I spent 7 months working in a warehouse where we tested, among other things, the radiation leakage of microwave ovens. Microwave energy is non-ionising, so it does not cause cancer, but it is still dangerous since it is mostly used for cooking things.
We used handheld digital sensors which go on alarm when the emitted radiation is greater than or equal to 5 mW/cm^2. Briefly: 1 watt (W) is the transfer of 1 joule of energy every second (energy applied over time is known as “power”). 1W could power a Raspberry Pi computer or a 4 inch screen. 5mW is 200 times less than that (not very much).
1cm^2 is the area of the very end of your index finger if you point it at your face. The radiation limit means that, each second, a leaking microwave should not be able to apply more than 5 millijoules of energy to each square centimetre of your body which is close enough to be affected. If you lay on top of a leaking microwave, it would be applying that energy to every exposed square centimetre, every second.
Many brand new microwave ovens leak more than the recommended dose of radiation by the time they hit the shop.
This problem was so endemic that our employer eventually asked us to stop testing microwaves because they negatively affected testing statistics. Instead they would dump the whole stock in bulk. The sensors alert at 5 mw/cm^2 but only go up to 10; after that they just say “OL” for OverLoad. At least 1 in 20 of those I tested were OL. I have tested domestic ovens less than a year old which fail with readings of 7 or 8.
This level of radiation is not worth replacing your oven for. Anyway, new ovens are still likely to leak.
The safest ovens are small, simple models with analogue dials. I don’t know why. The highest risk are larger capacity models, which tend to be modern and digital, though this is probably a coincidence with their size.
The worst areas for leakage are the top of the door, and the left vent. Don’t rest your head near these areas ;)
Take 1 step away from the oven when it is on. This is enough to cut the power per area to less than 1 unit! (This is basically the main preventative step).
Look inside your unit. There is a sparkly piece of card on the right side – this is a filter. If it is burnt or missing, you should replace or return your oven.
While only anecdotal evidence, I hope this article has helped to inform about the metrics involved in microwave leakage.