Occupational exposure to mineral oils, in particular hydraulic or motor oil, increases the risk of developing rheumatoid arthritis by 30%. These are the results of a study published today in Arthritis Research & Therapy.

Berit Sverdrup and Lars Alfredsson, from the Karolinska Institutet in Stockhom, Sweden, and colleagues selected a group of patients diagnosed with rheumatoid arthritis (RA) between May 1996 and December 2003. They matched these patients with people of the same age, gender and residential area, who acted as controls. Shortly after they had been diagnosed with RA, the patients were asked to complete a questionnaire regarding occupational exposure to different types of mineral oils, such as cutting oil, motor oil, form oil, hydraulic oil and asphalt. The same questionnaire was sent to controls.

In total, the study included 1419 cases and 1674 controls. Only men reported high occupational exposure to oil, mostly motor and hydraulic oil. A group of 135 men diagnosed with RA and reporting high exposure, as well as 132 matching controls, was retained for further study.

Sverdrup et al.'s results show that men highly exposed to motor or hydraulic oil have a 30% higher risk of developing RA than unexposed men. Exposure only increased the risk of developing 'rheumatoid factor positive' (RF+) rheumatoid arthritis, a more severe form of RA. It didn't increase the risk of developing rheumatoid factor negative (RF-) rheumatoid arthritis. Exposure to oil is also linked to a 60% increased risk of developing 'anti-citrulline positive' (anti-CP+) rheumatoid arthritis, another type of the disease.

This study confirms results found in animals - exposure to mineral oil had been shown to induce arthritis in rats - and raises questions regarding exposure to other environmental or occupational agents, such as infectious agents that contain molecules that may activate the immune system in similar ways as mineral oils, and a possible link with arthritis.