Unless the airplane makes a low pass over the immediate area you should be OK. The amounts of radiation being detected in the US are what they call measurable which to me means if you graph out the last 2 weeks you see a bit of a spike but still way below any significant amount.
You are right, the air does come from outside and is pressurized by the engines then air conditioned and filtered before entering the cabin. Don’t think for a second the filters catch radiation particles, they are no bigger than the O2 or N or any other molecules so there is no way to filter them out and still be able to breath!
If things get worse then the aviation authorities should route aircraft around the affected area much like routing around a volcano. I say should because you know how bureaucracies work sometimes.
Legal: The above is my opinion and is expressed only in laymans terms. If anybody reading this and is radiated thereafter you can’t catch me!