So for the time being, the only viable answer is complete avoidance.
So how far do you take this? Volcanic activity is always present somewhere on earth, so volcanic material is always in the air, in unknown concentrations. The only
complete avoidance is to stop all flying forever.
If flying visually, even light snow, tiny ice particles, wisps of cloud or fog, haze or smoke, such as that from coal fired power stations, can be seen and avoided; we do this routinely. If volcanic material were suspected to be present, common sense prevails.
A mandatory complete avoidance, such as occurred last time the Icelandic volcano erupted, is greatly over-cautious and a great over-reaction.