Dose conversion factors linking concentrations in water to resulting radiation dose, recommended by the International Commission on Radiological Protection (ICRP 1996) were used in deriving the MAV concentrations. This approach is consistent with that of other organisations such as the World Health Organisation (2004). The MAVs are deliberately conservative. If the natural radionuclides radium-226 and radium-228 were present in drinking-water at the MAV level (worst case scenario), the annual radiation dose would still be less than 5 percent of the total annual natural dose.
It's set up in reference to international standards - the ICRP reference is worth looking at. So what are the limits?
The MAVs for radiological determinands are:
- alpha: 0.10 becquerel per litre, excluding radon-222
- beta: 0.50 becquerel per litre, excluding potassium-40
- radon-222: 100 becquerel per litre.
Well, 1Bq is one nuclear disintegration a second. The equivalent USEPA standard for beta concentration is about 0.2Bq (using 5pCu) so their standard is actually tougher than ours. But ... is it big - the authors go on to point out:
In the radiological context, the MAV is intended to indicate a level above which the radioactive content of the water should be investigated further and an assessment of all relevant radiological issues undertaken. Radiation protection issues are often complex and many factors would have to be taken into account before a water supply could be classified as unacceptable even though a radiological MAV might have been exceeded.
The DWSNZ therefore emphasise that further assessment by the National Radiation Laboratory of the Ministry of Health is required in such cases. The MAV is thus more of a guideline than necessarily an absolute maximum. It is also intended to be clear however, that at levels below the MAV, there is no need for further assessment.
... in other words - we have adopted a safety limit of 20% of what we'd get anyway just walking around and the US prefers 10% as a safety margin.
Exceeding the margin just means that further investigation is needed.
But how about danger levels?
Here's an old paper - (also see the links to the dial-painter studies in that paper)... the dial-painter studies suggest tumors do not appear below 3.7 million Bq - we could be skeptical about this by 6 whole orders of magnitude and still be pretty safe.
What has been interesting to me about this trawl through data sources is the difference between what the US government provides it's population and what the NZ government does. The US official sources are all calm and reassuring language with a lack of much that can be checked - it takes quite a bit of hunting to reach the level where you can find out anything verifiable and I usually had to go to non-government sources. On the NZ trawl there seemed to be more interest in actually informing the public to the point that I could reach verifiable data much more quickly.
It's only a subjective impression, but it would seem that the US public's general feeling that the government is not telling them something has some foundation. OK It is certainly true - no government (or anybody) tells you everything. However: if you want to look trustworthy you should include checkable facts in your information bits ... how would someone reading the document check what you are saying?
ICRP (1996). Age-dependent Doses to Members of the Public from Intake of Radionuclides: Part 5 Compilation of Ingestion and Inhalation Dose Coefficients. Publication 72, International Commission on Radiological Protection. Pergamon Press.
WHO (2004). Guidelines for Drinking-water Quality 2004 (3rd Ed.). Geneva: World Health Organisation.