I was thinking the other day about the eruption of Eyjafjallajökull in Iceland, and wondering what the actual levels of ashfall in the UK had been. This was prompted in part by the 30th anniversary of the eruption of Mt St Helens, in the northwestern USA, on 18 May 1980, and my childhood memories of walking through piles of ash and scooping it into jam jars (we lived in Washington state at the time); and also by a question that we received at IHRR about the health effects, if any, of Icelandic ash. After the current eruption began on 14 April, my sons and I put pans out in the back garden to catch any Icelandic ash that fell, and they were disappointed to find only leaves and dead bugs. But there have been a few reports of ashfall at ground level, and I wondered what the concentrations had been.

I should say at this point that I am not doing any research on the health effects of the Iceland ash plume – so what follows is no more than the musings of an interested citizen. I spent some time looking around on the web, and came across the UK Air Quality Archive, which has several good summaries of ground measurements taken during different phases of the eruption:

http://www.airquality.co.uk/

What those data show pretty convincingly is that the increase in levels of particulate air pollution measured by ground stations (typically expressed as the concentration of particles that are less than 10 microns across – known as PM10) has been nil to minimal, even at times when the plume has been observed over the UK. Any increase – for example, that seen in Northern Ireland on 4 May – is still within the typical daily noise.

For comparison, the German aerospace agency DLR measured peak concentrations of 60 micrograms per cubic meter over Germany and the Netherlands on 19 April (mention is made in this report):

http://www.nature.com/news/2010/100427/full/4641253a.html

This is measured quite high in the atmosphere, at typical heights at which commercial aircraft fly, and would therefore be much higher than the levels measured at the ground. (Bear in mind, too, that a microgram is a millionth of a gram). On the same day, the Met Office measured concentrations of up to 130 micrograms per cubic meter over Scotland, but only 5-25 micrograms per cubic meter over England – these were limited to high elevations and did not extend down to the surface:

http://www.metoffice.gov.uk/corporate/pressoffice/2010/volcano/plane.html

It’s useful to look at those numbers in contrast to levels of particulate air pollution in the world’s major cities, which are often greater than 100 micrograms per cubic meter even when averaged over a full year. There are lots of data on this in a World Bank report available here:

http://econ.worldbank.org/WBSITE/EXTERNAL/EXTDEC/EXTRESEARCH/0,,contentMDK:20785646~pagePK:64214825~piPK:64214943~theSitePK:469382,00.html

The maximum allowable 24-hour concentration of PM10 in the US is 150 micrograms per cubic meter, which is less than any of the ash measurements at altitude over the UK, let alone at the ground.

This is not to say that the ash poses no health risks at all, but to point out that we need to keep this in perspective. A straight comparison between ash concentrations and PM10 measurements is not really fair, because particulates in cities come from a diverse range of sources, and many studies have shown that it is not just the particle concentration that matters in determining health hazard, but also the particle shape and composition. But the basic message is that the levels of ash concentration, even high in the atmosphere, are generally lower than those experienced daily in many cities worldwide. It actually turns out that proper toxicological assessment of volcanic ash is very difficult to do, and published studies can come to very different conclusions using data from the same volcano. One thing that is clear, however, is that short-term and incidental exposure, even at fairly high concentrations, is very unlikely to cause significant problems – the majority of documented health effects from ash come from long-term exposure (e.g. quarry workers, residents of active volcanoes where dust storms are frequent, etc.). And this is where we can breathe a little more easily (so to speak), because in volcanological terms, the exposure of the UK and Ireland has been quite brief – measured in terms of weeks, rather than years. A small source of comfort if you find yourself stuck in an airport terminal somewhere because of a cancelled flight.