why are there so many units of measurement for radiation? Rads, roentgens, greys, severet, and so on?
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Rads are an English unit of radiation absorbed dose; grays are the metric unit for the same thing. They quantify how much energy was absorbed per unit mass of tissue.
Rems (English units) and sieverts (metric units) multiply rads/grays by a quality factor to calculate the effective dose. The quality factor accounts for the fact that different types of radiation is more or less damaging, and different types of tissue responds differently to radiation. Rems / sieverts are a measure of how likely it is that harmful effects such as cancer will result from exposure the radiation.
The roentgen is a very old measure of radiation exposure. It’s historically interesting, but isn’t used much any more.
There are so many units partly because we have been studying the effects of radiation on human health for well over a hundred years, and radiation itself for even longer. Early ideas about how to measure and quantify radiation and its effects have been supplanted by newer ideas and techniques (such as the metric system).
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