How clean is clean enough?
This is the question, which we need to answer when we design appropriate remedial measures of contaminated sites. As humans are exposed to natural background radiation all the time, it is not practicable to reduce exposure from radionuclides below the level of background radiation. Often, we are dealing with remediation of large amounts of wastes or contamination. Therefore, the closer levels get to background radiation, remedial measures become more and more expensive and often are not feasible from a practical or economical view. This is why we have to define the limit of exposure, which is still acceptable and practically and economically achievable. But how can you derive such a limit?
First we have to derive the relationship between the concentration of the radionuclide in exposure situation and the risk to health. This can be achieved by converting the concentration of the radionuclide to effective dose. The unit is Sievert (Sv), where 1 Sv represents a 5.5% chance of developing cancer. The effective dose shows the probability of cancer induction and genetic effects of low levels of ionizing radiation. It takes into account the type of radiation and the nature of each organ or tissue being irradiated, and it enables the summation of organ doses due to varying levels and types of radiation, both internal and external, to produce an overall calculated effective dose. The annual average background effective dose is about 2.4 mSv, but it can vary among different locations on Earth, for example up to 200 mSv/year in Ramsar, Iran.
To calculate effective doses for inhalation and ingestion of radionuclides, dose conversion factors are used. Dose coefficients enable easy
calculation of effective doses if we know the activity concentration of radionuclides in ingested food or water, or inhaled air. If we want to calculate the annual ingestion effective dose for a specific radionuclide due to ingestion of drinking water or food, this equation can be used:
Where E is the annual effective dose in Sv/year, A is the activity concentration of a specific radionuclide in Bq/kg, e(1) is the dose coefficient in Sv/Bq and m is the annual intake of food in kg/year.
The negative effects of radionuclides or ionizing radiation produced during their decay can only be easily determined for high doses. Below effective doses of 300 mSv/year of chronic exposure or 100 mSv/year of acute exposure, the evidence on excess of cancer incidence is weak or inconsistent. At the moment, it is believed that no threshold linear model should be applied for exposures below which we have no firm evidence that they pose a risk due to ionizing radiation. This means that as soon as the effective dose is higher than background, there is a risk for humans, which linearly increases with increasing dose. However, so-far there is no definite scientific evidence available if this is true or not. Based on this assumption, the current recommendation is that the general public should not receive more than 1 mSv additional dose from exposure routes other than natural background radiation. As such, regulators usually set up 10 or 20 times lower limits for exposure for a specific site to be on safe side to comply with the limit of 1 mSv.
Nevertheless, there is no scientific evidence why these limits should be applied and the general public often ask questions on whether or not this is enough or if lower limits should be applied. This is why researchers are working on the effects of low doses to humans and biota. The aim of these studies is to re-examine the no-threshold linear model and perhaps, in the end, to define risk thresholds which could very certainly vary depending on the pathologies and the populations. As effective dose only takes into account cancer, it should be ensured that we are properly evaluating the whole range of risks, and not just cancer as is the case today. Other risks arising from ionizing radiation might include the risks to the cardiovascular system, digestive tract, immune system or brain function.
It is not easy to conduct scientific studies on the effects of chronic exposure to low doses. The effects are not very visible and the possible health impact can only be observed over the long term. Moreover, the studies are difficult to set up, either because they require vast cohorts to be followed up over decades, or because experimental research is expensive and difficult to carry out. Nevertheless, this work is essential in order to discover and prevent risks to people.
When setting up limits on how much contaminated sites should be cleaned up, all this has to be considered and until new scientific evidence becomes available, the best option is to reduce risk as much as reasonably practicable. Once we have defined the dose criteria, remedial measures can be designed in such a way that we can meet the limits.
