This publication (Documents of the NRPB volume 9 number 1) gives generalised derived limits (GDLs) for seven radioisotopes.
Generalised derived limits (GDLs) were calculated in various environmental materials for the radiologically significant isotopes of strontium, ruthenium, iodine, caesium, plutonium, americium and curium, derived from the annual effective dose limit of 1 microsievert.
The most recent age-dependent dosimetric models were used and the methods and data used in calculating the GDLs were reviewed. The values presented here were intended as convenient reference levels against which the results of environmental monitoring could be compared.
GDLs are calculated using deliberately cautious assumptions and are based on the assumption that the level of environmental contamination is uniform over a year. If a measured environmental concentration exceeds about 10% of the GDL then the doses should be examined more closely, taking account of site-specific factors and the length of time the measured level is likely to be maintained.