Table 2 Subgroup analysis for the relationship between PM2.5 and COPD hospitalizations

Study location

 Asian

[31, 34,35,36,37,38]

5

1.008 (1.005,1.010)

 < 0.001

218.34

98.16%

 < 0.001

 Non-Asian

[25,26,27,28,29,30, 32, 33, 39]

14

1.006 (1.003, 1.010)

 < 0.001

130.46

90.03%

 < 0.001

Study design

 Case-crossover

[27, 32, 33, 38, 39]

8

1.014 (1.010, 1.018)

 < 0.001

3.97

0.00%

0.087

 Time-series

[25, 26, 28,29,30,31, 34, 36, 37, 35]

11

1.005 (1.003, 1.007)

 < 0.001

329.87

96.97%

 < 0.001

Season

 Cold season

[25, 30, 39]

3

1.033 (1.006, 1.061)

0.017

3.758

46.78%

0.153

 Warm season

[25, 30, 39]

3

1.037 (1.003, 1.074)

0.035

1.600

0.00%

0.449

Lag days

 0 day

[25, 27,28,29, 31,32,33,34,35,36,37,38,39]

17

1.007 (1.005–1.009)

 < 0.001

321.751

95.02%

 < 0.001

 1-day

[25, 34, 38, 39]

4

1.005 (1.000–1.009)

0.034

9.446

68.241%

0.024

 2- days

[25, 34, 37,38,39]

4

1.006 (1.001–1.010)

0.101

5.437

44.824%

0.142

 3-days

[25, 34, 37,38,39]

4

1.005 (1.001–1.010)

0.017

0.421

0.000%

0.936

 0–1

[25, 34, 37, 38]

4

1.009 (1.004–1.013)

 < 0.001

6.696

55.194%

0.082

 0–2

[25, 34, 38]

3

1.008 (1.003–1.014)

0.004

1.934

0.000%

0.380

 0–3

[25, 34, 38]

3

1.010 (1.004–1.016)

0.002

1.120

10.686%

0.290

Confounders

 Humidity and temperature

[26,27,28,29,30,31,32, 34, 35, 37,38,39]

16

1.007 (1.005, 1.009)

 < 0.001

346.628

95.67%

 < 0.001

 Others

[25, 33, 36]

3

1.012 (1.005, 1.019)

0.001

1.228

0.00%

0.0541

Ages

 All ages

[25,26,27,28,29,30,31, 31,32,33,34,35,36,37,38,39]

18

1.007 (1.005–1.009)

 < 0.001

325.721

94.781%

 < 0.001

  ≥ 65 years

[31, 34, 39]

3

1.030 (1.023–1.036)

 < 0.001

97.503

97.949%

 < 0.001