As soon as David J. Brenner heard about the undersea earthquake and subsequent tsunami that devastated northern Japan on March 11, he checked a map of the region’s nuclear power plants. One, because of its coastal location and reactor design, looked particularly vulnerable: Fukushima Daiichi. He hoped he was wrong.

Less than a day later, ominous reports of failed cooling systems and radiation leaks at that plant began to emerge. Dr. Brenner, director of the Center for Radiological Research at Columbia University — the oldest and largest such center in the world — found himself called on repeatedly to explain what was happening with the failed reactors and to assess the radiation risk to public health, both in Japan and around the world.

Dr. Brenner, 57, a native of Liverpool, England, is a physicist who has spent his career studying the effects of radiation on human health. He has published research showing that CT scans increase the cancer risk in children, and he recently testified before Congress, saying that the widespread use of whole-body X-ray scanners at airports would produce 100 extra cases of cancer each year in the United States.

He thinks CT scanners and the people who use them need more regulation to make sure the scans are medically needed and the doses of radiation as low as possible. He believes that even low doses increase the risk of cancer, and that there is no “safe” level or threshold below which the risk does not rise — even if that risk cannot be measured statistically.