I looked at this research carefully. Most studies saying that low-level ionizing radiation is safe are written by people who are not experts or are written by people payed grants by the nuclear industry or DOD related government funding.
The use of the study I'm going to paste below to argue that low level ionizing radiation is "not bad," illustrates serious bias that could be accidental or deliberate.
Let us for moment presume that the study itself was not flawed or fraudulent.
Let us presume that the study did empirically discern that people in this area of northern Iran, exposed to ionizing radiation continuously at higher rates than normal, were particularly less likely to get cancer than projected for the normal population when exposed to that same level.
Here is my read on the glitch in the argument that this study demonstrates we won't be harmed by low-level ionizing radiation.
The people studied in A CITY in northern Iran were no doubt people whose families had lived in that area for generations. Thus, they would have developed biological tolerances for radiation that would have been transmitted epigenetically, across generations.
These Iranian people who had been epigenetically prompted to have higher levels of cell repair or immunity to radiological damage, these people are NOT REPRESENTATIVE OF THE NORMAL POPULATION.
The average, normal person lives under conditions of normal radiation and therefore would be extremely vulnerable if suddenly exposed to a significantly higher level of "background" ionizing radiation.
Abstract—People in some areas of Ramsar, a city in northern Iran, receive an annual radiation absorbed dose from background radiation that is up to 260 mSv /year, substantia¬lly higher than the 20 mSv /year that is permitted for radiation workers. inhabitant¬s of Ramsar have lived for many generation¬s in these high background areas. Cytogeneti¬c studies show no significan¬t difference¬s between people in the high background compared to people in normal background areas. An in vitro challenge dose of 1.5 Gy of gamma rays was administer¬ed to the lymphocyte¬s, which showed significan¬tly reduced frequency for chromosome aberration¬s of people living in high background compared to those in normal background areas in and near Ramsar. Specifical¬ly, inhabitant¬s of high background radiation areas had about 56% the average number of induced chromosoma¬l abnormalit¬ies of normal background radiation area inhabitant¬s following this exposure. This suggests that adaptive response might be induced by chronic exposure to natural background radiation as opposed to acute exposure to higher (tens of mGy) levels of radiation in the laboratory¬. There were no difference¬s in laboratory tests of the immune systems, and no noted difference¬s in hematologi¬cal alteration¬s between these two groups of people. Health Phys. 82(1):87–9¬3; 2002 http://www¬.probeinte¬rnational.¬org/Ramsar¬.pdf