Because the sound of X in English is that of a vowel sound. What this means is that you pronounce it "EX", with the sound of the "E" first. Ergo, one says an "ex". It's actually logical in this case...
An x-ray because X is pronounced as eks , that is, it starts with a vowel.
X-Rays are part of the Electromagnetic spectrum, along with radio waves, microwaves, infra-red, visible light, ultraviolet rays and gamma radiation.
George Washington
they can be; in large amounts or with no protection.