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Neither ANSI/ESD S20.20 or its ESD Handbook ESD TR20.20 mentions hats.
Wearing hats should be no problem, and likely is preferable to exposed hair which also can be a source of contamination.
ESD Handbook ESD TR20.20 section 2.3 Nature of Static Electricity includes “human hair” in its Triboelectric Series.
Ideally, all the hair would be gathered under the hat and not exposed. It would be better if the hat material was low charging. It would be better if the hat was left on at all times in the ESD protected area. But all these should not be necessary as even if the hat was high charging, the rule would be to keep the hat at least 12” away from ESD sensitive items. This should be easy to do with a hat on one’s head.