James my car was covered in sand in the south of England before the eruption! A week ago!
What we need are density levels that are a threat to the flight not to long term engine life.
What density level of ash causes ash to be visible to the naked eye ? How does that compare with the existing maximum permitted levels.
Regardless the proven track record over 50 years shows that volcanic ash is not a major threat to life and certainly far less rhan many proven threats that we accept and live with.
Pace