l'Otters
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It does matter if you're asymptomatic and infectious.I often don't bother in supermarkets as it's not actually effective unless most people are doing it. I'm sort of slack on the train too even though I always have them on me. I did wear one at the opticians while having glasses fitted but that involved someone adjusting glasses on my face.
Same goes for every other individual who isn't wearing one.
It matters if you're on the train as well, because it's aerosolised and whatever you've breathed out will be hanging around in the air after you've gotten off.