Table 3 Levels of airborne endotoxin (unit: EU/m³) and heavy metals (Al, Cr, Fe, Mn, Ni, Zn, and Pb) (unit: µg/m³) in different indoor places in subway environments.

Materials	N^a	Driver room			N^a	Station office			N^a	Underground tunnel			p value
Materials	N^a	GM (GSD)	Min	Max	N^a	GM (GSD)	Min	Max	N^a	GM (GSD)	Min	Max	p value
Endotoxin	16	0.9219 (5.6)	0.250	7.439	22	0.392 (1.5)	0.134	1.993	11	1.437 (6.1)	0.275	6.053	0.392
Al	16	0.856 (1.9)	0.232	2.177	22	0.292 (1.6)	0.104	1.689	6	0.930 (7.5)	0.144	18.315	0.014*
Cr	16	0.414 (1.2)	0.181	0.946	22	0.173 (1.1)	0.048	0.267	7	1.318 (2.0)	0.744	2.491	0.015*
Fe	16	11.360 (2.2)	4.335	53.427	22	1.853 (4.6)	0.317	5.112	9	14.003 (9.9)	1.656	722.384	0.013*
Mn	16	0.131 (1.3)	0.042	0.873	22	0.036 (1.1)	0.005	0.300	9	0.137 (13.5)	0.017	7.992	0.013*
Ni	16	0.054 (1.0)	0.030	0.092	22	0.018 (1.0)	0.010	0.028	9	0.046 (1.4)	<LOD^b	0.997	0.009*
Zn	16	0.079 (1.1)	0.022	0.419	22	0.066 (1.3)	0.013	0.892	4	0.448 (7.4)	0.063	4.286	0.017*
Pb	16	0.010 (1.1)	0.003	0.036	22	0.004 (1.0)	0.001	0.040	6	0.047 (1.1)	0.001	0.284	0.010*

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