A 10% chance of getting cancer is not small at all. It's pretty significant - thousands of products and substances have been banned for much less. The chance of getting cancer for current smokers, for instance, is said to be 10-15%. Do you think that tobacco smoke is a really puny carcinogen?

No, I don't think tobacco smoke is a puny carcinogen. Dosage matters. To get that 10% chance of cancer, you need to have what would be an almost certainly lethal dose of acute radiation.

It's easy to get cancer by smoking. It's really really hard to get cancer from radiation without giving yourself a lethal acute dose of radiation in the process.

Have you a source for that? I could well believe that the cancer risk is low and obviously is is uncommon for a dose of radiation to be large, but 'comparatively large' doses of radiation are given regularly in hospital catheter labs, CT/PET scanners. And while generally these would be in the 10-20-30 mSv range, for many patients scans are repeated regularly for long time periods, rapidly reaching the 150-500mSv range discussed in the below article. Of course this is a fractionated dose and it would be much more dangerous as a single hit. I cannot find any reference to radiation dose verses cancer risk. Somewhat supporting you is this article which discusses over estimation of dose risk. Interesting discussion too: http://www.nejm.org/doi/full/10.1056/NEJMc073513

Unfortunately that doesn't correspond to any reading I've done on this topic. I'm pretty sure that chronic, sublethal exposure to radiation significantly increases your chance of various types of cancer.

Would you happen to have a source for this? I'm finding it hard to imagine how a study like this can even take place given that there have (probably) not been many survivors among those who have received a dose of 4 Sieverts.

There's a recent book called Physics for Future Presidents, written by a physics professor at Berkeley, which has a lot of material on radiation risks. He says you can calculate your risk of cancer from radiation by dividing your exposure by 2500 rem. The level that gives you acute radiation sickness is 200 rem.

This is called the "linear hypothesis" and is widely used. They use it in medicine to decide if diagnostic scans are worth the risk. The risk could actually be lower; some scientists think there's a threshold below which there's no risk. There's too much statistical uncertainty at low exposure levels to know for sure.

The class is available as open courseware, too. It is a no math physics course that focuses on concepts.

This is the course site, click webcast lectures on the side: http://muller.lbl.gov/teaching/physics10/pffp.html

I don't know of any studies on people with such large doses of radiation, no -- but it's believed that cancer risk from radiation is proportional to the dosage received, and lots of studies have been done looking at populations in areas with higher/lower levels of background radiation.

It should be noted that UNSCREAR use the linear no threshold theory despite the lack of evidence regarding lower doses . Err on the side of safety.

Do you realize that official estimates in Europe hold chernobyl responsible for about 500000 deaths caused by secondary radiation effects? Do NOT play this down. Radiation kills!