call to action: Surgeon General Call to Action Healthy Homes 6-9-09.)

One key housing trend identified in the AHS is the increase in central air conditioning; in 2002, nearly 60 percent of U.S. households had a central-air-conditioning system, up from approximately 15 percent in 1972. In that same 30-year time period, the NHANES reveals how asthma rates more than doubled, from 3.4 percent to 7.8 percent, and the average body mass index climbed from 24.9 to 27.7; a BMI of 25 and above is considered overweight; 30 and above, obese. (See the charts, right and below.)

The study authors acknowledge that various risk factors contribute to weight gain and respiratory problems as well as other health issues, but they make a good case that air conditioning and heating are at least partially to blame. For one thing, there's the lifestyle impact. "The increase in central air conditioning and its associated improved thermal comfort could provide an incentive for people to remain indoors and thus exercise less and/or to exert less energy through lower metabolic rates," they wrote.

As for indoor-air quality, the authors contend that central-air systems "increase air velocities and resuspension of particulate matter" and reduce fresh-air introduction, since "windows that would otherwise be open in the warm months are more likely to be kept closed in the presence of central air conditioning." They also observe that air conditioning and closed windows can help reduce pollen and dust mite-inducing humidity in some environments.

Any broad-stroke study such as this one will have its caveats and counterarguments. The study's authors point out that television and junk food also contribute to changes in health. But this analysis takes the discussion of housing and health relationships beyond lead-based paint and other familiar hazards, especially as they relate to disadvantaged groups. Do the study's findings mean you should jettison your air conditioning? Probably not. But they're a good reminder that you should:

Maintain your forced-air system. If you have central air, change the system filter when it's dirty, maybe about once a month or so, to prevent dust from accumulating on the evaporator fins and being dispersed throughout the house. If your home has window air conditioners, clean the filters once a month. And during heating season, replace furnace air filters monthly.

Ventilate adequately. Exhaust fans in the kitchen, bathroom, and laundry area are best at expelling combustion gases, odors, and excessive moisture, which can breed mold and other allergens. In a temperate climate, turning off the AC and opening the windows on cool days can also help ventilate, provided there's not a pollution or pollen advisory in effect. Install a whole-house fan to expel hot indoor air and bring in fresh, cooler outdoor air when the weather is mild.

Exercise regularly. Good cardiovascular health not only keeps weight down but can also help prevent several chronic diseases, including hypertension and type 2 diabetes. During summer months, limit outdoor activity to before 10 a.m. and after 4 p.m. Review these additional tips for staying safe in the sun and read our latest report on sunscreens (available to subscribers).—Daniel DiClerico | e-mail | Twitter | Forums | Facebook

Essential information: Read our latest report on air conditioning and cooling your home (available to subscribers), which includes ratings of 29 window air conditioners and brand reliability of 11 central-air-conditioner manufacturers. And learn more about air purifiers and indoor air.