With concern growing about the potential health risks from radiation being released by the Fukushima nuclear reactors in Japan, University of Texas M.D. Anderson Cancer Center radiation oncologist Dr. James Cox spoke with Chronicle medical writer Todd Ackerman last week about the dangers associated with the potent force.

Q: How does Fukushima compare to Chernobyl and Three Mile Island? What's the potential?

A: It's probably as bad or worse than Three Mile Island. It's certainly not as bad as Chernobyl, not by quite a bit, at least at this point. I don't think it would ever become as bad as Chernobyl because of the different way the reactors are made, the back-up safety systems, the containment.

Q: What are the ways people are exposed to radiation?

A: Radioactive emissions, released into the air, can be inhaled, but they also pose a danger when they fall to the ground around the reactor site. At Chernobyl, emissions fell on the grass, which in turn was eaten by cows that then produced milk containing radioactive iodine. Thousands of children who drank the milk later developed thyroid cancer. That's well understood in Japan. That's why they evacuated a 12.5-mile area around the site. Radioactive emissions can have an effect on the skin, but that can be handled by taking a shower.

Q: How does radiation cause harm?

A: Radiation preferentially kills rapidly dividing cells, whether in a tumor or the body. The cells in the bone marrow and gastrointestinal tract are the two rapidly dividing normal cell populations, which is why you get blood count drops and symptoms such as nausea, vomiting and diarrhea. Children are most at risk because they're growing and have more rapidly dividing cells.

Q: Isn't there concern about genetic effects?

A: That's really still theoretical. Even at Hiroshima and Nagasaki, where the dropping of the atomic bomb had a powerful effect on people, including the development of cancer years later, there were no genetic effects in those people or their offspring. Genetic effects have been seen in laboratory animals such as fruit flies and mice, but it was never demonstrated in Hiroshima and Nagasaki survivors. That is something not very widely appreciated.

Q: At what point does the amount of radiation become unsafe?

A: The recommended limit for individuals is .1 rem (a measurement unit.) Most of the radiation people get is from medical uses of radiation — a chest X-ray exposes a person to .01 rem, a (full-body) CT scan to 1 rem. In Japan, anywhere from 25 rem to 40 rem are being released per hour above the reactor. That's what going in the air, then dissipating and falling to the ground. The further you get from the site, the less radiation there is.

Q: Are potassium iodide pills an antidote?

A: They block uptake. They go to the thyroid gland, where it's normally utilized, and fill it with normal iodine so radiation can't come in, at least for a period of time. Fortunately, radioactive iodine doesn't last very long. It has a relatively short half-life - about 8 days.

Q: How much danger are the on-site workers in?

A: I think they're in some danger. I have every reason to believe they're being monitored closely, both in terms of radiation detection devices they're wearing and blood counts, the first clinical evidence one would see. Some people have gone to the extent of saying they're on a suicide mission. I don't believe that's the case.

Q: What about the risk to non-workers in Tokyo and other parts of Japan?

A: They might get some exposure that could be measured. But I don't think they're at any health risk, certainly not people hundreds of miles away.

Q: So there's no need for people in the U.S. to get iodide potassium pills, as has been reported?

A: No. There's reason not to do that. There's a greater health risk from taking iodine potassium pills unnecessarily than the threat of exposure.

Q: Given all the screaming headlines about "Japan nuclear crisis worsens," is the threat being overhyped?

A: I think it's being overplayed in certain ways. The reactor malfunction and radiation release is a very serious problem - I'm not suggesting it's not - but what's being oversold is the risk to people quite far away, including in the U.S. There will be measurable levels throughout the world eventually, just as the particles that were blown from the volcano in Iceland went around the world. But they will not be clinically significant. They will not cause harm to people.