Cosmic Radiation and Airline Crew Radiation Exposure
Published on Jul 17, 2006 at 11:12 pm.
10 Comments.
Filed under aeronautics, cosmic rays, physics, travel.
I had an idea to write about cosmic rays and cosmic ray exposure, but this post turned out to be a much larger project than I thought. Below you will find information about cosmic rays themselves, as well as the exposure that airline crews experience due to cosmic rays.
A century ago, scientists were just learning about radiation. Different substances were shown to emit radiation of various types and energies. Other substances were seen to act as shields to radiation. How effective a shield something turned out to be was shown to depend in part of the type of radiation and in part upon the intensity and energy of the radiation. But, there soon appeared a mystery. No matter how much shielding was used, there was always some radiation. Believing that perhaps the source of this residual radiation was from terrestrial background sources, radiation experiments were conducted on the surface of lakes, but this had no effect on the residual intensity. But, when the experiments were conducted deep underground, the residual radiation decreased. This did not seem to make sense if the source was terrestrial. Following a hunch, Victor Hess lifted radiation meters several kilometers into the air aboard balloons. He found that the radiation level dramatically increased with altitude. Furthermore, the intensity seemed to increase exponentially with altitude. This is what you expect if the atmosphere were acting as a radiation shield. So, this mystery radiation was apparently coming from outside the Earth’s atmosphere. He coined the term cosmic rays to describe this extraterrestrial radiation. Victor Hess won the Nobel Prize in 1936 for his work with cosmic rays. Later experiments showed that the radiation level actually peaks at an altitude of about 15 kilometers, somewhat higher than commercial aircraft fly. Though the radiation level peaks higher than commercial aircraft fly, they fly close enough to the peak that cosmic radiation levels can be hundreds of times higher than experienced at the surface of the Earth.
So, what are these cosmic rays? Well, we need to differentiate between what Hess discovered and what we find at very high altitudes. It turns out that that cosmic rays are extremely high energy atomic and subatomic particles moving at high speeds through space. When they enter Earth’s atmosphere, they collide and interact with atoms in the atmosphere. These interactions can result in the creation of new particles that then can either interact with more atoms or decay directly into even more particles. The original cosmic ray particle might even go blasting through interacting with many air atoms before slowing to the point that it doesn’t have much effect. Each interaction can create more particles or radiation. So, a single such particle can create a shower of other things. This multiplying factor is why radiation levels peak at about 15 kilometers. So, we differentiate between the original particles, called primary cosmic rays, and the created particles, called secondary cosmic rays.
So, what are these cosmic rays? Where do they come from? What effect do they have on us? In particular, since commercial aircraft fly at altitudes where the radiation level is much higher than on the ground, what effect does it have on the passengers and crew of those aircraft?
To start with, let’s think about the origin of the cosmic rays. That means looking at the primary cosmic rays to start with. These can be classified as three general types: galactic cosmic radiation, solar radiation, and anomalous cosmic rays. Each have a different origin, and they have a different effect when they strike the atmosphere. What are the galactic cosmic rays? Most of them, about 89% are pretty ordinary protons, only moving at extremely high speeds. About 10% are helium nuclei, and about 1% are heavier particles, mostly nuclei. In fact, the primary cosmic rays can consist of just about every imaginable type of stable nucleus, though nuclei heavier than iron are exceedingly rare. Most of the galactic cosmic rays are particles moving as speeds ranging from a little under half the speed of light up to speeds of about 99.6% the speed of light. A few, though, have much higher energies. One particle was found with an energy corresponding to a proton moving at 99.99999…995% the speed of light (the … there signifies that it is 99 followed by twenty three nines and then a five). This particle is nicknamed the Oh-My-God particle. Imagine a baseball moving at about 60mph. That is about the same energy as this super energetic particle, except that it is a subatomic particle having that same energy! It and a few others that have been found have energies that exceed what theoretical astrophysicists had believed was the maximum energy that a particle could carry without interacting with the cosmic background radiation, slowing down the particle and producing more subatomic particles. These super high energy cosmic rays are a complete mystery. But what about the ordinary galactic cosmic rays?
