We focus a lot on dose whereas what we should be focusing on is dose rate particularly in the dose rate range that we need to worry about when we’re faced with the possibility of a contaminated environment. Most of our understanding about the health effects of radiation come from the a-bomb survivor study that is our biggest most general population and it has been studied very deeply for many decades. The dose rate to the people in Hiroshima Nagasaki however was very high. all of the dose that they thought that the survivors received came in about two seconds and so that’s a very big dose rate. In fact, it’s a million, ten million a hundred million times what we see at background whereas when we’re making a decision as to whether or not to leave an environment for instance, the 20 millisieverts per year that the Fukushima residents used as a trigger for when they evacuated that’s only eight times background and so the big question is how do we take what we know about the a-bomb survivors when they got irradiated at a dose ten million times background. how do we take that information and make it useful for us when we’re trying to make these difficult decisions about leaving our homes, leaving our communities, leaving our towns, breaking families apart as we’ve seen in the Fukushima situation so that’s the big question. the way we do that is to use something called a dose rate effectiveness factor. The risk of radiation that we calculate from the a-bomb survivor study divide that by some number and then we say okay that’s from low-dose radiation.
Influence of Dose
The seminal document that describes the dose rate effect was written by the NCRP in 1980. this is NCRP 64. This has remained the most important reference for choosing the dose rate effectiveness factor. It’s important to note that NCRP 64 separated low dose rate effects into two categories. one category they called protraction and that was a situation in which an animal was irradiated for most of their lifetime at low dose at relatively low dose rates. The experimental studies are all carried out still a few orders of magnitude maybe a thousand times sometimes 10,000 times higher than background but there still may be a thousand times lower than the a-bomb survivor data. So they’re still looking at the sparing effect of low dose rates but the low dose rates they investigated were still very high. they looked at protraction in which case the animal is subjected to radiation for a big chunk of their lifetime this allows not just for DNA repair to take place but also changes in susceptibility to tumorigenesis adaptation to the radiation all sorts of physiological responses get taken into consideration. they distinguish that protraction effect from a true dose rate effect which is a much shorter duration of radiation not taking into consideration any changes in the whole organism over time such as Radio adaptation or changes in susceptibility that kind of thing but only considering the effect of DNA repair. All agencies since then have chosen to use a value of either 2 or 1.5 and in some cases there are explicit explanations as to why they’re using a low number and mainly to be conservative.
This strategy has been used and implemented for coming up with standards for radiation workers. It is said that, that is an inappropriate value of the dose rate effectiveness factor for the situation when the dose rate is really quite low a handful of multiples or 10 times background maybe even up to 100 and 200 times background in that case we should be using something closer to those protraction factors which when they were looked at by the NCRP ranged from 6.6 to over 12 with an average of about 10. when the NCRP looked at even lower dose rates than they looked at in protraction factors they couldn’t find any increase in tumorigenesis they couldn’t find any decrease in lifespan in fact they found the opposite. they found increases in longevity they make it very clear that at quite low dose rates still above background many multiples of background you get in many animal models increased longevity probably due to some kind of systemic stimulation that the low-dose radiation is doing to the organism. so just as just to summarize I think that we are when we’re looking at low dose rates and we’re looking at the problem of a contaminated environment we need to do away with all of our conservative assumptions and we need to provide the most accurate data we can and by using (a DREF) a dose rate effectiveness factor of only two we are diluting the accuracy of the data that were reported in interesting NCRP 64 and we are basically doing it this service to the population that has to make a decision as to when to evacuate is it 8 times background or could we stay two perhaps 10 20 50 times background.
Conclusion
so I would like to just conclude by making a couple of recommendations. It’s vitally important that we consider using more than one value of the dose rate effectiveness factor when the dose rate is really quite low and people are exposed to radiation for abstention periods of time much longer than needed for DNA repair long enough for these systemic changes and susceptibility for instance we need to use a number more appropriate to protraction in which case the risk estimates would fall perhaps by a factor of five from what we’re using and that would be of enormous benefit be more accurate and be highly beneficial to people having to make the decision of whether to stay or whether or not to stay. It is also said that in the dose rate regime of interest to this article here contaminated environment when to stay when to go it is not the total cumulative dose that’s the important issue, it’s the dose rate that the individual is experiencing. and the reason I say that is because you can elicit a certain response in an organism and a given dose delivered acutely but if you reduce the dose rate you can completely eliminate that negative effect and that’s been seen experimentally many times and also it is believed that we need to generate radiation risk estimates specific for the condition this specific for the situation of a contaminated environment. in this case we would be eliminating many of the conservative assumptions that seem more appropriate for radiation in the workplace scenarios and in addition we would choose dose rate effectiveness factors that data show are much higher than the values we’re using.
