The reason for this association is unclear. In the
hospital-based series, referral bias may have contributed,
as farmers who find themselves functionally limited by
symptoms of RA may seek earlier diagnosis and treatment
than members of some other occupations. However, the
finding of an association in studies of different design
makes bias a less likely explanation. No plausible mechanisms
have been proposed to explain the possible link
with organic solvents, pesticides and crops, and no data
have been presented so far on which among a heterogeneous
group of chemicals might be responsible, but this
remains an area of active research inquiry.