We don’t really know for sure the source of cosmic rays. However, we suspect that most come from supernovae explosions. Actually, they aren’t likely accelerated to such energies directly in the stellar explosion, but rather they are accelerated in the expanding supernova remnant that follows. As the cloud of gas expands for thousands of years after the supernova, particles within it collide with one another. These collisions randomly move energy from particle to particle. Some speed up and some slow down. A tiny few just by change speed up far more often than they slow down, and these become cosmic rays. This explains most of the ordinary galactic cosmic rays, but not the high energy ones, and certainly not the super high energy ones. Various models have been proposed for the high energy cosmic rays, including particles from jets from black holes, from hypernovae explosion jets, from particles given off by neutron star spin offs, and various other more exotic hypotheses (like those aren’t exotic enough!).
Solar cosmic rays, as the name suggests, come from the Sun. The Sun is always shedding particles into space, what we call the solar wind. Most solar wind particles are not really all that energetic, and so they are not a concern except to astronauts. Even solar cosmic rays that strike the atmosphere fail to produce sufficient secondary cosmic rays to be a major concern. Normally, that is. The Sun occasionally has a very violent explosive release of energy called a solar flare. These solar flares can bathe the Solar System in X-rays, and they can accelerate large chunks of the Sun’s corona into space, in what we call a Coronal Mass Ejection (CME). The more flares, the more CME’s. The bigger the flare, the bigger and more energetic the CME and the more other particles are flung around the solar system. If sufficient particles run into Earth’s near space environment, we have what is called a Solar Radiation Storm. NOAA’s Space Environment Center monitors such solar activity, and issues radiation storm watches and warnings. Normal solar wind isn’t much of an issue for Earthbound or aircrews. However, during and energetic solar radiation storm, the intensity and energy of these solar particles, mostly protons, drastically increases. For those of us on the ground, even solar radiation storms are not a problem. But astronauts can be exposed to very dangerous levels of radiation, and aircrew can experience major radiation dosage. During major solar radiation storms in 2003, aircraft flying at about 40,000 feet in North America and northern Europe experience a radiation exposure equivalent to about 2 or 3 chest X-rays, per hour of flight. Transatlantic flights, which flew quite far north on the so-called Great Circles, could be exposed to even higher radiation levels, with a single flight producing upwards of 25 chest X-rays worth of radiation. Just 4 such flights would expose passengers and crew to as much radiation as is generally allowed for an entire year for the general population from all sources. Just one flight would nearly reach the maximum radiation exposure that a pregnant woman should experience during a single trimester of pregnancy. That’s one flight, not a round trip. Fortunately, such radiation storms are quite rare. Most radiation storms occur near solar maximum. The Sun goes through a cycle of activity lasting about 11 years on average. However, statistically, the most powerful solar flares often occur on the downside of the cycle. We are just at the cusp of starting a new solar cycle, which should peak in about 2012, give or take a year. But, solar cosmic rays have an interesting interaction with the galactic cosmic rays. The interaction between solar cosmic rays and galactic cosmic rays tend to rob the galactic rays of their energy. So, when the solar cosmic rays go up in intensity, the galactic cosmic rays tend to decrease. This effect is called the Forbush decrease, after the physicist Scott Forbush, who first discovered this effect. At peak solar activity, the galactic cosmic rays can decrease by up to 30%. Since most solar cosmic rays are low energy, this has the strange effect that aircrew actually experience lower cosmic ray exposure during solar max than they do at solar min (we are now at solar minimum), assuming that they are not flying during the occasional radiation storm.
As mysterious as the super high energy galactic cosmic rays are the anomalous cosmic rays. We really don’t know where they come from. We suspect that they come from the edge of our Solar System, what we call the heliopause. This is the boundary between the region of space dominated by the Sun and the interstellar medium. It is believed that neutral particles may enter the heliosheath (the region around the magnetic boundary of the heliopause) and become ionized. They are then accelerated and become low energy cosmic rays. An alternate hypothesis is that some galactic cosmic rays interact with the buildup of particles along the heliosheath and slow down to be anomalous cosmic rays. As my post of a couple of days ago said, there are four spacecraft leaving the Solar System right now. Two are still working. Voyager 1 passed into the heliosheath a bit over a year and a half ago. Voyager 2 should do so in about two or three years. Unfortunately, the data from Voyager 1 is not really consistent with our models of anomalous cosmic rays. We don’t know if that is because of something strange in the particular part of the heliopause that Voyager 1 went through, or if perhaps we are totally off base on where these things come from. Hopefully, Voyager 2 will clear up some of the confusion.
