In answer to Mr Happy; Jet planes and turboprops are affected more severely than prop driven aircraft. The reason is that the rock and silica particles melt at jet engine temperatures, then solidify on the turbine blades in the engine. This can cause imbalance, vibration, seizing and engine shutdown. In a piston engined plane, the air filter takes care of most of this, and if that is changed regularly there shouldn't be a problem.
On a wider point; there is still no solid research evidence one way or the other about whether it is safe to fly at the moment, just predictions from Met Office models and inconclusive findings from a couple of publicity stunt flights by the airlines. This is counterbalanced by reports of military aircraft having problems. The decision to reopen air travel seems to have been taken on the grounds that the shutdown is costing the airlines a great deal of money (they have lobbied very hard indeed), and the travelling public are being greatly inconvenienced. History suggests that when we let profit-motivated business organisations take the prime role in what are essentially moral decisions, people suffer and occasionally die. I have a bad feeling about this one.
Tim