eli5: Safe radiation levels


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?

In: 11

The unit is Sv, sieverts, which is exposition to radiation measured in J/Kg so Sv/h is the exposure over time… mcSv/h means microsievert, the safe limit for working is considered 50mSv per year, so that’s 50.000mcSv so at 8mcSv/h it’s even considered safe for working

Over the course of human radioactive research there has been several units / measurements, the current one most used is Sievert.

*mcSv/h* would be a bit strange, because milliSievert is usually *mSv* and microSievert is usually abbreviated as *μSv*. I assume that *mcSv* is meant as *microSievert*.

8.4 mcSv/h would mean that the human body in that location takes 8.4 microSievert worth of radiation per hour (so 84 microSievert in a 10 hours perioud, and 201 microSievert in a 24h period.

As long as you do not stay in that area for very long this is not a dangerous amount of radiation. Around 10 μSv / microSievert is the normal daily dosis a normal human being received from natural background radiation each day (so you are just fine).

This was discussed already [here](https://old.reddit.com/r/explainlikeimfive/comments/phatb3/eli5_how_do_we_measure_radiation_like_the_unit_of/), so just a quote

> Ionizing radiation (at least for us humans) is measured in *Sievert*, most commonly in *mSv* (MiliSivert). from there it is further differentiated with the time frame you are exposed to radiation (Immediately, per hour, per year, lifetime etc). Science differentiates between different types / sources of radiation, from normal natural background radiation to external radiation

> The base assumption for many countries, depending on international regulations, is *1 mSv per year*from external sources as maximum. Natural Background radiation can be higher and comes from exposure to cosmic radiation and basically simply existing. At this level it is a normal part of human life and usually humans stay far below any dangerous levels.

> From there it can go in any direction. For example Astronauts on the ISS can get up to 80 mSv for a long stay, with 1000 mSv (or simply 1 Sv) being the lifetime limit. Firefighters in Germany can be exposed to 100 mSv in a single year, to 250mSv for a single incident and only if it is for saving lives, with 400 mSv being the absolute max for their professional life (they would usually be removed from the Hazard Unit after such a dosis). Countries may regulate that a single nuclear power plant may only add 0.3 mSv per year to the radiation exposure of their employees.

> Everything over 1000 mSv / 1 Sv should be firmly in the *Nope!* category.


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.

It might make more sense to think of exposure to loud noises damaging your hearing. We are all exposed to radiation all the time, but it’s low, like hearing noises under 90 dB all the time. It’s not likely to damage you. But up the exposure and you get damage. 100 dB once isn’t going to cause hearing loss, but work at a place with that constant exposure and you’re going to damage your hearing. Massive exposure can cause instant damage you will never recover from, like 160+ dB, same for radiation.

The very best chart explaining radiation doses was created by xkcd.