Metal fume concentrations inside the welding helmet and in the personal breathing zone in 23 experimental welding exposures were studied to explore whether welding helmet use substantially attenuates exposure to airborne metal fume. Observations produced a mean ratio of inside to outside metal fume concentrations of 0.9 +/- S.D. 0.2 with a highly variable effect. Iron fume concentration was inversely correlated with this ratio, representing greater helmet-associated attenuation with heavier exposure (r = -0.70, p < 0.001). In contrast to previous reports, these data suggest that welding helmet use provides marginal and highly variable reductions in fume exposure and cannot substitute for standard respiratory protection.