Can I just make a few comments as a Geologist who has worked on a few active volcanoes? Apologies if similar comments have already been posted.
First, in no way does the Icelandic eruption compare with Pinatubo in 1991 or Galunggung (the BA B747 ash encounter) in 1982. Pinatubo had a VEI (volcanic explosive index) of 6, and was a once-in-a-century eruption; Galunggung was probably a VEI 4 to 5. The Icelandic eruption was probably a VEI 2 to 3 event. The VEI scale is logrithmic, so the Icelandic eruption was probably hundreds of times less powerful than Pinatubo. The Icelandic eruption took place over a week or so, which increases the total ash emission somewhat, whereas Pinatubo had a major highly explosive climactic eruption lasting only a day or so, but even so, Pinatubo and Galanggung emitted vastly more ash, and to a much higher altitude (50000ft+).
In the context of historic explosive eruptions, the Icelandic volcano is totally insignificant (at least so far!) In terms of human disruption, of course it's different matter. However, due to the location of Iceland on top of a major "hot spot" plume, or rising column of magma from the earth's upper mantle, eruptions in this part of the world can emit large quantities of lava or ash over time, due to the persistence of the magma column feeding the volcano, such as happened with the Laki fissure eruptions in the late 1700's. Now if the present volcano turned into a similar sized long-lasting event, we could have problems not just with ash but with vast volumes of SO2 and fluorine gases emitted, which could seriously impact food production due to reduced sunlight and toxicity.
A problem of assessing ash concentrations is that we are dealing with a four-dimensional problem, ie the eruption intensity and explosiveness is continually changing through time, as well as having a spatial effect. Volcanoes are notoriously unpredictable and explosive events are often in pulses with quieter periods in between.
I'm surprised that more effort wasn't made to obtain advice from Alaskan operators and the local VAAC who have to deal with explosive volcanoes from time to time.