Not all radiation exposure packs the same punch.

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Don’t worry too much about the hint of radiation reaching U.S. shores from the damaged nuclear reactors in Japan, experts say.

So far, it’s much less than we’d get from a chest X-ray.

But consider this: Every day, all day long, we’re bathed in low levels of radiation — cosmic rays from outer space, radon in our houses, uranium deposits in the soil, radio signals from every AM and FM station in range, airport full-body scanners, dental X-rays, cellphones, even tiny hints lingering from the A-bomb tests of the 1940s and ’50s.

And remember that radiation is cumulative. Most scientists agree there’s no such thing as a harmless dose.

Now relax. It’s less scary than it sounds.

“It’s absolutely true there’s no safe dose. But we’re likely to be OK if we remain vigilant,” says Dr. Nagy Elsayyad, a radiation oncology expert at the University of Miami Sylvester Comprehensive Cancer Center.

Not all radiation is equal. What comes from medical X-rays, airport scanners, leaking nuclear plants and similar sources is ionizing radiation — the dangerous kind, capable of causing cancer, increasingly so as time and dosage increase. But much of the radiation around us, from radio waves to cellphone signals to leaky microwave ovens, is the non-ionizing type never proven to cause illness, say scientists — well, most scientists.

World War II newsreel footage from Hiroshima and Nagasaki shows the doomsday effects of nuclear bombs exploding over cities, killing hundreds of thousands from the immediate blast and tens of thousands more later on, from high-dose radiation poisoning. And the current nuclear crisis in Japan has prompted new fears of radiation contamination.

But lower, everyday levels we experience routinely are less easy to grasp. For example, the U.S. Federal Aviation Administration estimates an airline pilot spending a 25-year career flying at 30,000 feet between Chicago and New York will experience enough extra cosmic radiation through the thinner atmosphere to increase his or her cancer risk by about 0.3 percent.

Another natural source, radon, is an invisible, odorless, radioactive gas emitted from soil.

In all, natural sources expose the average earthling to about 6.2 millisieverts (mSv) of radiation annually. The millisievert is the common unit for measuring radiation exposure; one millisievert equals about 10 chest X-rays. There are other ways of measuring radiation, but the millisievert is the most widely used.

The trick is to limit our exposure to ionizing radiation above that universal 6.2 mSv, Elsayyad says. The National Council on Radiation Protection recommends we get no more than one additional unit per year.

To put it in perspective, measuring in millisieverts, a chest X-ray adds 0.1, a mammogram adds 0.7, a high-altitude, cross-country airplane flight adds 0.05 and a medical CT scan adds 10. The new full-body scanners at airports may be intrusive, but their radiation level is low — far less than 0.1 mSv, according to the American Cancer Society.

Although scientists differ on the effects of low-level exposure to ionizing radiation, most believe no dose is safe.

“There is no safe level of exposure, whether from food, water or other sources. Period,” said Jeffrey Patterson, immediate past president of Physicians for Social Responsibility, in a statement calling for a nationwide moratorium on new nuclear reactors in the United States following the incidents in Japan.

A few differ: “There is no way to determine precisely the risks of low-level radiation,” writes Kenneth Strubler, director of medical physics at the Greater Baltimore Medical Center, in the online journal “To date, no proven body of evidence has established an increase in human disease as a consequence of radiation rates comparable to those of earth’s natural background — or even 10 times higher.”

U.S. Environmental Protection Agency scientists say there’s no practical way to measure the risk of lifetime chronic exposure to low levels of radiation. But they can measure the effects of extremely high doses. So they extrapolate backward to create very rough estimates of low-level risks. Their conclusion: A person exposed to 10 extra millisieverts of radiation, the equivalent of about 100 chest X-rays, in small doses over a lifetime, would have a 0.3 percent greater chance of dying from radiation.

Studies of the body’s reaction to low levels of radiation have shown that as exposure increases, it causes the same cancers as very high doses although in far smaller numbers.

Thyroid cancer and leukemia can follow after years of chronic overexposure. Later, in 10 or 15 years, come lung cancer, skin cancer, multiple myeloma and cancers of the breast and stomach, according to the American Cancer Society. And children are at much higher risk from exposure to radioactivity, the ACS says, because their tissue is growing faster and because they have longer life expectancy during which cancers can develop.

Avoiding unneeded radiation involves trade-offs. Because of the soaring use of CT scans and other diagnostic tools, the amount of medical radiation received by the average person has increased by more than 600 percent since the early 1980s, Elsayyad says.

“Doctors often have no idea how much radiation they’re exposing their patients to,” he says.

He’s not against such procedures, agreeing they’re often necessary.

“If I had angina and I needed a stent, I would want it done, and I wouldn’t worry about the radiation,” he said. “But doctors must be sure it needs to be done, and it isn’t just a fishing expedition.”