Web Extra - Relative Radiation Doses

Radiation Exposure in Context

Talk about the ongoing problems with the Fukushima plants, following the earthquake and tsunami disaster in Japan, along with comparisons to Three Mile Island and Chernobyl. Radiation levels are often described as “X times the normal level” or “Y% over the legal limit" as in the case of this BBC article on the subject.

Such comparisons look alarming, make good news and can be very confusing.

A Chart to Aid Understanding:

Click the oversized thumbnail below and then click the resulting picture again to view it at it's largest size

I found this assessment of the relative risks, laid out in an easy to understand chart form, on the Webcomic site XKCD, which is frequently entertaining and thought-provoking.

The author, Randall Munroe, has kindly waived his copyright, so I thought it worth repeating here together with some of his commentary.

Just to be clear the chart is covered by a Public Domain Creative Commons License.

Randal Munroe disclaims any expert knowledge of radiation or its effects, where I think he has scored with this is in graphically representing comparative information in a way that makes it comprehensible. He provides a list of the sources from which he compiled the information for those who may wish to check them.

You can see further details, including a discussion page, on the relevant part of the XKCD site.

Note that there are different types of ionizing radiation; the “sievert” unit quantifies the degree to which each type (gamma rays, alpha particles, etc) affects the body.

It is also worth noting that the chart is not attempting to enable you to predict risk, as it takes no account of distance from a radiation source or the half-life of the radioactive substance(s) to which you may have been exposed. What it is doing is comparing the effect on the body of various doses of radiation from various putative sources, so that you can put into context the relative risks to human health of reported levels.

Another Perspective

A very different exploration of the same issues was featured on the BBC Radio 4 programme More or Less, which specialises in getting behind the use of numbers. This essay by David Spiegelhalter Professor of the Public Understanding of Risk, Cambridge University uses some similar data and even leads to some similar conclusions but goes about it quite differently.