The case for 2 mSv per day

Moved from 1 mSv/d to 2 mSv/d, 2024-01-23. 2 mSv/d is the correct 1934 ICRP limit. There is a properly referenced PDF of this piece at gordianknotbook.com. This is true of most of the Gordian Knot News posts.

Figure 1. US Dose Limits through time

Through 1951, the International Commission on Radiological Protection (ICRP) dose rate limit for the general public was 2 mSv/d. However, in 1951, the ICRP changed the recommended limit to 3 mSv/week. This was based on claims of genetic mutations at low doses which turned out to have no foundation.\cite{calabrese-2011} In 1957, the American counterpart of the ICRP, the National Council for Radiation Protection(NCRP), added a limit of 50 mSv/y for nuclear workers and 5 mSv/y for the public. As the NCRP itself acknowledged, this humongous change, a reduction of 30, was not based on any new data.

The changes in the accumulated MPD [Maximum Permissible Dose] are not the result of positive evidence of damage due to use of earlier permissible dose levels but rather are based on the desire to bring the MPD into accord with the trends of scientific opinion.\cite[page 1]{nbs59-1957}

Opinion trends that are not based on data are hardly scientific.

In 1980, Lauriston Taylor, founding chairman of the NCRP, said:

Collectively, there exists a vast array of facts and general knowledge about ionizing radiation effects on animal and man. It cannot be disputed that the depth and extent of this knowledge is unmatched by that for most of the myriads of other toxic agents know to man. No one has been identifiably injured by radiation while working within the first numerical standards first set by the NCRP and then the ICRP in 1934. [2 mSv/day]

Table 1 shows that this is still the case. To see detectable harm --- get out of the green --- requires dose rates of 20 mSv/d or more. Taylor's 2 mSv/d provides a factor of 10 margin. It's a good rule. But why is this a good rule? To answer that, we must bring in some basic biology.

Table 1. Populations who have received very large doses

The DNA in our bodies is constantly being assaulted by Reactive Oxygen Species(ROS). These chemical active molecules such as OH- are the by product of oxygen based metabolism. About one-billion ROS micro-bombs per day per cell leak from our cell's mitochrondia into the rest of the cell. Roughly 1 in 20 thousand of these molecules chemically damage our DNA. This is the price we pay for an oxygen based metabolism.

This damage can take the form of Single Strand Breaks(SSB) and Double Strand Breaks(DSB) of the DNA double helix.¹ Table 2 shows some estimates of the number of SSB's and DSB's each of our cells endure per day from normal metabolism.

In response to this onslaught, Nature has equipped us with a remarkably accurate DNA repair system. Without this system, we would not be here. SSB's are repaired almost automatically by the clever design of the double helix. The repair uses the intact strand as a template and is essentially error-less. SSB repair takes about 25 minutes.

DSB repair is far more difficult. In portions of the cell cycle, a backup template is available, and can be used to make a practically error-free repair. In the rest of the cell cycle, the attempts at repair, which take up to about 12 hours, cannot always be successful. This is especially true, if the cell is faced with a large number of DSB's at the same time.\cite{bissell-2011} A portion of the incorrectly repaired DNA will survive and a portion of those mutations will escape our immune system and a portion of those will lead to cancer. Clearly, Double Strand Breaks should be our focus.

Radioactive particles can also damage our DNA. Table 3 shows estimates of the DSB's per millisievert of radiation.

If we conservatively assume 10 endogenous DSB's per cell-day, and 0.04 DSB's per millisievert then it would take 250 mSv per day to equal the number of DSB's produced endogenously. If normal endogenous harm is equivalent to 250 mSv/d, then any harm associated with 2 mSv/d would almost certainly not be detectable. At the same time, it is not surprising that we start to see detectable harm at 20 to 50 mSv/d. At that point, the cell is forced to deal with a substantially higher than normal number of DSB's.

In short, the Table 2 and 3 numbers are consistent with Table 1, and the pre-1950 tolerance dose rate, and its implied repair period of a day. 2 mSv per day is 700 times higher than the NRC limit of allowed exposure to the public of 1 mSv/y and 3000 times higher than the EPA limit of 0.25 mSv/y, both of which are erroneously regarded as harm based numbers.

Since we see no detectable harm at 2 mSv/d, there is no argument for enduring the very real costs including the health risks of evacuation at this dose rate. At Fukushima, this means no member of the public would have been evacuated, avoiding the nearly immediate killing of 50 frail, elderly people, and the 2300 plus premature deaths associated with the multi-year evacuation. The radiation release would have been a footnote to all the damage caused by the tsunami, including the loss of three multi-billion dollar reactors. The other three reactors at Fukushima Daichi would have been back on line in a matter of weeks or months. None of the other Japanese nuclear plants would have been affected, avoiding a 30 billion dollar per year increase in electricity costs,\cite{neidell-2019} 2000 to 7000 premature deaths per year due to people being unable to afford heat or cooling,\cite{he-2019}, and a 150 million ton per year increase in CO2 emissions.\cite{kharecha-2019}.

If an overly conservative 2 mSv/d were the evacuation trigger at Chernobyl, only the 50,000 people in the immediate neighborhood of the plant would need to be evacuated; and they could have returned to their homes in a matter of weeks. Almost all the harm that was inflicted on 340,000 permanent evacuees would have been avoided. It is telling that Chernobyl Units 1 and 2, which share the same building as the obliterated Chernobyl Unit 4, were back on line in about six months.

If our legal and regulatory systems recognized that dose rates of 2 mSv/day and below produce at most negligible harm, nuclear power could be cheaper than coal, as it was in the mid-1960's, when nascent nuclear was just starting down a steep learning curve. A solution to the Gordian Knot of energy poverty and global warming would be within reach.

1. Some of the damage involves chemical changes other than a break in the strand; but, as long as one side is intact, we call it a Single Strand Break.