- Dr B Lundbäck, Respiratory Epidemiology Unit, Department of Occupational Health, National Institute for Working Life, P O Box 7654, SE 907 13, Umeå, Sweden.
- Received October 19, 1998
- Revision requested December 1, 1998
- Revised March 1, 1999
- Accepted March 24, 1999
- First published July 1, 1999.
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