But, unless you are an astronaut, or you own or operate a satellite, primary cosmic rays are not really a direct concern. They rarely survive to the ground. Rather, they interact with the atmosphere to produce secondary cosmic rays. These are the ones that you will normally have an interaction with. As I said earlier, a single cosmic ray particle, if energetic enough, can produce a large number of secondary particles. In fact, a single cosmic ray particle hitting the atmosphere with sufficient energy can sometimes shower nearly a square kilometer of the ground with particles. At sea level, a single square meter gets about 8 cosmic ray showers per second. You can see this effect with very high efficiency cooled digital cameras, such as astronomers use. Taking a image of nothing, with the cap on the CCD camera, you should see nothing. Instead, you get an image that looks like a sparse star field. There are little dots here and there across the image. These are pixels that are charged by interactions with these secondary cosmic rays. It is something that has to be taken into consideration if you are doing real science, and not just taking pretty pictures.
But, what are these secondary cosmic rays? Some are things like protons, electrons, and neutrons. However, these typically don’t go very far through the atmosphere, and comparatively few reach the ground. Most are a short lived particle called a muon. The muons that make it to the ground are very high energy particles, moving at very near the speed of light. They are very penetrating particles, too. You can’t really effectively shield against them. When I was an undergraduate physics major, I did a senior lab project in which I measured cosmic ray muons. I did the experiment in the basement of the physics building. So, these muons went through many miles of air, through two floors of a concrete and steel building, and into the basement where I measured them. Now these things are very short lived, so about the best way to shield against them is to go deep underground; that way they’ll have longer to reach you and will have a chance to decay. But, if you go higher, you are closer to the source, and so more will reach you. Muons are not the only secondary cosmic ray, though. There’s a whole zoo of things that can be produced. You can get a shower of protons, neutrons, pions, and muons. The pions don’t last long before they interact with another atom or decay into a muon and a neutrino. You also get high energy gamma rays. These are very high energy gamma rays, in fact, and were among the first type of secondary cosmic ray identified. When I was a child, the textbooks listed “cosmic rays†as the shortest type of electromagnetic wave. Now, we realize that these are just very high energy gamma rays. Something else interesting is what we call Cherenkov radiation. Many of these cosmic ray particles are moving at very close to the speed of light. That is, they are moving very close to the speed of light in a vacuum. Light moves a bit slower when passing through a medium. When a particle moves faster than light in a medium, it slows down and produces a form of light that we call Cherenkov radiation. This produces the blue glow around water moderated nuclear reactors. It also makes the entire sky glow a little bit. Astronomers are now using telescopes specially designed to study this Cherenkov radiation.
So, when I started this (rather long) entry, I indicated that I would talk about the effect of cosmic rays on aircrew and passengers. I’ve got several really good friends who are flight attendants or pilots. Naturally I am interested in their health if they are going to be irradiated like this. So, how much radiation do they actually get? A lot.
I did a little research to prepare this posting. I asked some of my flight attendant friends for some information on average flying time for their airlines, and I looked at documents from several government agencies, airlines, and aircrew associations. I found numbers all over the place for radiation exposure. I think that I will continue to research the topic even after this posting. To make the topic even more confusing, everyone was using different units. As my poor physics students figure out, this whole radiation business is confusing, since there are a multitude of ways of measuring radiation exposure. They don’t always give the same answer, even if you are measuring the same radiation. That is because they have different effects that they are looking at. Most of the figures I found from the airlines or the FAA, used a measure of radiation energy absorbed. I found some other data from scientists using relative exposure (this is a measure that includes the radiobiological differences due to different types of radiation). I converted from one to another to try to compare the results. So, the numbers that I give here should be taken with a grain of salt. They are definitely a back-of-the-envelope type calculation, but they should be in the ballpark. The FAA actually has devised a very good program to compute likely actual exposures per flight. This is really the best thing to use, since there are so many factors involved. Radiation exposure depends upon altitude flown, solar activity, and flight path. The flight path matters because the cosmic rays are charged particles, and so they are affected by Earth’s magnetic field. On average, the closer that you are to the Earth’s magnetic poles, the more the cosmic radiation you are experience. The FAA program to do the calculations is called CARI-6, and it can be found here. It isn’t terribly user friendly, but it is supposed to be very good. Another, web-based, calculator for determining radiation exposure per flight can be found here.
