I think it means that, whatever his health issues are, being indoors with a very large number of people (so limited / recycled air circulation etc) is a bigger risk than being outdoors (with comparatively fresh air).
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How fresh the air is on Sunday at Glastonbury is another question.
Never managed to get tickets so haven't been on the forum much bit are they looking like a done deal?Â
If so I am gonna have to make sure I'm there one way or another!