Table 1. Summary statistics of follow-up time and known confounders by PM2.5 (µg/m³) quantiles.

		Quartile of PM2.5
Variable	Level	6.6–12.2	12.2–13.6	13.6–14.8	14.8–21.0
Temperature (C),Mean(SD)		17.4(5.0)	17.9(3.5)	15.9(3.4)	16.2(3.1)
Follow-up (days),Mean(SD)		1675(736)	1672(674)	1682(662)	1772(649)
Season, %
	Spring	29.9	24.9	27.9	18.5
	Summer	29.1	30.2	27.3	29.5
	Fall	18.1	22.4	20.7	26.7
	Winter	23.0	22.5	24.1	25.3
Incident stroke, %		1.9	1.8	2.1	1.9