How many Hiroshimas have you survived?

I've actually asked that question of friends, and it never fails to get a reaction. People are shocked to think it's even possible that they may have been exposed to the same levels of radiation as atomic bomb survivors.

But many of us have--and I'll tell you how to calculate your own "Hiroshima level" in a moment.

First, a new study finds you're not alone if you find the comparison hard to believe--because Americans are utterly clueless when it comes to the levels of radiation they've been exposed to through CT scans.

Most people don't even believe that these tests can cause cancer down the road.

Researchers asked 1,168 emergency room patients to agree or disagree with statements about CT scans and radiation, and then rank that statement on a scale of 0 (completely disagree) to 100 (completely true).

The majority of patients disagreed with this statement: "2 to 3 abdominal CTs over a person's lifetime can increase cancer risk."

That's despite research showing that every 1,000 CT scans produces one extra cancer case. The CT scans carried out in 2007 alone will eventually lead to 29,000 cancers and 15,000 deaths, according to government numbers.

More than three quarters of those surveyed also didn't realize that CT scans expose patients to more radiation than X-rays.

In reality, each CT scan packs the punch of more than 100 X-rays--and some can be 1,000 times more powerful.

Now let's get back to Hiroshima... because this one got the biggest reaction in the survey.

The statement, "approximately 2 to 3 abdominal CTs give the same radiation exposure as experienced by Hiroshima survivors" ranked at just 13 on the 100-point scale, meaning most people didn't think it was even remotely true.

But it is.

So here's how to figure out your own Hiroshima level: Take the total number of CT scans you've had and divide it by three--and that's roughly the number of "blasts" you've survived.

If it's any consolation, you're hardly alone--Americans are now exposed to far more radiation than ever, up 700 percent over 30 years. (Read more about that here.)

Even children are getting zapped like never before: A new study in the Archives of Pediatrics and Adolescent Medicine finds that the average child has more than seven scans involving radiation before the age of 18.

The news gets even worse, because 8 percent of kids have had a CT scan--and 3.5 percent of them have had two or more, giving them their first Hiroshima experience before they're even old enough to vote.

And while some children may really need those tests, the real tragedy is in the growing number cancer cases that will hit kids and adults alike because of tests they never needed in the first place.