When people with rheumatoid arthritis have been studied there's a connection between them having smoked for a long time and very high levels of antibodies.

Dr David Hutchinson, Rheumatologist

The team looked at 239 patients with rheumatoid arthritis, and 239 healthy people.

Those with rheumatoid arthritis were more likely to be smokers than the healthy group.

And over half had no family history of the condition, which the researchers said was a known risk factor.

Debilitating

Rheumatoid arthritis is a debilitating condition, which affects three in 100 people in Britain.

Joints become inflamed which means the body "turns on itself" and damages cartilage and tissues.

Osteoarthritis is different because in that condition, joints are worn away.

The reason a long-term heavy smoking habit makes developing the condition more likely is unclear, say the researchers.

But they said the link with heavy smoking could explain increased death rates amongst sufferers of the rheumatoid arthritis.

It is known that smoking produces an antibody called rheumatoid factor, a blood protein caused by a reaction in the immune system.

There is uncertainty about what the antibody does. Some researchers think it makes certain cells release their contents within the joint, or that it creates inflammation in the joint in some other way.

But rheumatology experts agree it is key to the condition.

Rheumatoid arthritis can affect people in many different ways, from aching joints to severely deformed joints.

'Improve understanding'

The researcher who led the work in Liverpool, Dr David Hutchinson, said making the link between heavy smoking and rheumatoid arthritis could help the medical profession understand how the condition worked..

Those identified in the study as having a family history of the disease were less likely to be heavy smokers, leading the researchers to believe there are two factors in the development of rheumatoid arthritis, the genetic and the environmental - such as smoking.

Dr Hutchinson told BBC News Online: "Certainly, when people with rheumatoid arthritis have been studied there's a connection between them having smoked for a long time and very high levels of antibodies.

"We're considering people who have never smoked and who smoked at the time of diagnosis as two different groups to see if there are and genetic differences between the two, and differences in their responses to treatment."

There are subtle differences in the way the condition manifests itself dependent on whether people are heavy smokers or have a family history.

'Further knowledge'

Deborah Symmons, professor of rheumatology and musculoskeletal rheumatology at the Arthritis Research Campaign's epidemiology unit in Manchester, said the Liverpool study built on the findings of an ARC research team four years ago - and was even more conclusive.

She said: "We found in a study of 180 people in Norfolk that people who smoked were two to three times more likely to develop rheumatoid arthritis than those who didn't - the Liverpool study puts it at 13 times higher."

"That difference could be explained by the fact we took people at the start of their disease whereas the Liverpool study took people who were already attending hospital so probably had more advanced disease.

"The study takes our knowledge just that bit further, and builds up an even stronger case against smoking and its association with rheumatoid arthritis."

Amanda Sandford, research officer at Action on Smoking and Health said: "This is yet another disease to add to a long long list of smoking related diseases. This is an unpleasant and debilitating condition."

This paper is published in The Annals of the Rheumatic Diseases.