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Exposure time, physical proximity, air flow, amounts of saliva droplets in the air (i.e. is the The Plague Person breathing through their nose gently with a mask on or shouting at the top of their voice in full spittle-spray Alex Ferguson mode about why masks are useless). Those are the risk factors as I understand it. So if someone walks in without a mask and you tell them to fuck the fuck off ASAP, you're probably OK.
Yes, the level of risk is mostly related to exposure time. If you just pop into shops very quickly, it's low.