Hials, if you do a search on Pprune, you will find some Sixty odd pages of thread regarding the recent Icelandic volcano event. Somewhere in there are the permissible maximum concentrations in micrograms per cubic metre, as well as numerous bleatings from uninformed idiots whose holiday trip to Benidorm was disturbed.
Suffice to say that levels of ash that are invisible from the ground and may be invisible in CAVOK conditions can seriously shorten the life of first stage turbine blades and nozzle guide vanes.
You should also be aware that it may be lessors and insurance companies that are calling the shots.