I was watching this show called dark tourist on Netflix and in their Japan episode they visited the Fukushima area and received readings of up to 8.4 mcSv/h. My question is firstly what do the units even mean, and secondly how dangerous are these levels of radiation?
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The units are a bit complicated, but 8.4 microSieverts per hour isn’t something to worry about for short periods. I wouldn’t hang out there, but I also wouldn’t be worried about spending a significant amount of time there. Your normal dose of background radiation from living on the Earth is about 3 milli-Sieverts which is 3000 micro-Sieverts. You’d have to be in that area for a little over two weeks to pick up a year’s worth of radiation, so a visit to pretend you’re brave for being there isn’t a significant dose.
Anyway, radiation units. There are two things to measure when it comes to radiation: exposure and dose. Exposure is the amount of radiation energy you absorbed. It’s measured in Grays and one Gray is 1 Joule of radiation per kilogram. This is a tremendous amount of radiation and getting it all at once would give you pretty serious radiation sickness, but you’d probably survive. To make things more manageable we use milli-gray and micro-Grays.
That’s not the whole story because different types of radiation will affect you differently. For example, absorbing 1 Gray of neutrons is 2-10 times more damaging to your cells than 1 Gray of gamma rays. The Sievert is just the Gray times that damage factor, so 1 Gray of gamma rays is 1 Sievert and 1 Gray of neutrons is 2-10 Sieverts.
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