Tiny particles in polluted air may be doing more than harming the lungs and heart. A large South Korean study suggests they could also make rheumatoid arthritis (RA) more active, raising the risk of painful flare-ups in people already living with the disease.

The findings, published in the Annals of the Rheumatic Diseases (ARD), the official journal of the European Alliance of Associations for Rheumatology (EULAR) and published by Elsevier, add fresh evidence that clean air could become an important part of managing RA, a long-term autoimmune disease in which the body's immune system mistakenly attacks healthy joints and sometimes other organs.

Researchers from Seoul National University followed 1,070 patients through 12,583 outpatient visits between 2021 and 2024. They compared monthly exposure to six common air pollutants with disease activity while accounting for age, medicines, weather, income and other factors. They also used a second statistical method comparing changes within the same patient over time to strengthen the findings.

"Our study found that higher PM2.5 concentration was associated with increased disease activity and flare risk, notably prolonged exposure to elevated PM2.5 over more than two weeks," said lead investigator Dr Eun Bong Lee.

PM2.5 refers to extremely tiny particles produced by vehicle exhaust, smoke, burning fuel and dust. Because they are far smaller than the width of a human hair, they can travel deep into the lungs and enter the bloodstream. Scientists believe they can trigger harmful unstable molecules that damage cells and fuel inflammation throughout the body.

In an accompanying editorial, Dr Jeffrey A. Sparks of Harvard Medical School and Mass General Brigham described the work as one of the largest studies linking air pollution with RA disease activity. He said avoiding poor-quality air might help reduce flare-ups and could explain why symptoms sometimes worsen without an obvious cause.

The findings also have wider public health implications. "We would recommend these patients avoid prolonged exposure to poor air quality, particularly high PM2.5 levels," Dr Lee said, while noting that further studies are needed to determine whether cleaner air directly reduces disease activity.

Not all evidence is identical. Reviews published in journals including RMD Open and earlier epidemiological studies have reported mixed results across countries, suggesting local pollution levels, genetics and climate may influence outcomes. Because this was an observational study, it shows a strong association rather than proving pollution directly causes RA flare-ups.

ARD Editor-in-Chief Dr Josef Smolen called the findings "an important wake-up call", while cautioning that studies in other populations are needed to confirm whether the results apply globally.