So, what did I get from all my research? Well, it seems that the average pilot or flight attendant gets a lot of radiation exposure. A whole lot, that is. As I said, the amount of radiation depends significantly upon the actual flights flown (altitude, flight path, and solar activity). However, using sort of average numbers, and assuming an average number of flying hours per month from my friends that I asked, I came up with some interesting numbers. Assuming that an average flight attendant flies about 85 hours per month, and about 1/3 of those hours are transatlantic flights, 1/3 are transcontinental flights, and 1/3 are medium range flights, and assuming typical altitudes for those flights, I came up with a figure of a little under 600 mrem per year for radiation exposure. More transatlantic flights drastically increases that figure, since those are the highest radiation flights. More flights in the New England, Canada, or South America also significantly increase radiation exposure, since those areas receive more cosmic radiation due to anisotropy in Earth’s magnetic field. Incidentally, this figure is near the upper range of what United Airlines has published as “typical†for a flight attendant, once all unit conversions are made. It is also almost 6 times the level at which someone is considered to be occupationally exposed to radiation. It is about 5 times the level that a typical nuclear reactor technician experiences. It is about what nuclear waste handlers get. This value is also just under 1/3 of the latest value recommended as the maximum recommended exposure for nuclear workers. The difference is that those people get a lot more education and a lot more monitoring of radiation exposure. And this is typical exposure, excluding solar radiation storms. When I compute a somewhat higher percentage of transcontinental and transatlantic flights, I get numbers of about 1 rem per year. This is the threshold value established by the EPA for radiation exposure to the general public for requiring emergency measures following a nuclear accident. Hmm. I guess this is why all my flight attendant friends have such “glowing†personalities! Lol!
So, what effect does all this radiation have? Health Canada reports that radiation exposure due to cosmic rays can produce about a 1% chance of a fatal cancer after 30 years of flying about 1000 hours per year. This is about what my friends report as average flight attendant flying hours. But, the human body has a remarkable repair mechanism for single gene radiation damage. However, certain cells are more susceptible to radiation damage than others. Bone marrow, intestinal linings, and hair follicles (and incidentally cancer cells, too) are particularly susceptible. That is why radiation sickness, or whole body radiation treatment for cancer, produce nausea, anemia, and hair loss. I once spoke with astronaut Susan Helms, who reported that after extensive time aboard the International Space Station, her hair had gray streaks in it (which gradually went away after returning to Earth). However, radiation can also damage women’s eggs or men’s sperm. This can lead to all sorts of reproductive problems. Also, the developing fetus is highly susceptible to radiation damage. The FAA Office of Aviation Medicine has released a document that is suggested reading for female aircrew who either are or expect to become pregnant. It can be found here. The International Commission on Radiological Protection recommends a maximum exposure during the entire course of pregnancy of about 05. milliSievert, which corresponds to 50 mrem. Using my typical data, this corresponds to under 10 transatlantic flights, or under 40 transcontinental flights. Short haul flights produce less exposure, so up to about 80 of them are OK, and it would take nearly 400 very short haul regional flights to yield that sort of exposure. Health Canada recommends less than 200 hours of flying for pregnant women. That figure obviously does not consider high radiation transatlantic flights.
So, with all this radiation exposure, what are we doing about it? Well, European airlines seem to be taking measures to limit radiation exposure to their employees and passengers. They are even assigning pregnant employees to ground jobs. During the major solar radiation storms of 2003, some transatlantic flights were held on the ground for several hours until the storms subsided. Flights in the air flew lower so that they’d get less radiation exposure. So, what are US airlines doing? Well, I guess that they are of the opinion that if you can’t see or feel radiation then it must not be anything to worry about. They don’t seem to be doing much. In fact, they seem to actively lobby the FAA to keep them form imposing the sort of restrictions that Europe uses. Some US airlines that fly to and from Europe, or who have code sharing agreements with European airlines are being forced by those agreements to do at some minimal radiation education activities. Looking around the internet, I find that the Association of Flight Attendants had been active in trying to get something done to protect aircrews. They have a web page here with some links on radiation issues for aircrew. The International Federation of Air Line Pilots is trying to promote a policy limiting radiation limits for flight crews to 20mSv per year. That sounds good, until you realize that 20mSv per year corresponds to 2 rem per year, the recommended maximum radiation exposure for commercial radiation workers and double the level that requires emergency response from a nuclear accident.
So, why so little response? It has been suggested by more than one person that perhaps the airlines are afraid of how the general public might respond to the news of radiation exposure on airplanes. They are already hurting economically, so they might fear anything that might reduce paying passenger flights. The reality, though, is that while the radiation levels due to cosmic rays are substantially higher in high flying aircraft than on the ground, we get a lot of radiation exposure due to various other sources all the time. For most passengers, the exposure on a single flight isn’t really all that much. They’d get about as much sitting too close to their TV. But, even most frequent fliers don’t fly even remotely as much as pilots and flight attendants. So, the aircrew are the ones that really get the high doses of radiation. A passenger would have to fly something like 60,000 miles to start to get significant radiation exposure. Most flyers don’t fly that much.
