Table 5.

Spatial-scale based upper limit warnings of $N_{event}$.

Type	ACH, h−1	Speaking		Breathing
		$N_{event}$-w	$N_{event}$-h	$N_{event}$-w	$N_{event}$-h
Buses	5	NRa/6/2	NR/NR/4	NR/19/6	NR/NR/NR
	10	NR/NR/4	NR/NR/7	NR/NR/NR	NR/NR/NR
	15	NR/NR/6	NR/NR/NR	NR/NR/NR	NR/NR/NR
Subways	10	NR/NR/7	NR/NR/12	NR/NR/NR	NR/NR/NR
	15	NR/NR/9	NR/NR/NR	NR/NR/NR	NR/NR/NR
	20	NR/NR/11	NR/NR/NR	NR/NR/NR	NR/NR/NR
Trains	5	NR/7/3	NR/13/6	NR/NR/11	NR/NR/22
	10	NR/12/5	NR/NR/9	NR/NR/NR	NR/NR/NR

^a‘NR’ represents not being required once spatial-scale based limit warnings of $N_1$ and $N_2$ are satisfied.