I.-The employer shall carry out measurements in the workplace when the results of the risk assessment show that exposure is likely to reach or exceed one of the following levels:
1° For the whole body: 1 millisievert per year;
2° For the crystalline lens: 15 millisieverts per year;
3° For the extremities and skin: 50 millisieverts per year;
4° For the radon activity concentration in the air for the occupational activities mentioned in 4° of article R. 4451-1: 300 becquerels per cubic metre as an annual average.
II-The purpose of these measurements is to assess:
1° The level of external exposure;
2° Where appropriate, the concentration of radioactive activity in the air or surface contamination.