Concerning radon, some comments on paras 279 - 303: (279) Of course it makes sense to have an action level in the context of radon to deal with the small number of homes with very high radon concentrations and with other situations where the exposure is substantial. However, we now know that, in countries where an action level has been set, the vast majority of radon-induced lung cancers (almost all of which are fatal) occur as a result of exposure BELOW the action level. In the light of this, the last two sentences of this paragraph seem unduly negative and need revision, possibly along the following lines: “Below the relevant constraint, action may be cost-effective in terms of avoiding radon-induced lung cancer and, wherever this is so, such action should be undertaken. There is no predetermined level below which no action is warranted to reduce the dose further.” (280) Intuition can be misleading. Radon in the home is a good example of this, in that in many countries the installation of basic radon reduction measures in new homes is often highly cost-effective even in situations where the radon concentration would be relatively low and the risk to an individual living in the home would be small – the point being that there are a very large number of such homes and the installation of radon reduction measures is often very cheap. Perhaps this paragraph should be deleted. (294) No justification is given for the last but one sentence “Due to the wide variation …exclusion levels should be set 40 Bq/m3.” In an area with low outdoor radon concentration and mean indoor radon concentration level of 40 Bq/m3 it may well be cost-effective to install basic radon protection measures in new homes. Perhaps the sentence should be deleted. (296) This paragraph is a nice summary of recent work (except that the word “measurement” should be changed to “measurement error”). It might, however, be worth adding in another sentence after it saying something like: “In the European pooling study, the finding of a statistically significant dose-response relationship did not depend on the inclusion of individuals with very high radon concentrations in the analysis. When the data were analysed considering only individuals with observed radon concentrations below 200 Bq/m3, the dose response relationship remained significant, with slope similar to that based on the full data. The same was true when the data were analysed considering only individuals with observed radon concentrations below 400 Bq/m3 and below 800 Bq/m3” (298)-(303) These paragraphs need supplementing by some further paragraphs pointing out that in many countries, the installation of basic radiation protection measured in new buildings at the time they are constructed is likely to be a highly cost-effective way of avoiding radon-induced lung cancer deaths. This will be the case, even in areas where few, if any buildings exceed the existing action level.