RoR: Forum

I read somewhere that the average dose world wide was about 2.4 mSv but that in the U.S. it was between 3 and 6 mSv varying by where in the U.S.

In addition to "dose equivalent," it is further complicated because you can safely handle a larger dose if it is received over a longer time (on TV they gave an example of being able to spend 15 minutes a day in the sun for weeks and get no sunburn, but not be able to spend 2 hours in the sun at one time. I assume that it's because the body has a better job of keeping up with repairing the damages in smaller doses over time. And different people have different tolerance levels and effects vary by age as well.

Post 21

Saturday, March 19, 2011 - 7:47am

Steve,

You're right; there is a big difference between receiving a given dose of radiation over a short vs. a long period of time. An acute dose (over a short time) can lead to more immediate effects. The effects of radiation exposure are divided into two categories: deterministic and stochastic. Deterministic effects are due to acute doses and are usually due to the death of cells. These are the effects usually described in radiation poisoning and there is a threshold dose below which the effect doesn't happen. And the effect gets worse with higher doses.

Stochastic (or probabilistic) effects are the result of lower doses and include cancer, genetic damage, and risk to a fetus. Here the radiation dose only affects the probability of the effect occurring. And the severity of the effect is independent of the dose; it either happens or it doesn't. The model usually assumed for stochastic effects is the linear, no threshold model. This means that no matter how low the dose is, there is still a non-zero probability of the effect happening; and the probability increases in direct proportion to the dose. The cutoff between these two regimes (obviously a blurry boundary) is about 0.5 Gray (Gy) or 50 rad.

Since I'm talking about units, let me elaborate a little. The original unit for radiation measurement was the Roentgen (R). This was a measure of how much ionization of air in a fixed volume the radiation exposure would produce. This is based on the operation of the ionization chamber method of measuring radiation exposure, which is still the method used for measuring radiation exposures in x-ray exams and other lower-level exposures. But the ionization of air is different from the ionization of, for example, tissue, so the rad was introduced. The "rad" is an acronym for "radiation absorbed dose" and it's the amount of energy deposited in a given amount of mass (of tissue). The fancier unit is called the Gray (Gy) and 100 rad = 1 Gy.

Now, not all types of radiation cause the same biological damage for a given dose. There are the x-rays which are produced in x-ray machines and in radioactive decay (gamma rays are just higher energy x-rays), there are the beta particles (electrons or positrons) released in radioactive decay, and there are neutrons and alpha particles released in radioactive decays. So, the equivalent dose was introduced to take into account the effectiveness of the different forms of radiation. It's related to the absorbed dose by a multiplicative factor. The units of the equivalent dose are the rem ("radiation equivalent man") and the Sievert (Sv); 100 rem = 1 Sv. To add to the confusion, these are the same units as the "effective dose", which is a different quantity. As I said in another post, the effective dose is the whole-body dose that would have the same biological effect as the particular exposure being studied. So, this is really physically different from the equivalent dose, but it has the same units.

The kinds of doses received by the emergency workers at Chernobyl were acute doses which led to radiation sickness which led to death in many, but not all, of them. Others exposed to lower levels, through fallout, directly or indirectly (drinking milk from cows who had eaten contaminated grass), would be subject to the stochastic effects. These may take many years to happen and again you can only talk about the probability of it happening.

Thanks,
Glenn

Post 22

Saturday, March 19, 2011 - 10:15am

Glenn,

Your description is far, far cleared than any of the descriptions I've heard or read elsewhere. Thanks, Steve.

Post 23

Saturday, March 19, 2011 - 9:41pm

http://pajamasmedia.com/tatler/2011/03/19/fukushima-update-get-a-grip-part-2/

Post 24

Sunday, March 20, 2011 - 6:00am

http://xkcd.com/radiation/

Post 25

Sunday, March 20, 2011 - 7:08am

I noticed in Ed's table, and in the first link Robert gave, that the annual limit for a radiation worker in the US is incorrect. It should be 50 mSv (or 5 rem) per year, not 100 mSv/yr. If a radiologist's radiation badge readings add up to more than 50 mSv in less than a year, he/she is not legally allowed to perform any procedures that would expose them to radiation for the rest of the year. That's why some radiologists don't wear their badges all the time. : )

Post 26

Sunday, March 20, 2011 - 10:57am

Glenn,

My newspaper reported a 5-year cumulative exposure of 100 mSv as the limit. That would make the limit on annual exposure out to be 20 mSv (for 5 years in a row), wouldn't it?

Interesting aside:
Ann Coulter thinks that more than 50 mSv a year in radiation is actually good for you (better than getting less than 50 mSv a year) -- and she's coming out with a new book documenting that. Assuming she's right (and that all the experts are wrong on the matter), this is not the first time that a layman outsider had to step in in order to tell the experts how to think about their subject matter.

In the mid-20th Century, psychological science was such a terrible joke that it took a layman (and her daughter) to create what is perhaps the most significant advancement ever in personality typology: the MBTI. And before that, we had Spinoza -- a shoemaker who knew philosophy better than any of his officially-educated peers.

:-)

Ed

p.s. While not necessarily a layman, Rand can at least be considered an outsider who had better answers than any of the working experts in the field.

(Edited by Ed Thompson on 3/20, 11:15am)

Post 27

Tuesday, March 22, 2011 - 4:30pm

We live in a world that used to, until very recently, conduct full scale nuclear weapons tests in the atmosphere.

http://www.brookings.edu/FP/PROJECTS/NUCWCOST/TESTS.GIF

Over 500 of them, the largest being a 50 MT burst by the Soviet Union.

It would be interesting to put the Japanese circumstances in the context of these former test programs, to understand the potential marginal impact of the event.

As well, as a direct consequence of the earthquake and tsunami, some number of order 10,000 dead has resulted, and hundreds of billions in damage.

In a worst case scenario at these plants, do those numbers go up by some few percent, or by factors of 2 or 10?

Adding the potential additional catastrophe of these nukes is not helping anything, but the incremental costs seem to be dwarfed by the enormity of what has already occurred.

Swimming pools and Barbie dolls are killing more Americans than exposure to radiation, by far. We kill 40,000 of each other on the highways every year over nothing more important than rushing home from our workout at the gym in time to watch American Idol. And yet, we aren't terrified by that prospect in the least.

It's hard to fathom.

Post 28

Tuesday, March 22, 2011 - 6:46pm

Fred,

I couldn't agree more. The world has seen more people die over the decades from vending machines tip-overs, than have been killed in nuclear power plant accidents.

Post 29

Wednesday, March 23, 2011 - 8:32am

Good post Fred. This reminds me of that tour bus accident in NY that killed 15 people. Tragic as it is for these people to have died, 40,000 people as you say die every year on America's roads. But now what are the state governments in my region proposing? More regulations on tour bus companies. One proposal, requiring them to place costly electronic devices on their buses for regulators to be able to track them. Before they're done, they'll make it expensive enough to drive out a lot of tour bus companies, hitting my area with more economic misery as we depend on these tour buses.

They're also focusing on this bus driver's history. They note he was convicted of manslaughter. But the conviction had nothing to do with motor vehicles, and was rather associated with a stabbing. So regulations proposed to conduct better background checks on drivers would probably not have even prevented this man from driving that bus. Why would a prior conviction unrelated to motor vehicles prevent him from operating a motor vehicle? Would the court system uphold such a regulation that any prior criminal convictions, no matter what the crime, should preclude you from employment as a commercial bus driver?

No rationality to this whatsoever.

Post 30

Wednesday, March 23, 2011 - 10:01am