If you have rheumatoid arthritis or RA, you probably already know that certain things can make your joints feel worse. Many conditons — diet, stress, infections, missed medications, even changes in weather — appear to set off an RA flare.

There is another trigger worth paying attention to — the air you breathe. People with rheumatoid arthritis appear to be more likely to experience increased disease activity and flares, researchers in South Korea found, if they had been exposed to higher levels of air pollution. The biggest triggering effects seemed to come from the fine particles left in the air by smoke, soot and dust. This fine particulate matter (PM) is known as PM2.5.

PM 2.5 emerged as the pollutant most strongly associated with RA activity. Higher concentrations of this fine particulate were associated with increased activity and flare risk.

Their study looked at 1,070 people being treated for RA at a tertiary medical center in South Korea. Over 86 percent were women, and their average age was about 61. The participants had been living with RA for an average of about nine years.

To get a clear picture of how polluted the air had been at the time people in the study experienced RA flares, the researchers linked their medical records to air-quality measurements for the areas where they lived. The air quality measurements showed monthly levels of six pollutants: sulfur dioxide, nitrogen dioxide, ozone, carbon monoxide, PM10 and PM 2.5.

The rheumatoid arthritis patients made over 12,500 outpatient visits during the study, giving researchers repeated snapshots of both their exposure and their arthritis activity. At each visit, the investigators assessed measures including tender and swollen joints, C-reactive protein — a marker of inflammation — and other standard measures of RA activity. They recorded medications the patients took, including steroids and antirheumatic drugs. Their statistical analysis also took into account factors that could influence RA activity, including age, sex, smoking, socioeconomic circumstances, temperature and humidity.

The researchers additionally performed a second analysis that compared pollution exposure before flare visits with exposure before non-flare visits in the same patients. This case-crossover approach helped the investigators examine whether changes in pollution were associated with flares within an individual rather than simply comparing different people.

For people with RA, the findings suggest a simple precaution: Pay attention to air quality. On days when the air is especially polluted, limit your air exposure to the air outside.

PM 2.5 emerged as the pollutant most strongly associated with RA activity. The researchers reported that higher concentrations of this fine particulate were associated with increased activity and flare risk, particularly when exposure remained elevated for more than two weeks.

“Our study found that higher PM 2.5 concentration was associated with increased disease activity and flare risk, notably prolonged exposure to elevated PM 2.5 over more than two weeks,” investigator Eun Bong Lee, MD, PhD, a professor in the Department of Molecular Medicine and Biopharmaceutical Sciences at Seoul National University, said in a press release.

Why might dirty air affect joints? PM2. 5 particles are extraordinarily small and can travel deep into the lungs and enter the bloodstream. The researchers propose that these particles may increase oxidative stress and stimulate inflammatory responses throughout the body.

The study noted an association between exposure and disease activity. Because RA is driven by abnormal inflammation, the added inflammatory burden of polluted air could potentially aggravate the condition, but that connection does not prove that pollution causes RA flares. What's more, the participants were all treated in South Korea, so the findings may not apply exactly to people living elsewhere. The researchers say more work is needed to determine whether reducing pollution exposure reduces disease activity.

For people with RA, however, the findings suggest a simple precaution: Pay attention to air quality, particularly during periods of high PM 2.5. On days when the air is especially polluted, limiting your air exposure may be sensible. That doesn't mean staying indoors every time air quality dips, but it does give people with RA one more potentially modifiable factor to discuss with their doctors.

For someone living with a disease that can flare unpredictably, knowing that the environment may play a role could offer another way to take an active part in managing it.

The study is published in Annals of Rheumatic Disease.