It has been suggested that perhaps 60% of birth defects of unknown cause could be due to occupational and environmental teratogens. Maternal exposure in pregnancy has been well studied but paternal exposure less so. The relationship between paternal occupation and risk of birth defect has been reviewed (S-E Chia and L-M Shi. Occupational and Environmental Medicine2001;59:149–55).

Ten studies reported between 1989 and 1999 were reviewed. Men found to be at increased risk of having a child with a birth defect included janitors, painters, printers, those exposed to solvents, fire fighters, and agricultural workers. Suggested teratogens include pesticides, solvents, and wood preservative but the writers of this review point to methodological weaknesses in most of the studies and question the strength of the evidence. Paternal occupational exposure could act either directly by an effect on spermatozoa or indirectly by contamination of the mother. Maternal contamination could be a result of contamination of the home or of teratogens secreted into the father's semen.

Certain paternal occupations seem to be associated with an increased risk of birth defect. Future studies need to be concentrated on men in these occupations.