...any level....
Perhaps no aircraft should ever fly?
I remember about 10 years ago, when the dust storms in Beijing reduced visibility to less than 500m. Airbus was asked whether it was safe to continue. They said there was no limit. And, in my experience, there was no problem.
I hasten to add that sand and volcanic are quite different. So, I am not suggesting that flights into volcanic dust should be equated.
However - the density of volcanic plume is currently what? It seems to me that it is immeasurable. The sky is clear.
Where is common sense?