An air advisory has been issued for Mammoth Cave National Park, not the cave itself. Looing at the link with the camera, it seems the problem is particulate matter.
http://www.wbko.com/news/headlines/7797031.html
Mammoth Cave National Park has issued another air pollution advisory. This advisory comes just days after their initial findings.
Park officials don’t believe the elevated pollution levels will affect the general public, but they could be harmful to certain groups. These groups include those with respiratory problems, those engaging in strenuous exercise and others who may be especially sensitive to pollution.
Officials also advise visitors and area residents to refrain from strenuous activities until the pollution drops to a lower and safer level.
The article also has a link to a live camera overlooking the Green River Valley:
http://www2.nature.nps.gov/air/webcams/ ... acacam.cfm
Again, this advisory is for the National Park, not the cave specifically.