Paris Catacombs Air Quality

Well are you concerned about catching Covid on the plane, in the metro, on a bus etc... seriously it's a major city so the risk is there. You could do what I do routinely, if I am in a crowd I wear a GOOD mask. But as we now know Covid does not live that long outside it's host so the tunnels would probably be no more dangerous than anyplace else.

(And covid questions go in that forum, so expect this to be moved in spite of your creative title)

You should be at least as concerned about the steep stairs and the damp floor, and if you or the others are the slightest bit claustrophobic, send them through the tour and meet them at the other end where it comes out of the ground...

As for covid (and the other viruses circulating), we wore our masks on the Metro, and in crowded museums, in Paris last year. Otherwise we felt OK going without, including eating in cafes...and I would not bother in the Catacombs, either.

Good luck.

I agree with JoJo - the greatest concentration of visitors is on the spiral exit strairs. When I was there (2019) only 200 visitors were permitted at a time although there was no time limit for visits.

I've been in the Paris Catacombs twice (and the Roman Catacombs).

I cannot answer your question about Covid because I'm not a medical professional. However, on one of my visits, my sister was with me, and she is a physician. She had no qualms.

You have the option of visiting the Paris sewers where all of the city's wastewater from toilets, sinks, showers, dishwashers and so on goes: https://musee-egouts.paris.fr/en/. A face cover is recommended for it.

I think underground sanitation museums got an uptick in sightseeing interest since the Richmond F.C. visited the one in Amsterdam during a friendly away game on Ted Lasso.

Should I be concerned?

No more so than normal. If you do have qualms about it though, just wear an N95 mask to put your mind at ease. No one will think anything about you doing so.