A historical cohort study on multiple sclerosis among offshore workers in the petroleum industry in Norway

Norway has one of the highest frequencies of multiple sclerosis (MS) in the world with a life-time risk of more than 1 per 1000. The cause of this disease is still unknown. It is commonly assumed that MS has a polygenetic and multifactorial etiology with possible interactions between the genetic and exogenous factors. Exposure to chemical hazards including mineral oil and organic solvents has been suggested as one possible environmental factor. Offshore workers on the Norwegian continental shelf have be en exposed to a number of chemical agents including a wide range of oils. During the last years there has been a concern both in the media, among authorities and in the petroleum industry about offshore workers having been exposed to hydraulic and turbine oils and later diagnosed with multiple sclerosis. We want to examine the frequency of MS in a historical cohort of 27 916 offshore workers and 424 584 referents from the general working population in Norway. This cohort is based on the Norwegian Registr y for employers and employees and includes all workers employed from 1981 to 2003. The cohort has previously been used to show an increased risk of leukemia in a subgroup of the offshore workers. The frequency of MS in each of the sub-cohorts will be esti mated by linking the whole cohort to the nationwide MS-registry located at Haukeland University Hospital, Bergen. The linkage will be performed at the Statistics Norway and the final file will be returned to the researchers with no personal identification of the study subjects. The main goal of the study will be to investigate whether there is an increased frequency of MS among Norwegian offshore workers compared to the general working population in Norway and to investigate differences in risk according to subgroups with varying levels of exposure.