One of the underlying reasons that we as a society have a fear of radiation is that x-ray radiation is invisible. You cannot tell you are being exposed to radiation through your five senses. It’s the original ‘Phantom Menace’, and it is precisely the reason that x-rays weren’t discovered until 120 years ago.
This fact also makes x-rays difficult to measure or quantify. The original unit of measure of radiation was skin erythema – or reddening of the skin. The only way that scientists could describe how much radiation they were working with was by how much it eventually turned their skin red. Luckily this is not what we use today.
Today, we measure x-rays by capturing the electrons that are knocked out of their atoms by radiation. These electrons create an electrical charge, which we know how to measure. In Geiger counters, this electrical charge is transferred to a speaker, which clicks. The first unit of measure of radiation based on this principle is named for the person who discovered x-rays– Roentgen. It is given the symbol R and refers to the number of ionizations (electrons being knocked out of their atoms) in air.
Radiation workers are often required to wear dosimetry badges that measure their exposure to radiation. Most dosimetry companies use a unit of measure called the rem, which stands for radiation equivalent man. This unit takes into account how much relative damage the radiation does to the human body. We all receive around one-thousandth of one rem, or one millerem, of background radiation each day living on planet Earth. Radiation workers are allowed to get up to 5,000 millerem (or 5 rem) of whole body dose per year.