I found this funny tool on the net while surfing. Now you can see how harmful a flight might be... @:-)

European Program Package for the Calculation of Aviation Route Doses

With this computer program you may calculate online the dose, which you would receive along a specified flight due to cosmic radiation. Additionally, you may determine the dose, which is accumulated during a stay of one hour at any flight position in the atmosphere.

http://www.gsf.de/epcard/eng_start.php