“It’s well known, and it probably increases risk by between 60 and 80 percent. It’s a significant risk factor. There have been changes in smoking rates, particularly in women and that’s the group in which we really see this increase. Women are smoking more than they used to,” Dr. Mikuls says. But, he adds, “is that the explanation for this? It’s really speculation.”

Another possible environmental factor, researchers say, is vitamin D deficiency. Several studies have shown it could also be a cause of RA, and more people have developed this condition in recent years, especially women.

Newer oral contraceptives are another possibility. They contain less estrogen than older pills. Researchers think estrogen has a protective effect against the development of RA, so reducing it may lead to a greater risk of developing the condition and may account for differences in rates between men and women.

“There’s some data that oral contraceptives influence the risk of getting RA,” Dr. Mikuls says, but he adds, “that’s not been as consistent a relationship as we’ve seen with smoking. “

Researchers admit there may also be other factors at play like infections, immunizations, obesity or socioeconomic status or other causes they haven’t identified yet.

While the medical community tries to pin down exact reasons for this increase, Dr. Mikuls says he’d like to see what the data looks like over even more time.

“It would be nice to see where this is going to go,” Dr. Mikuls says. “But it’s interesting and it’s a cautionary tale that it may be on the rise.”

And Dr. Gabriel says despite the unknowns, there is something to be learned from this study for people at risk for RA because their relatives have it.

“It is likely that environmental factors affect your RA risk,” she says. “Smoking certainly does – another reason not to smoke. Vitamin D deficiency may, so be checked for this. Hormonal factors may also play a role. Talk to your doctor about what you can do to decrease your risk.”