After reading much of this thread, along with some other research, I can only conclude that it's going to be difficult to define a "safe" concentration.
Any outpourings from a volcano, encountered by a turbine powered aircraft, are likely to pose some sort of long term risk.
From:
http://www.caa.co.uk/docs/375/fsd_may93_p1-9.pdf
..these gases may remain suspended in the stratosphere for years after the solid rock particles have settled. The sulfur dioxide in the clouds absorbs water vapor and is converted into droplets of sulfuric acid.
From:
http://www.nasa.gov/centers/dryden/p...ain_H-2511.pdf
There was no evidence of engine damage in the engine trending results, but some of the turbine blades had been operating partially uncooled and may have had a remaining lifetime of as little as 100 hr.
(Interesting photos on page 1).
Sounds like flying in any aeroplane that has ever been flown through volcanic ash and/or gas is going to be less safe than one that has not.
I realise that most things in life involve risk assessments - I'm just glad I'm not the one with the responsibility of making the decisions here. I think it may be a little unfair to be slating people for taking the safe(r) route of maintaining the airspace suspensions.
Just my point of view.
OC619