I heard that flying on an airplane exposes you to lots of radiation. true or false, and if true then why?

Radiation in planes: People in airplanes are exposed to more radiation than people on the ground, especially those at sealevel, because there is less atmosphere to lessen the radiation from space. Those living at high altitudes are also more exposed. (amount doubles with every 6, 000 feet of altitude)the amount of radiation depends on the length of flight & number of flights. Single trip should not be a concern.

Somewhat true: It is true that you are exposed to radiation when on an airplane, but something like 160 hours of flight per year would be needed to expose you to enough radiation to be considered over the maximum recommended per year. The reason for this exposure is due primarily to being at higher altitude. This decreases the absorption of radioactive particles in the atmosphere the higher up you get.

Bottom line: I would not be concerned unless you fly extremely often and at higher altitudes. The higher the altitude, the less the atmosphere absorbs radiation but it takes over 150 hours of flight to achieve any significant levels of exposure.