No scientist or scientific based agency will ever commit to stating that any level of radioactivity is “safe”. The best way to look at things is to decide at what level you personally consider “safest”.
In theory, a level of zero radon might be considered “safe” but is a level of zero actually achievable? Because uranium is found everywhere, so is radon. That includes in the outdoor air as well. Outside of uranium mining deposits, where the concentrations of both uranium and radon at the surface are much higher, the average outdoor air level of radon fluctuates between 25 and 41 Bq/M3. Trying to get the indoor air to below this level is impractical because every time you open a door or window you would let the higher exterior levels in.
The general consensus is, that if radon levels are high in the building and need mitigation, repairs should be made that get the radon levels to As Low As Reasonably Achievable (known as the ALARA principle). The term “reasonable” is subjective. What is reasonable to one person, might not be reasonable to another.
For example, lowering a radon level from a 200 Bq/M3 result to 25 Bq/M3 might involve spending tens of thousands of dollars on specialist engineering and extensive ventilation equipment (or it might not). Lowering the level from 200 Bq/M3 result to 41 Bq/M3, however, might only involve installing a regular ventilation system or simple sub-slab depressurisation system in an existing sump.
Which one is reasonable? If the exterior air samples fluctuate between 25 and 41 then spending an inordinate amount of money on a high-tech and high-cost solution would not generally be considered “reasonable”.
When it comes to “safe” or “not-safe” the problem is confounded even more by the terms “action-level” and “reference-level”. Health-Canada, along with a number of other countries use the term “action-level”. This, to some, gives the false belief that anything below this “action-level” must be OK. After all, if the government is telling you it’s OK not to take action below their specified level, then it must be OK, right?
Remember, this is the same government that for years told everyone that Asbestos was safe, mainly because Canada produced so much of it and its production boosted our economy.
The World Health Organisation uses the term “reference level”, and indeed, recommends all countries switch to this term. They go on to state “A reference level does not specify a rigid boundary between safety and danger but defines a level of risk from indoor radon is considered to be too high if it continues unchecked into the future.” they continue by specifying “protective measures may also be appropriate below this level to ensure that radon concentrations in homes are well below that level.”
This to our viewpoint identifies that the belief that there is a “safe” level for radon is erroneous and that everyone should have their homes tested and where required remediated to the level that is achievable without breaking the bank.
We also recommend ensuring that, as with all environmental testing, the individual (or company) who performs the measurement is not the same individual (or company) who provides the remediation. That would be like putting the fox in charge of security at the hen-house.