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I'm obviously talking about realistic scenarios. There, the time is the one crucial factor--people sitting in an office in South Korea with air conditioning blowing the virus all over them, people sitting on trains, etc. Obviously, it's perfectly conceivable that someone might aggressively try to infect someone else, and I'm sure that's already happened (we had those spitting cases, although they were ruled not to have been the cause). It's not very important for general risk.
Short exposure has not been shown to be very risky. It's only prolonged exposure that appears to spread a sufficient viral load. The usual exceptions prove the rule.