The Gordian Knot Group is pushing Sigmoid No Threshold (SNT) as a replacement to the Linear No Threshold (LNT) radiation harm model. SNT requires dividing an individual's dose rate profile into repair periods (currently set at a day), fitting an S-shaped response curve to the dose in each repair period, and treating each repair period as an independent event. Once you have an estimate of the daily dose rate profiles and a computer, SNT is as easy to implement as LNT.

In our last post, we pointed out LNT failed the Huxley one-ugly-fact test. LNT's prediction of bone cancer incidence in radium dial painters who had received massive doses over a ten year plus period is off by orders of magnitude. Therefore, LNT is wrong and must be rejected. It is only fair that we subject SNT to the same test.

Unlike LNT, SNT does not ignore how rapidly or slowly the dose was received. SNT requires a daily dose rate profile. Radium dial painting started about 1915. In the late 1920's, the ladies were advised to stop licking the tips of their brush. That pretty much put a stop to the bone cancers.

If we assume the ladies received their dose evenly over 10 years, which is almost certainly conservative, then we can add an SNT prediction to the picture, Figure 1. //

SNT claims we should have seen nearly no cancer up to about 2 mSv/day, but then the prediction turns sharply upward. The actual jump upward takes place at about 20 mSv/day. Moreover, at 20 mSv/d, SNT predicts a 99% incidence rate when the observed was less than 30%. However, unlike LNT, SNT avoids the absurdity of a cancer incidence greater than 1.0.

The charitable view of this is SNT is conservative by a factor of ten, in predicting the point at which dose rates become seriously harmful. This is close to a factor of 100 better than LNT. //

My takeaway is SNT is wrong; but it is a qualitatively different wrong than LNT. Furthermore, it is quantitatively acceptable at dose rates up to 1 mSv/day, even when that daily dose is repeated for years. Fortunately, the dose rates experienced by the public in a nuclear power plant release are almost never above 1 mSv/day and then only for a few weeks at most.

If we use SNT in a compensation scheme, the few people who do get hit with more than 1 mSv/day will be over-compensated. From a societal point on view, this is bad. Not only does this represent an inefficient allocation of resources; but it will cause unnecessary psychic trauma in the over-compensated. In the wrong hands, it could lead to unnecessary evacuations. But perhaps this is a price worth paying for SNT's simplicity and its multi-order of magnitude improvement over LNT. //

Jack Devanney 19 hrs ago Author

LNT (like SNT) bills itself as a model that works for ANY radiation exposure. But the BEIR7 fit is based mainly on the bomb survivor (RERF) data which is a single acute dose. It is not a good fit to the RERF data. No smooth curve could be since the RERF data bounces up and down in a very weird way., which should tell you something about the quality of that database.

But the point is that LNT it is off by orders of magnitude when you try to apply it to chronic doses received over protracted periods such as the dial painter data. A regulatory model cant change every time you get a new exposure data. We must have ONE model that does a reasonable job both on large acute doses and large chronic doses. LNT cant do that. SNT can.