Mercury vapour (Hg0): Continuing toxicological uncertainties, and establishing a Canadian reference exposure level

There are four published reference exposure levels (RELs) for Hg0, ranging from 0.09 μg/m3 to 1 μg/m3. All RELs were derived from the same toxicological database, predominantly of male chloralkali workers. Some key factors are apparent which make the use of that database questionable for REL derivation. Occupational studies of chloralkali workers are not an appropriate basis for a REL for Hg0. Concomitant exposure to chlorine gas (Cl2) diminishes uptake and effects of Hg0 exposure. There are gender differences in Hg0 uptake, distribution and excretion, with females at potentially greater risk from Hg0 exposure than males. Studies of chloralkali workers focused almost exclusively on adult males. Recent investigations of dental professionals (dentists, technicians, assistants) have failed to define a threshold in the dose–response relationship linking Hg0 with neurobehavioural outcomes, an observation generally ignored in Hg0 REL development. Finally, there is a growing database on genetic predisposition to health effects associated with Hg0 exposure. Based on these considerations, we propose a different key study for REL derivation, one that involved male and female dental professionals without concomitant Cl2 exposure. Adjusting the LOEAL to continuous exposure and applying appropriate UF values, we propose a Canadian REL for Hg0 of 0.06 μg/m3.