Every day we live we receive a small dose of radiation from cosmic rays. This is an inevitable part of life. But when we rise into higher altitudes, for example when flying on an airplane, the doses of radiation we receive become a lot higher. To the extent that European flight attendants who happen to be pregnant are prohibited by law from spending more than a certain number of hours in the air. It turns out that the “safe” number of hours you can spend in flight while pregnant is not that large, so you don’t have to be a flight attendant to exceed that threshold in nine months. Let’s look at the numbers to determine how much flying is safe.
Sievert (Sv) is the unit commonly used to measure radiation. The amount of background radiation we accumulate simply by living is about 1/1000 of a Sievert per year, or 1 millisievert (mSv). On a normal transcontinental flight at 10-12,000 meters, the radiation is 100-300 times as high as at sea levels. During a roundtrip flight from Paris to San Francisco, the amount of radiation you receive is about 140 microsieverts (uSv). On a roundtrip flight between the East and West coast of the US, you receive about 80. Interestingly, you receive lower radiation on flights closer to the southern hemisphere, because the Earth’s magnetic field, which reflects radiation, is stronger in that part of the world.
So let’s say you are a modestly frequent flyer, and you take an equivalent of 1 roundtrip flight a month across the North American continent. As a result, you accumulate about an additional 1 mSv of radiation. So in total you get twice as much radiation as you would normally receive as part of living.
How much extra radiation is safe?
If you are a “normal”, i.e., non-pregnant person, you probably shouldn’t worry about in-flight radiation. Permanent damage to organs occurs when a person receives around 1-2 Sv. And if you take 12 cross-North-America flights, you receive only 1/1000 of that. But, like many things, radiation is not so simple if you are actually pregnant. Radiation is a lot more damaging to a developing fetus than it is to an adult. So “safe” radiation doses become much smaller if you are carrying a child.
While experts agree that really bad things like birth defects occur only if you approach about 20 mSv of radiation (e.g., more than 20 roundtrip cross-coast flights per month), there are other rather nasty things like early childhood cancer (see ), whose probability can increase if the fetus is exposed to radiation while in the womb. And here, it’s a lot more difficult to say how much radiation is really safe, but the consensus is that the more you get, the higher the odds. So people who study these issues recommend pregnant women to stay below 1 mSv of extra radiation during pregnancy. This is probably a conservative limit, but if the stakes are so high, it’s probably best to be conservative.
In Europe, it’s actually a law that prohibits pregnant flight attendants to exceed 1mSv of radiation during pregnancy. In the US, there is an equivalent FAA recommendation. So the rule of thumb, if you are pregnant, not to exceed 12 cross-continent round-trip flights or 7 long trans-continental flights during your term. We could stop right there, but there is a caveat. And that has to do with solar flares.
Solar flares and in-flight radiation:
The way I understand it, a solar flare is a period of increased activity on the Sun when the Sun spews out more particles than it normally does. If you happen to be in flight during a solar flare, you can receive as much as 200 uSV of radiation per hour. So on a 5-hour flight between the East and West coast you absorb a whopping 1000 uSV, or 1mSV – your threshold for the entire pregnancy! Pretty bad.
Solar flares don’t happen every day, but they do occur quite regularly. There are websites that report them, and you can even get a smartphone app that alerts you whenever there is a solar flare. Women who are pregnant in 2013 should especially watch out, because NASA predicts a massive solar flare in 2013 .
So, to conclude, don’t fly too much if you are pregnant (12 round-trip cross-coast flights is the threshold to go by), watch those solar flares and cancel your flight if one is happening, especially in 2013.
 In-flight radiation and European laws: http://www.sievert-system.org/WebMasters/en/exposition.html
 Radiation during solar flares: http://hps.org/publicinformation/ate/faqs/solarflare.html
 In-flight radiation exposure during pregnancy, Barish RJ. Obstetrics and Gynecology. 2004 Jun;103(6):1326-30.
 In-flight radiation calculator: http://jag.cami.jccbi.gov/cariprofile.asp
 NASA predicts massive solar flare in 2013: http://space.about.com/od/sunsol/a/Solar_Flare_2012_Conspiracy.htm