For ISS astronauts, measurements from the crew’s personal dosimeters indicate a range from 12 to 28.8 milli rads per day.
The number is not a constant because radiation comes from more than one source. There are galactic cosmic rays (GCR) and solar cosmic rays (SCR). There is also radiation trapped within the magnetic field, auroral precipitation, and albedo radiation. Activity of the sun and location of the ISS both cause significant variations in the amount of radiation that impinges on the International Space Station. Radiation is higher at higher latitudes, so, since the ISS flies at an inclination of 51.6 degrees, it experiences more radiation than it would at 28 degrees (standard inclination for Space Shuttle flights). Altitude also affects the dosage. We’ve set a danger line at 500 km (312 mi). At 500 km, the average daily dose is around 50 milli rad (5x10E-4 Sv). Shielding within the different ISS habitable modules also varies.
There are locations over Earth, such as the South Atlantic anomaly (SAA), where radiation is higher at lower altitudes. In fact, a significant portion of the radiation experienced by the crew occurs when their orbit takes them over the SAA. At 440 km, within the SAA, the radiation can be 1000 times as intense.
Much of the solar cosmic radiation is deflected by the Earth’s magnetic field, which, fortunately, is above low Earth orbit (LEO). An astronaut on the moon could receive a lethal dose of radiation in just a couple of hours, during a solar flare.
The Multilateral Medical Operations Panel (MMOP) monitors and sets the standards for crew exposure. They track to separate limits that are referred to as deterministic and stochastic. Deterministic refers to acute effects such as rashes and burns and stochastic to long-term effects, such as cancer. It can get a little confusing because they use different units for each concern. But, for example, they set a 30 day exposure limit of 0.25 Sv and an annual limit of 0.5 Sv and a career limit of 1 Sv. A sievert (Sv) is equivalent to 100 rads.