What is the difference between rems and grays in radiation?

Which one is more commonly used to express medical radiation? Are there age/sex cohorts for radiation thresholds? IE If i'm 35 and 200 lbs are there higher limits to the amount of radiation that can be received? Looking for an algorithmWhat is the difference between rems and grays in radiation?
REM comes from the term "rontgen equivalent man". It's a unit of radiation dose equivalent. What that means is it's a way of converting radiation exposure from different radiation sources into one system. It is equal to the absorbed dose in rads x a weighting factor for the type of radiation. For most radiation people think of, the weighting factor is simply 1. So, basically 1 rad = 1 rem for all intents and purposes. Conversion from rads to gray: 1 rad = 1 centigray (cGy).



As far as radiation thresholds... there is no clear cut radiation threshold. Most of the standards are defined as what would cause 1 excess cancer death per 10,000 people. However, those thresholds are backwards extrapolated from data from Hiroshima survivors - and it's debatable whether it applies at low doses. You can read about radiation hormesis if you have more interest in that. I've put links at the bottom.



As far as weight goes - it doesn't affect how much ABSORBED radiation you can receive. However, lower energy radiation won't penetrate as far. So you would have to have a much higher skin dose to receive the same internal body dose. So that's hard to say to... but in general when discussing radiation induced toxicity and death, age and weight isn't a factor (unless you're talking about a child or fetus - they are more susceptible because their cells are dividing more rapidly).



Here is a website that lists radiation exposure limits in the US.



http://www.physics.isu.edu/radinf/risk.h鈥?/a>



there's a table at the bottom with limits. It also discusses risk estimates and ways of discussing exposure and dose.What is the difference between rems and grays in radiation?
radiation absorbed dosage

SI unit

1 gray = absorption of 1 joule radiation energy per kg of weight

1 gray = 1 J/kg



conventional unit

1 rad = absorption of 100 ergs radiation energy per gram of weight

1 rad = 0.01 J/kg

1 rad = 0.01 gray



radiation dosage with biological effects Q

SI unit

sievert = (gray)(Q)

conventional unit

rem = (rad)(Q)

Q=1 for x-ray

Q=20 for alpha radiation

Q=10 for unknown radiation



example: 20kg body received x-rays 0.40w for 10sec

dose = 0.40*10/20 = 0.20w-s/kg = 0.20 J/kg

1 rad = 0.01 J/kg

dose = (rad)(Q)

dose = (0.20/0.01)(1) = 20 rem