http://www.swpc.noaa.gov/info/RadHaz.html

Bruno,

This site has a table identifying min and maximum exposure levels at
varying altitudes in 10,000 foot increments and for 35 and 70 degrees
latitude. It offers a sample calculation for increased cancer risk,
and they estimate about 1 additional death per 100,000 people for
aircrews which may have experienced a maximum solar event. It is my
interpretation that the additional exposure risk is quite low. They
also give a link to a site which tracks Radiation levels. If you're
really concerned, you might want to add that site to your pre-flight
list of resources, and then if a really good day corresponds with a
maximum solar event, try and stay low that day.

-Kevin