In radiation protection, the rem (an abbreviation for Roentgen Equivalent Man) is the non-SI unit of the equivalent dose, which is used predominantly in the USA. The rem represents the equivalent biological effect of the deposit of one hundred ergs (one rad) of gamma rays energy in a kilogram of human tissue. The rem is not derived from the unit of exposure, the roentgen. The acronym is now a misleading historical artifact, since 1 roentgen actually deposits about 0.96 rem in soft biological tissue, when all weighting factors equal unity.
Examples of Doses in rems
We must note that radiation is all around us. In, around, and above the world we live in. It is a natural energy force that surrounds us. It is a part of our natural world that has been here since the birth of our planet. In the following points we try to express enormous ranges of radiation exposure, which can be obtained from various sources.
- 0.005 mrem – Sleeping next to someone
- 0.009 mrem – Living within 30 miles of a nuclear power plant for a year
- 0.01 mrem – Eating one banana
- 0.03 mrem – Living within 50 miles of a coal power plant for a year
- 1 mrem – Average daily dose received from natural background
- 2 mrem – Chest X-ray
- 4 mrem – A 5-hour airplane flight
- 60 mrem – mammogram
- 100 mrem – Dose limit for individual members of the public, total effective dose per annum
- 365 mrem – Average yearly dose received from natural background
- 580 mrem – Chest CT scan
- 1 000 mrem – Average yearly dose received from natural background in Ramsar, Iran
- 2 000 mrem – single full-body CT scan
- 17 500 mrem – Annual dose from natural radiation on a monazite beach near Guarapari, Brazil.
- 500 000 mrem – Dose that kills a human with a 50% risk within 30 days (LD50/30), if the dose is received over a very short duration.
As can be seen, low-level doses are common for everyday life. The previous examples can help illustrate relative magnitudes. From biological consequences point of view, it is very important to distinguish between doses received over short and extended periods. An “acute dose” is one that occurs over a short and finite period of time, while a “chronic dose” is a dose that continues for an extended period of time so that it is better described by a dose rate. High doses tend to kill cells, while low doses tend to damage or change them. Low doses spread out over long periods of time don’t cause an immediate problem to any body organ. The effects of low doses of radiation occur at the level of the cell, and the results may not be observed for many years.