On Nov. 11th, Tony Phillips of spaceweather.com flew from California across the USA to attend a science communications meeting in Washington DC. As an experiment, he decided to take a radiation sensor onboard the plane. The results were eye-opening. During the apex of his flight to DC, cruising 39,000 feet above the desert between Reno and Phoenix, he recorded a dose rate almost 30 times higher than on the ground below:

There was no solar storm in progress. The extra radiation was just a regular drizzle of cosmic rays reaching down to aviation altitudes. This radiation is ever-present and comes from supernovas, black holes, and other sources across the galaxy.

In a single hour flying between Reno and Phoenix, the passengers on Phillips's flight were exposed to a whole day's worth of ground-level radiation--or about what a person would absorb from an X-ray at the dentist's office. That's not a big deal for an occasional flyer, but as NASA points out, frequent fliers of 100,000 miles or more can accumulate doses equal to 20 chest X-rays or about 100 dental X-rays. 

If you plan a long flight, maybe San Francisco to Australia, for example, one might just want to check the status of solar activity.  During a big solar flare, a long flight at 40k feet might not be the best idea.