Table 3 Excess risks (ERs) and 95% confidence intervals of preterm birth per IQR increment in air pollutant concentrations in warm and cold periods in Ningbo, China

PM_2.5^b

4.84 (1.77,8.00) ^c

4.92 (1.47,8.50) ^c

4.06 (− 2.55,11.12)

4.53 (1.09,8.09) ^c

4.65 (0.71,8.74) ^c

3.17 (− 4.23,11.15)

9.40 (0.74,18.80) ^c

12.24 (2.40,23.04) ^c

− 1.93 (− 18.95,18.67)

PM₁₀^b

3.56 (0.07,7.17) ^c

5.11 (1.04,9.35) ^c

1.87 (−5.26,9.53)

4.88 (0.88,9.03) ^c

6.74 (2.12,11.58) ^c

2.64 (− 5.42,11.39)

13.22 (2.55,25.00) ^c

16.73 (4.50,30.40) ^c

0.65 (−19.38,25.66)

SO₂^b

3.65 (0.86,6.51) ^c

4.93 (1.64,8.33) ^c

1.44 (− 3.70,6.85)

6.56 (3.39,9.82) ^c

7.04 (3.29,10.92) ^c

5.53 (−0.32,11.71)

8.46 (0.21,17.40) ^c

10.95 (1.37,21.45) ^c

2.79 (− 12.58,20.87)

NO₂^b

6.49 (1.86,11.34) ^c

9.25(3.45,15.38) ^c

3.12(−5.08,12.02)

6.66(1.42,12.17) ^c

10.32(3.69,17.37) ^c

2.7(−6.26,12.51)

14.20 (1.18,28.90) ^c

7.40(−7.79,25.09)

−2.94(− 23.32,22.87)

O₃^b

− 0.90 (− 4.76,3.11)

−6.35 (− 12.78,0.55)

3.23 (− 1.81,8.53)

− 2.76 (− 7.1,1.79)

− 3.86 (− 10.69,3.49)

1.55 (− 3.91,7.32)

−4.11 (− 14.14,7.10)

−8.96 (− 24.85,10.28)

−1.23 (− 14.53,14.15)

CO^b

3.36 (0.50,6.30) ^c

5.16 (1.33,9.13) ^c

1.89 (−2.71,6.71)

0.37 (−2.84,3.70)

2.29 (− 1.16,5.87)

0.40 (− 4.60,5.66)

7.82 (0.85,15.28) ^c

10.44 (1.71,19.92) ^c

4.14 (−7.37,17.08)