Some research has suggested a connection between exposure to air pollution and the risk of developing rheumatoid arthritis, but a large study of U.S. nurses has found no link.
Nearly one percent of U.S. adults have rheumatoid arthritis, an incurable joint-eroding disease that deforms patients' bodies, triples their risk of heart attack and raises their chances for certain cancers.
"Overall, we did not observe any evidence that increases in pollution levels were associated with increases in the risk of rheumatoid arthritis," wrote study leader Jaime Hart, an instructor of medicine at Brigham and Women's Hospital in Boston, in an email.
The triggers for the inflammation at the root of rheumatoid arthritis still elude researchers.
Some have theorized that pollution could set off an inflammatory response in the lungs that would then spread to the entire body. Genetic risk factors, interacting with the environment or hormones, are also thought to play a role because women tend to get the disease more than men.
For the study, Hart's group used data from the Nurses' Health Study, a registry that tracked 111,425 women - all originally registered nurses - across the United States every two years from 1976 to 2006.
Over the span of three decades, 858 confirmed cases of rheumatoid arthritis arose among the women, according to the report in the journal Arthritis Care & Research.
Using the nurses' mailing addresses, the researchers estimated how close study participants were to sources of air pollution such as traffic and power plants.
In past studies, Hart and her colleagues found increased cases of rheumatoid arthritis when Swedish women lived closer to roadways, suggestive that general outdoor air pollution might trigger the disease.
Unlike the earlier Swedish research, though, the current study examined specific outdoor air pollutants including the particulates in soot and components of smog such as nitrogen dioxide and sulfur dioxide.
The study also looked at the impact of smoking, which in the past has been identified as a significant risk factor for rheumatoid arthritis.
Hart told Reuters Health her team was surprised when it found no connection between rheumatoid arthritis and the components of outdoor air pollution, as well as little impact from risk factors like smoking.
But the study was based on mostly white, middle-aged and middle class women, so Hart said it doesn't necessarily refute the connection.
"Without additional confirmation from other populations, I think it is still too early to draw strong conclusions about the role of air pollution," she told Reuters Health.
Other researchers agreed.
"Until we find that one big triggering event, what starts everything off, we're going to have this situation where we see conflicting data in different populations," he said. "It doesn't change what we do to treat it."