Answer to Question #13060 Submitted to "Ask the Experts"

I have read answers to questions about radiation dose and safety for anyone flying at high altitudes. The answers did not make clear that the measurements were inside the plane rather than outside the plane. Some shielding is provided by the aluminum and Plexiglas of the plane. How does that shielding compare with that provided to us by our atmosphere at sea level?

To start, I've made measurements of cosmic radiation levels when I fly—mostly because I'm a geek and that's what we do. For what it's worth, I was also the guy wandering the black sand beaches in Hawaii with my radiation detector. Although my own radiation detectors only measure gamma dose, there's no doubt that the dose rate increased markedly at altitude compared to at sea level. If you're interested in the nitty-gritty details, dose rates dropped from sea level to about 3,000 meters (m) elevation, as the plane increased its distance from natural radioactivity in the ground. By about 4,000 m radiation levels were back to what I measured at the surface, and by the time we reached cruising altitude they were about 10–20 times as high as at the surface.

That's gamma radiation—but the majority of cosmic radiation comes from particles slamming into the atmosphere, and I can't measure those directly. The gamma radiation is an indirect measure of this particle radiation since photons (x rays and gamma rays) are produced during these collisions. So, let's think about what's happening to these particles as they encounter the airplane and the air in which it's flying.

The largest particles are things like the nuclei of iron atoms—these were likely formed by supernovae somewhere else in our galaxy (an exploding star produces as much iron as the mass of our entire sun). These will travel through space largely unchanged—until they plow into our atmosphere. There, they might hit an atom of oxygen or nitrogen and break up into smaller pieces, or they might just bounce around a little bit until they encounter the aluminum or composite skin of the airplane. Whenever they do break up, they produce a shower of particles and photons that continues to propagate through the atmosphere—in addition to smaller atoms, there are also neutrons, electrons, and x-ray and gamma-ray photons. Oh, and the smaller atoms that are formed when the iron atom breaks up will react similarly to the iron nuclei that started the whole thing.

Low-energy photons will be absorbed by the skin of the airplane—aluminum absorbs somewhat more than composite, but not that much more. So, the x-ray and gamma dose inside the aircraft will be a bit less than outside. But this isn't where most of the dose is coming from.

Neutrons will penetrate aluminum fairly well, but composites that contain lighter atoms can often deflect neutrons more effectively than will the metal. In all honesty, I'm not aware of any formal measurements that have been done along these lines—but I do know that plastics and waxes are very commonly used as neutron shielding, so it would not surprise me to find out that composites might provide a higher degree of neutron attenuation than aluminum.

And the atoms themselves—both the iron nuclei and whatever they break up into—are likely to either pass through the skin of the airplane or (more likely) to break up when they strike atoms on their way through. Here, too, I'm not aware of what studies have been done that specifically look at composite versus metal airplane bodies, so I have to guess. But since what's causing the cosmic rays to break up is their striking an atom, it seems reasonable to assume that it's the number of atoms they encounter that's important—not what those atoms are made of. In any event, it's likely that the material from which the airplane is constructed doesn't make much of a difference in the dose you receive on the inside of the aircraft.

Let's put all of this together to actually answer your question!

First—the skin of the airplane, whether aluminum or composite—will not provide much shielding from photon or neutron radiation. Composite might provide a little more shielding from neutrons and a little less from photons—but the difference is likely not very great. And the dose rate from both of these is likely to be similar both inside and outside the aircraft.

Second—the skin of the airplane offers a lot of targets to break up any iron nuclei that reach so deep into the atmosphere, as well as the fragments of iron nuclei that break up higher in the atmosphere. That being the case, the dose rate from these sources is likely no lower inside the airplane and might actually be a little higher.

Third—the fact that we can measure dose rates inside the airplane that are clearly higher than what we see at sea level (even the x-ray and gamma-ray doses alone) means that the skin of the airplane provides less shielding than about 10 km of air.

Having said that—the dose you receive flying doesn't seem to cause a serious health risk; according to radiation safety scientist and author Timothy Jorgensen, the risk that the world's most-traveled man (18 million miles and counting) will develop a fatal cancer from radiation received from his travels is less than 1%. The threat to your digestive tract and equanimity is perhaps a greater concern.

P. Andrew Karam, PhD, CHP