One microsievert is exactly 0.001 millisieverts. This is the scale of individual measurements — a single X-ray, an hour of flying, a survey meter reading — and it is where radiation reporting most often goes wrong in public communication.
About the units
The microsievert is a millionth of a sievert. It is the working unit of survey instruments and of dose rates, usually expressed as microsieverts per hour. Natural background is roughly 0.1 to 0.3 µSv/h, so a handheld meter in normal surroundings reads in that range. The distinction between a dose and a dose rate is where confusion arises: a reading of 5 µSv/h is a rate, and what matters is how long someone stays in it. Five microsieverts per hour for a full year would be 44 mSv, well above the occupational limit; for ten minutes it is 0.8 µSv, less than a dental X-ray.
The exact factor
The factor is exactly 1,000. Reference values: a dental X-ray 5 µSv; a chest radiograph 20 µSv; an hour at cruising altitude 3 to 5 µSv; background 2,400 to 3,000 µSv per year. News reports of contamination incidents often quote instrument readings in microsieverts per hour without a duration, which makes the figure impossible to assess — and quoting a dose rate as though it were a dose is the single most common error in radiation journalism.
Where you meet this conversion
Survey instruments, environmental monitoring and dose rate mapping. Radiation protection surveys around medical and industrial sources are recorded in microsieverts per hour. Personal electronic dosimeters display accumulated dose in microsieverts. Environmental monitoring networks publish hourly readings in the unit, and comparing a measured value to the local long-term average is the correct first step in assessing whether anything has changed. Nuclear medicine departments measure patient dose rates in microsieverts per hour at a metre to determine when someone can go home.