Earth Day slowly has expanded to Earth Week, and this year it brings to Georgia the administrator of the epitome of government mission creep, Lisa Jackson of the U.S. Environmental Protection Agency. But don’t expect a celebration of this nation’s environmental advances. It’s more likely to be an indictment of industry, power plants and Georgians’ lifestyle choices (driving) for “asthma-causing pollution.”
This criticism happens every year as the May-September “Ozone Season” arrives. Newspapers tie high ozone days to asthma, as they have for nearly 30 years. Headlines such as “Kids with asthma head indoors during smog season” reinforce the association. Unfortunately, it’s largely a myth, built mainly on statistical associations between high ozone on hot summer days and hospitalizations for asthma. Yet summer is the season of fewest asthma hospitalizations.
Hundreds of experiments show associations between ozone and asthma. Most show weak associations, probably confounded with unknown factors that affect asthma. Some studies of air pollutants showed no associations with asthma.
A throwaway phrase in a recent publication by the U.S. Centers for Disease Control and Prevention says a lot about the malady: “[T]he etiology of asthma is unknown....” Etiology is “the set of factors that contributes to the occurrence of a disease.”
Singling ozone out as the cause is nearly impossible, even using complicated statistical models that “control for other factors.” Often, other controlling factors are unknown, especially in the large and diverse populations that are studied.
By far the best association is the change from 1980 to 2009 in U.S. asthma prevalence and the U.S. average peak eight-hour ozone concentration. The statistical correlation is -0.86 (0 equals no association; 1 is perfect.) But this correlation is negative: Even as ozone declined, asthma increased.
The simple interpretation is that cleaner air causes asthma. What it really shows, though, is the fallacy of cavalier interpretations of associations. Interpretations based on associations lead to conflicting facts. Jackson’s EPA estimated that one of the benefits of reduced air pollution under the 1970 Clean Air Act was 850,000 “asthma attacks” avoided annually by 1990.
Yet the CDC reports the number of doctor’s office visits for asthma increased by about 3.9 million per year from 1975 to 1990-92. Did reducing pollution decrease or increase asthma? (Unfazed, the EPA last month proposed new power plant mercury and air toxics standards aimed at “preventing 120,000 cases of childhood asthma symptoms.”)
The weakness of the ozone-asthma connection is shown by other associations. After all, asthma experts consider dozens of “triggers” and “modifiers” for asthma, ranging from dust mites and cockroaches to household income and even violence. The complications make definitive statements about asthma and its causes dicey. And physicians don’t usually tell patients to stay inside. They know that most triggers are indoors, where EPA has concluded air pollution is often much higher than outdoors.
Urban areas are expected to have higher asthma rates because ozone, other air pollutants and traffic are higher there. But a CDC study of 2005 U.S. data concluded that “asthma prevalence is as high in rural as in urban areas.”
This is further complicated by low asthma prevalence in some countries with high pollution. In the early 1990s, East Germany was found to have much lower asthma prevalence than West Germany, although its air pollution was at least twice as high. A global report on asthma in 2004 cited prevalence for asthma symptoms for U.S. adults as 10.9 percent, but 2.1 percent for China and 2.2 percent for Russia, countries not known for their clean air.
A 2001-2004 CDC survey showed 14.6 percent of U.S.-born women reported that they had ever had asthma vs. 4 percent of women born in Mexico. For immigrants born elsewhere, it was 6.8 percent. The results were similar for men. The country of birth, and therefore the conditions of birth, was influential.
Associations with asthma are easy to find. Among Georgia counties, for example, it’s hospitalization rates and the percentage of land in cultivated crops. The correlation coefficient is statistically significant (+ 0.59) and as large as most of those for the asthma-ozone association. Yet nobody would conclude that crop cultivation causes asthma.
A further paradox in this paradox-laden condition: Several recent studies, mostly in Europe, show that children born on a farm, and especially those in contact with livestock, are less likely to have asthma, even as adults. The reason is speculated to be their contact with bacteria associated with farm conditions, especially animals.
It may be an example of the “hygiene hypothesis,” which holds that too “clean” conditions prevent childhood development of resistance to disease organisms or allergies and later cause serious problems. This may explain growing asthma rates as Americans moved from the farm and into cleaner houses. Clean houses are supposed to make children healthier, but perhaps that exposure on the farm strengthened earlier generations. It certainly holds more logic than blaming declining ozone and other air pollutants.
University of Georgia Professor Emeritus R. Harold Brown is an adjunct scholar with the Georgia Public Policy Foundation and author of “The Greening of Georgia: The Improvement of the Environment in the Twentieth Century.”
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