CT Scan doses vary wildly depending on the type of X-ray (chest, head, limb, etc.), the size of the person, and a lot of other things. The bigger the cross-section of the thing you put in a scanner, the greater the exposure. CT machines themselves also vary considerably. Newer machines are often more sensitive and employ better imaging software, so they can do their job while delivering a much smaller dose. Anyone wh…
As someone now in Hong Kong about to fly back home to SF, I have always tried to schedule my flights so that I minimize my exposure to day time radiation. I guess I'm in the minority, but it's something to consider when taking international flights.
http://en.wikipedia.org/wiki/Cosmic_ray
Then, in 1912, Victor Hess carried three enhanced-accuracy Wulf electrometers[12] to an altitude of 5300 meters in a free balloon flight. He found the ionization rate increased approximately fourfold over the rate at ground level.[12] Hess also ruled out the Sun as the radiation's source by making a balloon ascent during a near-total eclipse. With the moon blocking much of the Sun's visible radiation, Hess still measured rising radiation at rising altitudes.[12] He concluded "The results of my observation are best explained by the assumption that a radiation of very great penetrating power enters our atmosphere from above." In 1913–1914, Werner Kolhörster confirmed Victor Hess' earlier results by measuring the increased ionization rate at an altitude of 9 km.