Their view is that if you can't see it, it doesn't exist.
Perhaps I haven't been following the story as closely as I might have - Yours is the first mention that I've seen of caustic fumes emitted by the eruption. While I'd consider that highly likely, the only gas that I've seen mentioned is that dreaded pollutant, planet-killing CO2. :=)
Do you have any details as to the composition and concentrations of the other gasses produced?
You definitely were not following the story at all if you picked up on just the CO2 parts ~ http://www.philly.com/inquirer/front_page/20100423_Iceland_a_hot_spot_of_volcanic_activity.html
The amount of flourine emitted at this spot is unusually high. Deadly events elsewhere that appear to have involved the Iceland volcanic array include the destruction of Northern and Western Europe at the start of the Dark Ages (circa 535 AD) That event is reported in the Annals of the Kings of Britain. Later reports from British reports reveal that when resettlement was attempted in what is now called Brittany there were basically no animals, people or plants around anymore. Merlin found it necessary to replant all the vinyards (among other things).
More recently archaeologists have found that Sa'ami settlements in Southern Norway, Sweden, Finland and the Baltic states were present for thousands of years and then appear to have suddenly disappeared. This was before the Indo-European invasions!
If you read the article you'd notice that a modern era eruption killed people with poison gas in Britain (not just Iceland).
There's a reason almost all the people in the world with an Icelandic ancestor live in the United States or Canada.