In case of a nuclear explosion, FEMA warns to try to keep a 6ft social distance and wear a mask for covid.
https://www.ready.gov/nuclear-explosion (I took this screenshot today). It says: Go to the basement or middle of the building. Stay away from the outer walls and roof. Try to maintain a distance of at least six feet between yourself and people who are not part of your household. If possible, wear a mask if you’re sheltering with people who are not a part of your household. Children under two years old, people who have trouble breathing, and those who are unable to remove masks on their own should not wear them.