Incidentally, as I said, I asked some of my flight attendant friends for some information in preparing this. One of them, Flygurlual, actually got interested in the topic, and posted a blog entry of her own on the topic.
-Astroprof
Astroprof’s Page » Radiobiological Damage in Circumstellar Disks on October 9, 2006 at 2:11 pm: 1
[…] A while back, I posted about the effects of cosmic radiation on airline crew members. You can read that post, but the gist of it is that the higher you go, the less that the atmosphere protects you from radiation. The universe is filled with radiation of all types. Some of that radiation that hits Earth comes from the Sun, and some from outside the Solar System. The galactic cosmic radiation is going to be pretty much the same anywhere in the galaxy unless near a supermassive star or if there has been a supernova nearby. […]
Astroprof’s Page » The Aurora (Part 2: Origin of the Aurora) on November 12, 2006 at 4:57 pm: 2
[…] The magnetosphere deflects the solar wind past Earth. However, the solar wind itself is composed of charged particles, and these charged particles streaming past Earth produce an electric current whose magnetic field interacts with Earth’s magnetic field. An equilibrium is achieved, and the observed planetary magnetic field is in a sense a combination of both the magnetic field generated in the interior of the Earth and the magnetic field resulting from the solar wind. However, the solar wind is gusty, and so the amount of solar wind keeps changing. This means that the magnetic field in the magnetosphere keeps changing. The farther from Earth, the bigger the effect. Fluctuations in the global magnetic field are monitored and reported as a planetary K-Index. Major fluctuations in the planetary K-Index signify a geomagnetic storm. This data can be monitored on the internet at SpaceWeather.com or at NOAA’s Space Environment Center webpage. The bigger the planetary K-Index, the more likely the aurora, and the farther from the geomagnetic poles that an aurora may be seen. It takes a K value of about an 8 or 9 for an aurora to be visible from here in Texas. Also, the bigger the K-Index, the more radiation that airline flight crews are exposed to, and the further south and lower altitudes that intense exposure can be experienced. I did an extensive post on airline crew radiation exposure some time back. […]
Astroprof’s Page » A blast of radiation on December 8, 2006 at 2:40 pm: 3
[…] I’ve written before about airline crew radiation exposure. If you want to monitor current space weather, then you can do it at the Space Environment Center’s web page, and they have a special page just for aviation. […]
rose on July 13, 2007 at 5:24 pm: 4
I understand your rationalizing the radiation exposure in an airplane as somewhat normal, since we are always being exposed to some level of radiation anyway. However, being a flight attendant I am concerned, because I am being dosed twice. Yet, intelligent people are making radiation sound inconsequential being that it is a natural and unavoidable phenomenon.
rose on July 13, 2007 at 5:24 pm: 5
I understand your rationalizing the radiation exposure in an airplane as somewhat normal, since we are always being exposed to some level of radiation anyway. However, being a flight attendant I am concerned, because I am being dosed twice. Yet, intelligent people are making radiation sound inconsequential being that it is a natural and unavoidable phenomenon.
Astroprof on July 13, 2007 at 11:15 pm: 6
?????
Where did you get that I said anything about the radiation exposure being “normal” other than my saying that flight crew normally get excessively high levels of radiation far higher than is allowed more many other occupations?
Astroprof’s Page » Too much radiation? on March 31, 2008 at 11:58 am: 7
[…] People living at high altitudes, therefore, receive more radiation exposure than people living at lower altitudes. A couple of years ago, I wrote a rather long post about cosmic rays and radiation exposure for airlines. I was surprised in my research to find that airline crew members can receive more annual radiation exposure than many people working with radioactive materials. And, almost nobody wants to talk about that! […]
Zavatar on April 1, 2008 at 7:39 pm: 8
Very well written, Astroprof. Nice primer on cosmic radiation
Astroprof’s Page » GLAST: T minus 4 days on June 7, 2008 at 2:09 pm: 9
[…] the old charts of the electromagnetic spectrum showing cosmic rays at one end. But, the term cosmic rays has an entirely different meaning for us […]
Astroprof’s Page » IBEX on October 3, 2008 at 3:03 pm: 10
[…] are constantly exposed to radiation from space. The higher you are, on mountains or in aircraft, the more radiation you receive. And, changes in the heliosheath can change the amount of cosmic radiation that Earth receives. […]