Answer to Question #11997 Submitted to "Ask the Experts"

I am exposed to security screening machines twice or thrice per day as I use the underground train daily. Is that a huge dose that I should avoid? I have been exposed to it for eight months and I will be exposed to it for the coming three months.

You raise an interesting question, so let me see if I can help out. But first, let me digress a little bit.

Several years ago I was traveling with a radiation detector. Since it was fairly expensive, I packed it with my carry-on luggage. I took it out at the security checkpoint and explained its purpose. The screeners asked that I turn it on and pass it through the x-ray detector to confirm my story. At the other side, everyone clustered around and asked me how much dose it received going through the x-ray machine. I showed them—it was about 50 microroentgens (µR, the units of radiation exposure that my detector uses). Translated into dose to a person, that's about one-half of 1 microsievert (µSv). But I was puzzled by their interest.

So I asked if they were moonlighting by offering x rays to people on the night shift. They laughed and said no. Then they said that every now and again they would see a small child on their screen—it seems that sometimes a parent would put a child's car seat on the belt and, in the bustle of getting through the security scan, the seat would be swept into the machine before the kid could be removed!

They also asked me about the impact of these x rays on a child. I acknowledged that children are more sensitive to radiation than are adults, but I also pointed out that each kid was going to get much more radiation during the flight than in their passage through the x-ray machine. So, I'm not saying that this is something to be taken lightly, but we do have to keep it all in perspective.

Now, let's get to your question. There have been a number of studies done on this topic due to the ubiquity of x-ray screening, not just in the United States, but around the world. I have done surveys myself on a number of different types of x-ray screening machines—at a military base, in prisons, and on police equipment—and in every case the dose was very low (on the order of 0.1–0.2 µSv) and was certainly too low to cause any health effects.

I will point out that the level at which radiation can start to cause health effects is fairly high—a lifetime dose of 1,000,000 µSv gives you about a 5% chance of developing a fatal cancer sometime in the next few decades. This is not a trivial risk, but there's still a 95% chance that there will be no effect from the radiation. And to get to this level of risk would require over a million exposures in the machine.

Another thing I'd like to point out is that everybody is exposed to background radiation all the time—from radioactivity in the rocks and soils, from cosmic radiation, and even from radiation in their own bodies. This exposure to natural radiation comes out to about 10 µSv daily. If you pass through the x-ray screening machine five times a day and receive 0.2 µSv each time, then you're picking up another µSv every day and about 200–300 µSv every year. This is much less exposure than I got from flying last year and comparable to the amount of radiation you'd get from a medical x ray, but far less than you'd get from a whole-body computed tomography (CT) scan.

In addition, keep in mind the reason these x rays are being performed: to help reduce the very real risk of a terrorist attack. So as an individual, and as a society, we have to try to balance these risks. A successful attack against a subway can cause tens of deaths and hundreds of injuries; the x rays are meant to avert this threat.

Finally, here are a few documents that touch on this topic that you might be interested in:

• The Health Physics Society has written position papers on both low-dose radiation exposure and the use of radiation for security screening. These are both relatively short, nontechnical, and worth reading.
• Roger Clarke, past chair of the International Commission on Radiological Protection, commented on very low doses of radiation, suggesting that "if the most exposed representative individual is sufficiently protected from a given source, then everyone else is also sufficiently protected from that source." In other words, it's not appropriate to claim that giving a very small dose of radiation to a huge number of people might still result in harm to a couple of people—no more than we can say that throwing a million very small pebbles at a million people will crush a few people because the aggregate weight of all those pebbles comes out to a ton.

P. Andrew Karam, CHP, PhD