1. a National Institute of Occupational Health, P.O. Box 8149 Dep, N-0033 Oslo 1, Norway
2. b A.F.R.C. Institute of Arable Crops Research, Rothamsted Experimental Station, Harpenden, Herts, United Kingdom
3. c National Institute of Occupational Health, Solna, Sweden
4. d Microbiology Department, Swedish University of Agricultural Science, Uppsala, Sweden
5. e National Institute of Occupational Health, Umeå, Sweden