He’s an expert on nuclear security. His wife edits articles for the Bulletin. She was home with their two-month-old baby when the doorbell rang, the dog went nuts, and the baby — too young to roll over on his own, and protected by a pillow on the couch — somehow ended up wailing on the floor, with the crosshatched pattern of a rattan rug imprinted on his little head. (The incident remains under investigation, but circumstantial evidence points to psycho-dog.)

He’s an expert on nuclear security. His wife edits articles for the Bulletin. She was home with their two-month-old baby when the doorbell rang, the dog went nuts, and the baby — too young to roll over on his own, and protected by a pillow on the couch — somehow ended up wailing on the floor, with the crosshatched pattern of a rattan rug imprinted on his little head. (The incident remains under investigation, but circumstantial evidence points to psycho-dog.)

At the children’s hospital, the doctor did an exam and said the baby was probably fine but that there might be bruising on the brain. “If you’d like, I can do a CT scan,” he offered. “Do it,” said the mom. “Wait a sec,” said the dad. The mom remembers their subsequent deliberations: “He’s talking ‘lifetime exposure,’ and all I can think is: BRUISING ON THE BRAIN! — which for all I know is manageable but which sounds terrifying.” So, into the scanner went the baby.

The CT (computed tomography) scan — a series of x-ray images combined to create a three-dimensional picture — showed that the baby’s brain was uninjured. The mother and father were relieved, but also a bit worried about their son’s future. They know he is growing up in a world where medical radiation exposure is rising dramatically, and he’s already ahead of his class.

The biggest dose. “Remarkably to me, about one-third of all patients admitted to hospitals today are diagnosed or treated using radiation or radioactive materials,” Nuclear Regulatory Commission Chairman Gregory B. Jaczko said in a speech at the American Society for Radiation Oncology’s annual meeting last October. “Thirty years ago, these sources accounted for approximately 15 percent of the public’s total radiation exposure. Today, that figure has more than tripled, so that nearly 50 percent of the public’s overall radiation exposure can be attributed to medical sources.”

And it’s not just the United States. Japan, for example, has far more CT scanners (relative to its population size) than the United States does. As the UN Scientific Committee on the Effects of Atomic Radiation reported in 2008, “[H]igh-dose x-ray technology (particularly computed tomography scanning) is causing extremely rapid growth in the annual number of procedures performed in many countries and, by extension, a marked increase in collective doses.” For the first time in history, the report noted, exposures from diagnostic radiology in several countries exceeded those from natural background radiation or any other source. Some healthy people are even getting whole-body CT scans as “preventive” medicine, a practice that most doctors frown upon. (Unfortunately, it is also a practice once extolled by Oprah Winfrey, thus cementing its place in pop culture enthusiasms.)

Although the estimated lifetime risk of cancer from a single scan is small, the widespread use of scans means a significant increase in radiation exposure for the population as a whole. In a paper published in the New England Journal of Medicine in 2007, David J. Brenner and Eric J. Hall estimated that 1.5 to 2 percent of all cancers in the United States might be attributable to the radiation from CT scans.

Ironically, cancer therapy is another significant source of medical radiation. Rep. Edward J. Markey, who chairs the Energy and Environment Subcommittee of the House Energy and Commerce Committee, has raised concerns over Nuclear Regulatory Commission rules that allow cancer patients to go home or to check into a hotel immediately after being treated with radioisotopes, even though they are dangerous to the people around them for up to a week. And the more radioisotopes that are lying around hospitals, the greater the potential for a security breach.

The fear factor. So why aren’t more people worrying about medical radiation? Why are Americans instead fretting over trace amounts of radiation from sources such as the Fukushima accident and airport scanners, and even non-ionizing radiation from cell phones?

Social scientists tell us that people dread radiation because it is invisible, unfamiliar, beyond an individual’s personal control, fatal at high doses, and capable of affecting generations to come. In those respects, medical radiation seems no different than any other source of exposure. But as Paul Slovic points out in this month’s Bulletin, surveys have consistently shown that the public perceives medical radiation as less risky than experts do, and the opposite is true for nuclear power and nuclear waste. This perception gap probably occurs because Americans trust their doctors a lot more than they trust the Nuclear Regulatory Commission or the nuclear industry and because they have a better appreciation for the benefits they receive from medicine than from electricity.

You might think that the rise in medical radiation would be good news for the nuclear energy industry: It makes radiation exposure from power plants look small by comparison. It’s more likely, however, that a growing awareness of the dramatic increase in medical exposure will lead to calls for reducing other types of exposure. After all, turning off a few lights and erecting some wind turbines sounds a lot better than foregoing a possibly life-saving CT scan. When it comes to bruising on the brain, even the most well-informed moms aren’t going to take any chances.

Editor’s note: The May/June Bulletin is a special issue on low-level radiation risks and is available for free throughout the month of May.