How much ionising radiation is dangerous?

Radiation levels and their effects

The following table gives an indication of the likely effects of a range of whole body radiation doses and dose rates to individuals:

Sievert UnitsSeverity of Exposure
10,000 mSv (10 sieverts) As a short-term and whole-body dose would cause immediate illness, such as nausea and decreased white blood cell count, and subsequent death within a few weeks.
Between 2 and 10 sieverts in a short-term dose would cause severe radiation sickness with increasing likelihood that this would be fatal.
1,000 mSv (1 sievert)In a short term dose is about the threshold for causing immediate radiation sickness in a person of average  physical attributes, but would be unlikely to cause death. Above 1000 mSv, severity of illness increases with dose.
If doses greater than  1000 mSv occur over a long period they are less likely to have early  health effects but they create a definite risk that cancer will develop  many years later.
100 mSvAbove about 100 mSv, the probability of cancer (rather than the severity of illness) increases with dose. The estimated risk of fatal cancer is 5 of every 100 persons exposed to a dose of 1000 mSv (ie. if the normal incidence of fatal cancer were 25%, this dose would  increase it to 30%).
50 mSvIs, conservatively, the lowest dose at which there is  any evidence of cancer being caused in adults. It is also the highest dose which is allowed by regulation in any one year of occupational exposure. Dose rates greater than 50 mSv/yr arise from natural  background levels in several parts of the world but do not cause any  discernible harm to local populations.
20 mSv/yr Averaged over 5 years is the limit for radiological personnel such as employees in the nuclear industry, uranium or mineral  sands miners and hospital workers (who are all closely monitored).
10 mSv/yrIs the maximum actual dose rate received by any Australian uranium miner.
3-5 mSv/yr Is the typical dose rate (above background) received by uranium miners in Australia and Canada.
3 mSv/yr(approx) is the typical background radiation from  natural sources in North America, including an average of almost 2  mSv/yr from radon in air.
2 mSv/yr (approx) is the typical background radiation from  natural sources, including an average of 0.7 mSv/yr from radon in air.  This is close to the minimum dose received by all humans anywhere on  Earth.
0.3-0.6 mSv/yrIs a typical range of dose rates from artificial sources of radiation, mostly medical.
0.05 mSv/yrA very small fraction of natural background  radiation, is the design target for maximum radiation at the perimeter fence of a nuclear electricity generating station. In practice the actual dose is less.

Information courtesy of the World Nuclear Association