The roentgen is defined as 2.58 x 10-4 coulombs of charge produced by X-rays or gamma rays per kilogram of air.
A roentgen is a lot of radiation. A dose of 500 roentgens within five hours will kill you. So a place with a reading of 100 roentgens per hour or more is very dangerous. Shortly after the Chernobyl nuclear disaster, readings of up to 30,000 R/hr were recorded in some areas.
Devices are usually calibrated in tiny fractions of one roentgen. There are a thousand milliroentgens to one roentgen. The reading will often be given in mR/hr. Flying at high altitudes exposes passengers to around 25 mR/hr, due to cosmic radiation.
Rad stands for Radiation Absorbed Dose. The units used here relate to the amount of radiation absorbed by the irradiated material. This may be you. One rad indicates exposure equivalent to an energy of 100 ergs per gram. It is about the same as 1.07 Roentgen, or 1,070 mR.
Rads are useful when assessing whether acute radiation sickness is a risk. A dose of 10 rads (10,000 mrads) in less than an hour is dangerous. Treatment for ARS will be needed.
The absorbed dose, measured in rads, is adjusted to give the Roentgen Equivalent in Man. The type of radioactive material is taken into account, among other factors. This unit is used to assess the chances of getting cancer from exposure. It is used to calculate safe levels in industry and medicine.
A rem is a large amount, so readings are generally given in millirem (mrem). The general public should not be exposed to over 100 mrem per year. This is just over the natural radiation levels inside a building made of granite.
Roentgens, rads and rems are very roughly equivalent. As the adoption of international standards became important, they were replaced by other units. Some countries, particularly the USA, continue to use the old ones.
part three ‘Sieverts and Grays’ to follow