How much radiation? The Earth’s atmosphere shields out most of the harmful cosmic rays, but as we go higher in the air, there is less atmospheric shielding, and thus less protection. As a general rule, total atmospheric radiation doubles with every approximately 6,500 foot increase in altitude. So with a few math calculations, the atmospheric cosmic radiation at FL390 is about 64 times greater than at sea level! Descending to FL330 cuts the radiation level down significantly to about 35 times more than sea level amounts. Therefore flying at lower altitudes results in less atmospheric radiation exposure than at higher altitudes. Some may conclude, “If I just fly at night, I won’t have to worry about any of this, right?” Sorry, but no. Since we are talking about cosmic radiation, it is present all around the globe, day or night.

Latitudes between 50 to 60 degrees is about where radiation levels “max out”, and at these latitudes and above, there is about four times more atmospheric radiation than if you were flying at the same altitude at 25 degrees latitude. Crews with regular flights from North America to Europe and Asia are routinely the most highly exposed to in-flight cosmic radiation.

Time aloft of course also affects your exposure; the more you fly, the more cosmic radiation exposure you will receive.

The other exposure variable is from solar flares. A solar flare of sufficient intensity to be of health concern is rare, with an average occurrence of about one a year, which raises radiation levels about 10 times above normal. This is roughly the equivalent of about one chest x-ray per hour at about FL350. A solar flare can last from a few hours to a few days. Solar flares of even greater intensity are more rare still, but can--and have--occurred The most powerful one ever recorded was back in 1956, and produced radiation levels at jet cruising altitudes of the astounding equivalent of about 100 chest x-rays per hour! Mild solar flares occur more often, but are only significant for the disruption they may cause to radio operation.

2003 iswap.org article