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Studies have found that smoking, as well as being a strong risk factor for cardiovascular disease, lung cancer, and other respiratory problems, can also increase an individuals risk of developing rheumatoid arthritis.

The largest study on the impact of smoking on rheumatoid arthritis was conducted in 2001 by British researchers. In that study, 239 people who had been diagnosed with rheumatoid arthritis were compared to 239 healthy control subjects who were matched by age, sex, and job status.

In the rheumatoid arthritis group, 100 (42%) were current smokers compared to just 52 (22%) current smokers in the control group. 55 (23%) of the rheumatoid arthritis group were ex-smokers while 84 (35%) had never smoked at all. The average age of both groups was 60.5 years.

The researchers found that those people who had never smoked were almost half as likely to develop rheumatoid arthritis compared to those who had ever smoked. The researchers also found a strong correlation between heavy smoking and rheumatoid arthritis. Those who had smoked between 31 and 40 pack years had 5.72 times greater odds of developing rheumatoid arthritis. Those who smoked between 41 and 50 pack years had 13.54 times greater odds of developing rheumatoid arthritis.

A pack year was defined as a year of smoking 20 cigarettes per day. An individual who smoked 40 cigarettes a day for 30 years would therefore be considered to have smoked for 60 pack years.

It is believed that smoking may increase the levels in the blood of an antibody called rheumatoid factor (RF). RF antibodies are found in the majority of rheumatoid arthritis sufferers and the antibody is a strong indicator that a person will develop rheumatoid arthritis during their life. Just 5% of people without rheumatoid arthritis have RF while 70% of people with rheumatoid arthritis have the antibody.

While genetics plays a large role in whether a person will have the RF antibodies, some lifestyle factors are also thought to contribute to the development of RF. It is believed that smoking is one of these factors because RF is more commonly found, and found at higher levels in smokers compared to non smokers. An American study, published in the journal Annals of the Rheumatic Diseases in 1997 found that those who had smoked for more than 25 pack years were 3.1 times more likely to test positive for RF antibodies than non-smokers.