1. Environmental Health Science and Research Bureau, Health Canada, Ottawa, ON K1A0K9, Canada
2. National Research Centre for the Working Environment, DK-2100 Copenhagen, Denmark
3. Department of Public Health, University of Copenhagen, 1353 Copenhagen, Denmark
4. National Institute of Occupational Health, 0304 Oslo, Norway
5. Department of Biology, University of Ottawa, Ottawa, ON K1N 6N5, Canada