In the United States the Environmental Protection Agency issues “Protective Action Guidelines” (PAGs) which satisfy the same role that Emergency Reference Levels (ERLs) do in the United Kingdom. The Protective Action Guide (PAG) manual contains radiation dose guidelines that would trigger public safety measures, such as evacuation or staying indoors, to minimize or prevent radiation exposure during an emergency.
The 2017 PAG manual is a significant document (101 Pages) which has chapters devoted to:
- Early phase protective action guides;
- Emergency worker protection;
- Intermediate phase protective action guides;
- Planning guidance for the late phase.
For the early phase it provides a PAG for shelter in place or evacuation of the public of 10 to 50 mSv projected dose over four days. This is accompanied by a note that suggests that actions should start at 10 mSv projected dose and take whichever action or actions result in the lowest exposure for the majority of the population. This is broadly consistent with IAEA advice and UK practice although the UK ERLs are based on averted dose rather than projected.
The stable iodine PAG is 50 mSv to the infant thyroid from exposure to radioactive iodine.
Emergency workers should be limited to 50 mSv over the entire response or per year unless there are extreme circumstances (such as saving life or preventing a significant release).
The report warns that: “In the early phase, there may be little or no data on actual releases to the environment and responders may have to rely on crude estimates of airborne releases. Decision time frames are short and preparation is critical to make prudent decisions when data are lacking or insufficient.”
The report suggests that the public should be advised to cover their mouth and nose with “available filtering material” when particulate activity may be present. This advice is not given in the UK.
It is also suggested that where particulate activity is present but below the PAG level the public are advised to “stay indoors to the extent practical” to reduce their dose but make a judgement if they need to go out for any reason. Such advice is apparently given on days when air pollution is forecast to be bad. Again, this is not something I’ve heard mentioned in the UK.
A discussion about the relative merits of shelter in place and evacuation concludes that “Sheltering-in-place should be preferred to evacuation whenever it provides equal or greater protection. Sheltering-in-place followed by informed evacuation may be most protective”. It reports that populations that are aware of the evacuation alerting mechanisms and plans are more likely to successfully evacuate than unprepared populations and further reports that many areas are improving their communications processes and public training. The need to have special plans for needy groups such as schools, institutions and those without their own transport is noted.
On iodine tablets the report suggests that minimum doses for each age (weight) group is ideal but that, if this is too challenging logistically and in terms of communication, then a full dose can be given to all without great risk.
Interestingly it suggests that adults over 40 years old would only benefit from stable iodine if their projected thyroid dose is of the order of 5 Sv where it can be used to prevent hypothyroidism.
Stable iodine given to pregnant women also protects the foetus but repeat doses should be avoided if possible to prevent foetal thyroid complications. (In the UK it is expected that a single dose of stable iodine will be adequate because either the release will have stopped or the affected members of the public should be evacuated within the 24 hours that the first dose provides protection for).
Breast feeding mothers can reduce the radioiodine in their milk by taking the usual dose of stable iodine. This is useful for reducing the baby’s dose but does not protect them from directly inhaled radioiodine for which a small dose of stable iodine can be administered.
Section 2.3 discusses the process of estimating dose projections based on source terms, atmospheric dispersion and release prognosis. Section 2.4 discusses contamination and environmental monitoring.
Subsequent sections discuss the control and limitation of dose to responders and recovery workers and doses in the later phases of the event, including from food and water, once the release has stopped. Of interest is table 4-2 which summarises a discussion about the circumstances under which people can be allowed back into an area, either for remedial work, use of critical infrastructure, to access business and homes to recover property.
On the whole the advice given in this report is similar to that given in the NRPB’s ERL documents that are used in the UK. It contains some interesting and useful discussion.