This article is from the Static Electromagnetic Fields and Cancer FAQ, by John Moulder email@example.com and the Medical College of Wisconsin with numerous contributions by others.
There have been relatively few studies of cancer incidence in workers
exposed to static magnetic fields. Budinger et al  found no excess
cancer in workers exposed to 300 mT fields from particle accelerators,
and Barregard et al  found no excess cancer in workers exposed to 10
mT fields in a chlorine production plant.
There are also studies of aluminum reduction plant workers [8,9,10,61].
In general the studies of aluminum reduction plant workers were not
designed to analyzed the effects of static fields, but these workers are
exposed to static fields of 5-15 mT [2,3,4]. In the aluminum reduction
plant studies, the only excess cancer reported was lymphoreticular
tumors, and this was seen in one study . The only study to look
specifically at static field exposure and cancer reported no excess of
nervous system or hematopoietic cancers .