But it's harder at first blush to understand links to other conditions. In two studies reported last week, bad air was associated with higher rates of appendicitis and ear infections.
The new reports have been met with surprise because neither health problem seems obviously linked with the airway or bloodstream. At the same time, they represent a trend toward broadening the research scope of air pollution and health.
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"People are looking at everything and air pollution these days," says Francine Laden, an epidemiologist at Harvard School of Public Health in Boston.
Research on air pollution has been conducted worldwide for decades and is part of the basis for government regulation of air quality. Study after study has found more hospitalizations and higher death rates when certain pollutants are high. In addition to respiratory effects, research has established that air pollution increases the risk of cardiovascular events such as arrhythmia, heart attack and stroke, and the incidence of certain cancers.
In the appendicitis study, published Oct. 5 in the Canadian Medical Assn. Journal, researchers examined records for 5,191 adults admitted to Calgary hospitals for appendicitis from 1999 to 2006. The dates of the patients' admissions were compared to air pollution levels in the preceding week, using data from three air quality surveillance sites in the city.
The scientists found a significant effect of pollutants on appendicitis rates in the summer months among men, but not women.
The risk of going to the hospital with appendicitis more than doubled when summer pollution was at its highest, says study lead author Dr. Gilaad Kaplan, a physician-researcher at the University of Calgary.
The strongest effects were found when high pollution days preceded hospital admission by at least five days rather than a shorter period. This suggests there is a certain lag time between pollutant exposure and the development of appendicitis.
The study did not examine how pollution might cause appendicitis, but Kaplan speculates that inflammatory processes are involved. Substances the body produces to ramp up inflammation are implicated in appendicitis. Other research has found these substances in healthy volunteers after they breathed diesel exhaust.
A similar argument is used to explain cardiovascular risk factors associated with air pollution: that substances involved in blood clotting are produced after exposure to bad air.
In the ear infection study, presented at the annual meeting of the American Academy of Otolaryngology-Head and Neck Surgery in San Diego, researchers compared prevalence of the disease in 126,060 children with trends in air pollution from 1997 to 2006. Health information came from the National Health Interview Survey, administered by the U.S. Census Bureau, and air quality data came from U.S. Environmental Protection Agency records.
Four pollutants -- carbon monoxide, nitrous dioxide, sulfur dioxide and particulate matter -- decreased nationwide over the 10-year period. The number of children reported as having more than three ear infections in a year also declined.
Again, the study cannot say air pollution causes ear infections, only that the two are associated. And it did not investigate how pollutants affect the ear canal.
But it's not a stretch to go from respiratory illness to ear infection, says lead author Dr. Nina Shapiro, a pediatric otolaryngologist at UCLA School of Medicine. Pollutants have been shown to damage cilia -- tiny little hairs that line many of the body's passageways.
If that occurs in the ear, Shapiro says, then the cleansing process is damaged or slowed, which could set the stage for infection.
Study coauthor Dr. Neil Bhattacharyya found a similar association between air pollution and sinus infection in adults in an earlier investigation published in Laryngoscope in March.
An inherent weakness in both the ear infection and appendicitis studies -- and in many air pollution studies, for that matter -- is that air quality data for a geographical area are used as an estimate of what an individual actually inhales, says Derek Shendell, a public health researcher at the University of Medicine and Dentistry, New Jersey, in Piscataway.
Air quality measured at a site may not represent what someone living in that neighborhood is actually breathing. It will depend on levels they encounter in their house or workplace.
And even within a given neighborhood, pollution will be greater near busier roads.
Researchers must also be on the lookout for other unrelated factors that may affect the health condition being measured.
For example, Shapiro notes, there was a decline in cigarette smoking during the time period covered by her ear-infection study.
If the children also had less exposure to secondhand smoke -- a known risk factor for ear infections -- that could account for some of the decline in disease.
Pneumococcal vaccine, introduced in 2000 -- the middle of Shapiro's study period -- has also been credited with declining rates of ear infections.
In fact, both the new studies are just first steps. They are sure to stimulate more research on how air pollution might trigger these conditions as well as other nonrespiratory diseases.