Table 2. Comparisons of alveolar macrophages apoptosis among different subgroups based on age, smoking, initial working time, occupational time, and working age for occurrence in the exposed group.

Group	N	Apoptotic index	χ2/t or F	P
Age			0.597	0.551
< 40	156	27.22 ± 5.47
≥ 40	210	26.88 ± 5.25
Smoking (duration time, years)
0	165	26.52 ± 5.40	1.876	0.155
< 30	88	26.28 ± 5.07
≥ 30	113	27.62 ± 5.89
Initial working time (years)			0.791	0.429
< 20	144	26.69 ± 5.44
≥ 20	222	27.18 ± 6.00
Work years (years)			12.290	< 0.001
< 20	170	23.02 ± 5.12
≥ 20	196	29.98 ± 6.12
Working age for occurrence			1.403	0.161
< 10	130	27.33 ± 5.85
≥ 10	136	26.48 ± 5.48