The röntgen or roentgen (symbol R) is a unit of measurement for ionizing radiation (such as X-ray and gamma rays), and is named after the German physicist Wilhelm Röntgen. 1 R is the amount of radiation required to liberate positive and negative charges of one electrostatic unit of charge in 1 cm³ of air at standard temperature and pressure (STP). This corresponds to the generation of approximately 2.08×109 ion pairs.
The röntgen is accepted for use with the SI system but in this case its value is expressed in terms of the SI units charge divided by unit mass (C/kg) rather than as in the original definition. Although its use is allowable under the SI system, it is not itself an SI unit and continued use is "strongly discouraged" by the National Institute of Standards and Technology.
The röntgen was occasionally used to measure exposure to radiation in other forms than X-rays or gamma rays. To adjust for the different impact of different forms of radiation on biological matter, "röntgen equivalent man" or rem was also in use. Exposure in rems is equal to the exposure in röntgens multiplied by the Q value, a constant describing the type of radiation. The rem is now superseded by the sievert (see the latter for a list of Q values).
On this scale a dose of about 500 R in 5 hours is lethal for humans.
A typical dose of normal background radiation for a human is 200 mR per year.