Zap!

Happy Diagnosis Day! It's been 8 years since that fateful day when doctors discovered a 3 cm mass perched on my pineal gland. In that time, I graduated from college, went off to graduate school, married an amazing woman, got my PhD, and moved to France for a post-doc! In honor of the occasion, today I wanted to discuss radiation.

Part of my treatment included proton radiation, which I've talked about before (long before). While that exposure was medicinal, it's still best to limit overall radiation exposure to skin to about 50 millisievert (mSv) per year. For a couple years after my treatment, I avoided the TSA body scanners, opting for the manual screening. However, during a recent visit (to Michigan, not France) my friend Kevin wondered, How much radiation exposure do you get from flying in the upper atmosphere, compared to the security scans?

It turns out the TSA has actually used two different types of scanner in airports: x-ray backscatter, and millimeter wave. Up until 2012, the TSA mainly used x-ray backscatter machines, but switched to millimeter wave after manufacturer disputes. The EU actually banned x-ray backscatter in 2011 due to health risks.

Normal x-ray machines work by measuring the fraction of x-rays that pass through a material. Denser substances, like bone, absorb more of the x-rays and appear as white areas on the resulting photographic negative. Backscatter machines instead measure the rays that are reflected from an object, giving an image of the outer surfaces. The trouble with this is, x-rays are a form of ionizing radiation:

By Spazturtle - Own work, CC BY-SA 4.0, Link

Ionizing radiation means that it carries enough energy to damage cells, potentially resulting in cancer. This is why (in the EU anyway) these machines are no longer in use.

The alternative full-body scanning device is the millimeter-wave scanner, which use waves with a frequency of

On the chart above, this is just below the visible range, well into the non-ionizing region. Like the x-ray backscatter, these machines form an image by bouncing the waves off your body, and interpreting the reflected signal.

So, on to the exposure issue: The point Kevin brought up is that our sun puts out damaging ultraviolet light, and there are even more powerful cosmic rays coming from elsewhere in the universe. Typically, the Earth's atmosphere protects us from those sources, but by going high in the air, we strip away some of those protections.

The American Association of Physicists in Medicine released a report in 2013 comparing the exposure levels for a person of my size (5'10" and 160 lbs) on the ground, in the air, and in a scanner. The way they present their results though is a bit confusing (2.84 hour flight?). Instead, let's do some unit conversions to find how long, on the ground and in the air, it takes to equal the 11.1 nanosieverts they measured from an x-ray backscatter machine (which, remember, is no longer in use). On the ground, a person my size gets about 3.11 millisieverts per year, which means it takes 113 seconds to equal one scan. In the air, it's even less than that, 12.1 seconds!

There are lots of other issues with airport scanning machines, including privacy, and other possible health risks associated with radiation exposure. From the perspective of standard exposure limits though, you're a lot better off staying in the machine than going in the plane!