[palmer]

News flash: you don't need one indoors, either.

It's useful indoors when people are talking, singing, etc. There are well-documented super-spreading events for people spending a few hours together. When it is not useful is casual contact, 10 minutes or less.

[lepton]

When it is not useful is casual contact, 10 minutes or less.

---

Or when you aren’t coming close to others.