Rem

Rem, a unit of radiation dosage (from X-rays, for example) applied to humans. The unit rem, which is derived from the phrase Roentgen equivalent man, is now defined as the dosage in rads that will cause the same amount of biological injury as one rad of X or gamma rays. The rem, which had previously been poorly defined, was redefined in 1962 to clarify the use of the term relative biological effectiveness (RBE) in both radiobiology and radiation protection.

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Use of Rem 

The rem and millirem are the most widely used CGS units in the United States by the general public, industry, and government. However, outside of the United States, the sievert (Sv) is the standard unit, and it is becoming more common within the United States in academic, scientific, and engineering settings.

The standard unit for dose rate is mrem/h. Regulatory limits as well as chronic doses are frequently expressed in mrem/yr or rem/yr units, which are understood to represent the total amount of radiation allowed (or received) over the course of a year. In many occupational scenarios, the hourly dose rate can fluctuate thousands of times higher for a short period of time without exceeding the annual total exposure limits. Because of leap years, there is no exact conversion from hours to years, but the following are approximate conversions:

• 1 mrem/h equals 8,766 mrem/yr

• 0.1141 mrem/h equals 1,000 mrem/yr

The International Commission on Radiological Protection (ICRP) once recommended fixed conversion for occupational exposure, but this is no longer the case.

• 8 h equals 1 day

• 40 h equals 1 week

• 50 week equals 1 yr

Therefore, for occupational exposures of that time period,

• 1 mrem/h equals 2,000 mrem/yr

• 0.5 mrem/h equals 1,000 mrem/yr

The National Institute of Standards and Technology (NIST) of the United States strongly discourages Americans from expressing doses in rem, instead recommending the SI unit. The NIST recommends that the rem be defined in relation to the SI in every document that uses this unit.

Health Effect

Human health is affected by ionising radiation in both deterministic and stochastic ways. Only high doses (> 10 rad or > 0.1 Gy) and high dose rates (> 10 rad/h or > 0.1 Gy/h) cause deterministic effects that can lead to acute radiation syndrome. A deterministic risk model would necessitate different weighting factors (yet to be determined) than those used in the calculation of equivalent and effective dose. To avoid confusion, deterministic effects are typically compared to absorbed dose in rad rather than rem.

Stochastic effects, such as radiation-induced cancer, occur at random. The nuclear industry, nuclear regulators, and governments all agree that the incidence of cancers caused by ionising radiation increases linearly with effective dose at a rate of 0.055 percent per rem (5.5 percent /Sv). Individual studies, alternative models, as well as earlier versions of the industry consensus have all resulted in risk estimates that are dispersed around this consensus model.

There is a widespread agreement that the risk is much higher for infants and foetuses than for adults, for the middle-aged than for the elderly, and for women than for men, though there is no quantitative agreement on this. There is less data and much more debate regarding the possibility of cardiac and teratogenic effects, as well as internal dose modelling.

The International Commission on Radiological Protection (ICRP) recommends limiting artificial irradiation of the public to an average of 100 mrem (1 mSv) of effective dose per year, excluding medical and occupational exposures. Because of the uranium content of the granite structure, radiation levels inside the United States Capitol are close to the regulatory limit at 85 mrem/yr (0.85 mSv/yr). According to the ICRP model, someone who spent 20 years inside the capitol building would have a one-in-a-thousand chance of developing cancer on top of any other risk. (20 yr 85 mrem/yr 0.001 rem/mrem 0.055 percent /rem = 0.1% )

Difference between Roentgen and Sievert

The Gray represents the physical phenomenon of energy being deposited in matter by radiation. A different physical phenomenon, charge released by photon radiation in the air, is unitized by the Roentgen. The Sievert quantifies radiation risk and harm to humans.

The Roentgen (R) is a non-SI unit of exposure, which is the amount of charge released by photons in a volume of air. Exposure is relatively easy to measure with electronic instruments (such as ion chambers), which has historically been its utility. It is related to absorbed dose in materials in a fundamentally complicated way, but it is a happy accident of arithmetic that exposure of 1 R causes approximately 10 mGy of absorbed dose in human tissue. As a result, exposure remains a popular measured quantity in applied radiation protection. Only photons are defined as being exposed (gamma and x-rays).

In radiation protection for humans, the Sievert (Sv) is used to unitize various effective dose and equivalent dose quantities. Its dimensions are energy per mass, so it has fundamental units of J/kg as a SI unit. The quantities denoted by Sv have various meanings, but they all have one thing in common: they all quantify the risk or detriment (most notably the risk of fatal cancer) caused by radiation, and they can be thought of as modifications of absorbed dose that are weighted for biological significance (organ and radiation dependent).

Photons deliver a uniform absorbed dose of 1 Gy to the entire human body, resulting in an effective dose of 1 Sv (corresponding to about a 5 percent excess risk of fatal cancer). The effective dose is just one of the many quantities unitized in Sv.

Millirem to Roentgen

One millirem is equal to 0.00114 roentgen.