By Trevor Stokes
NEW YORK (Reuters Health) - Although previous research has suggested a connection between exposure to air pollution and the risk of developing rheumatoid arthritis, a large new study of nurses finds no link.
"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.
Nearly 1 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.
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 often than men.
For the new study, Hart's group used data from the Nurses' Health Study, a registry that tracked 111,425 women, originally registered nurses, across the U.S. 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 published 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, the current study examined specific outdoor air pollutants including the particulates in soot and components of smog such as nitrogen dioxide and sulfur dioxide.
Several groups, including Hart's, have also identified smoking as a significant risk factor for rheumatoid arthritis in the past. So in the current study, Hart's team also looked for any influence from cigarette smoking or lower socio-economic status on the nurses' risk.
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 other known risk factors like smoking.
The new study was based on mostly white, middle-aged and middle class women, however, so Hart said it doesn't necessarily refute any evidence linking pollution and rheumatoid arthritis.
"Without additional confirmation from other populations, I think it is still too early to draw strong conclusions about the role of air pollution," Hart told Reuters Health.
Similarly, with regard to smoking, she added, "It is far too reductionist to say that this paper contradicts the wide body of literature from our group and around the world on the adverse effects of smoking on (risk for rheumatoid arthritis)."
Dr. Christopher Morris, a rheumatologist with Arthritis Associates of Kingsport in Tennessee, who was not involved in the current study, 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."
Current rheumatoid arthritis treatments include drugs that target inflammation symptoms by inhibiting a key inflammation protein known as NF-kappaB or by suppressing the immune system generally - all of which can cause serious side effects.
"It is perplexing because now that they're looking at the specific air pollutants, they're not seeing an association," said Anneclaire De Roos, associate professor at Philadelphia's Drexel University School of Public Health, part of a separate group working on a study that explores the connection between outdoor air pollution and rheumatoid arthritis in British Columbia, Canada.
She noted, though, that the current study didn't examine components of outdoor pollution such as ground-level ozone or indoor air pollution, which could be potential triggers as well.
For now, De Roos told Reuters Health, "Based on this study, there's no reason to be concerned about outdoor air pollution (in connection with rheumatoid arthritis)."
SOURCE: http://bit.ly/XFojli Arthritis Care & Research, online February 11, 2013.