The annual limit for radiation dose for a member of the public is 1 mSv per annum or 1000 µSv per annum. If you are designated a radiation worker than you can receive up to twenty times this. I.e. 20 mSv per annum.
Not all radiation is the same, so scientists use the ‘sievert’ to measure the health risks of radiation. A one-sievert dose of radiation would cause immediate radiation sickness. But most radiation doses are much smaller, so you’ll see them measured in millisieverts or even smaller microsieverts.
1 sievert = 1000 millisieverts
1 millisievert = 1000 microsieverts