Yes, I’ve been in military parades too and have seen people around me keel over from the heat. They always told us to take in some salt beforehand, and to never lock your knees when standing there (I guess to help blood circulation).
That’s what we did.Though it did’nt help me.In ‘72 in NJ I keeled over during a COC ceremony and when I came back too, the medics were draggin’me (not carrying)to the ambulance.Those were the days.