You may be surprised to learn that you encounter far more radiation in your daily life than what's blown across the Pacific Ocean from damaged Japanese nuclear plants.

apan's ongoing nuclear crisis understandably has people around the world worried about radiation exposure and the potential health risks it may pose. According to the latest reports, radiation from Japan was detected in Southern California late this week, but experts are quick to point out that the levels are far from dangerous. The readings were "about a billion times beneath levels that would be health threatening," a diplomat with access to United Nations' radiation tracking told the Associated Press.

Nor is it unexpected. "Whenever radioactive particles get in the atmosphere, they have the potential to spread around the world," says James Thrall, MD, president of the American College of Radiology. "But they get diluted as they travel, so they're unlikely to pose any real health problem."

In fact, we're probably exposed to significantly more radiation every day than the miniscule fallout arriving from Japan. Here's a quick tutorial on radiation to put our collective anxiety in perspective:

What Is Radiation?

Radiation is a form of energy in waves. It exists on a spectrum, with low-frequency radiation (from radio waves and microwaves) on the low end and high-frequency radiation (from gamma rays and x-rays) on the high end. All radiation affects the cells in our bodies to some extent, but the lower the frequency of the waves and the lower the exposure, the less dangerous it is.

To understand the risks of high-frequency radiation — the kind we're talking about in this article — think back to high school physics: These waves have enough energy to knock electrons off molecules, which can cause damage to cell DNA that can ultimately lead to cancer.

How Are We Exposed to Radiation?

We encounter radiation each day from a variety of sources. The average American is exposed to about 6 millisieverts (mSv) of radiation annually, according to the United States Nuclear Regulatory Commission (USNRC). Half of this typically comes from background radiation that occurs naturally in the environment, and half comes from medical tests, such as X-rays, mammograms, and CT scans.

According to Kelly Classic, MS, spokesperson for the Health Physics Society, sources of environmental radiation include:

• Radioactive compounds in soil and building materials like concrete, brick, and stone
• Radiation from outer space that your encounter when you fly on airplanes or visit high-altitude places
• The mineral potassium in your own body (a small fraction of potassium, which our bodies need to function, is radioactive)
• Radon gas in the home, which accounts for about 2 mSv of exposure each year, and is the largest contributor of background radiation

Finally, there's the kind of radiation released during nuclear reactions, such as what's disseminating from Japan's Fukushima Daiichi plant.

Here's a look at various sources of radiation exposure (dose of radiation in millisieverts (mSv)), according to data from the Health Physics Society and the U.S. Environmental Protection Agency (EPA). By way of comparison, a single dose of radiation below 0.01 mSv is considered negligible by the National Council on Radiation Protection and Measurements.

• Banana: 0.0001
• Dental X-ray: 0.005
• Living within 50 miles of a nuclear power plant: 0.01 (per year)
• A flight from New York to Los Angeles: 0.04
• Smoking 1 ½ packs of cigarettes: 0.08
• Chest X-ray: 0.1
• Living at sea level: 0.25 (per year)
• Mammogram: 0.3
• Living in Denver: 0.5 (per year)
• Abdominal CT scan: 14
• Measures between reactors No. 3 and No. 4 during the March 15 explosion at the Fukushima plant: As high as 400 per hour

What Level of Radiation Exposure Is Safe?

It's well-established that exposure to large amounts of radiation at once can cause acute sickness and even cancer. (A 1,000 mSv-dose can trigger acute radiation sickness, causing symptoms such as nausea and vomiting; 3,000 mSV can be lethal, according to Thrall.)

But there's no good data on the long-term risks of the low levels of radiation to which we're continually exposed.

According to the World Nuclear Association, annual exposure to 100 mSv or greater carries a measurable, though small, increase in cancer risk. Below that level, it's believed that your body's cells are able to heal themselves from radiation. "There are enzyme systems in the body that repair damage from these low levels of background radiation," says Thrall.

But even small levels of radiation exposure may impact cancer risks later in life.

This has been of particular concern in the medical community, where some experts worry that increasing use of diagnostic CT scans (which has skyrocketed from 3 million annual scans nationwide in 1980 to 70 million in 2007, according to MedPage Today) will impact future cancer rates. For example, in one 2009 study, National Cancer Institute researchers estimated that one in 270 women and one in 595 men who had a heart CT at age 40 would eventually develop cancer related to the test.

While the health benefits of necessary diagnostic imaging usually outweigh the small risks of secondary cancers, it's always a good idea to talk to your doctor before any procedure involving radiation to understand exactly what you're getting, why you need it, and what the potential health risks may be.

Bottom line: Americans are exposed to far more radiation in their daily lives — and especially from certain medical tests — than from dispersed particles traveling across the Pacific. "With what we know now about the situation in Japan, there are no personal or public health risks apparent for people in the United States," Thrall says.