Table 3

Median maternal carbon monoxide levels and rates of breath carbon monoxide below and above cut-off by carbon monoxide sources

FactornCO ppm
(median, IQR)
CO <3 ppm (%)CO ≥3 ppm (%)P values
Smoking status†
 Never smoked1051.0 (1.0)93.36.7<0.001
 Ex-smoker1031.0 (1.0)86.413.6<0.001
 Current smoker4210.0 (8.5)***14.385.7<0.001
Exposed to passive smoking
 No1701.0 (1.0)85.015.0<0.001
 Yes381.0 (2.0)52.447.6NS
No of cigarettes smoked per day
 02081.0 (1.0)89.910.1<0.001
 1–5245.5 (8.5)***20.879.2<0.001
 6–101811.0 (6.5)***5.694.4
Time since last cigarette (hours)
 <1913.0 (11.0)0.0100.0
 1–21410.0 (8.0)0.0100.0
 3–695.0 (8.5)22.277.8
 >672.0 (11.0)*57.142.9
Smoked in previous pregnancy†
 No1911.0 (1.0)84.315.7<0.001
 Yes315.0 (10.0)***25.874.2<0.001
Uses a car or bus daily
 No312.0 (4.0)61.338.7<0.05
 Yes2191.0 (1.0)79.520.5<0.001
Lives beside main road
 No1041.0 (1.0)83.716.3<0.001
 Yes1461.0 (2.0)72.627.4<0.001
Lives in a built-up area
 No721.0 (1.0)84.715.3<0.001
 Yes1781.0 (2.0)74.225.8<0.001
Boiler serviced every year
 Yes1441.0 (1.0)86.014.0<0.001
 No501.0 (1.0)77.122.9<0.001
Uses fossil fuel fire
 No1181.0 (1.0)78.022.0<0.001
 Yes1321.0 (1.0)76.523.5<0.001
Chimney cleaned every year
 Yes531.0 (1.0)81.818.2<0.001
 No881.0 (1.0)81.118.9<0.001
Partner’s CO > 3 ppm‡
 <3 ppm281.0 (0.00)90.69.4
 ≥3 ppm261.0 (2.50)64.335.7<0.05
Partner/spouse’s smoking status†
 Non-smoker361.0 (1.0)87.512.5<0.001
 Current smoker151.0 (4.0)58.841.2NS
  • P values in final column indicate differences between CO ≤3 ppm and CO ≥3 ppm.

  • *0.05, **0.01, ***0.001.

  • †Based on self-reported smoking status.

  • -   The number of values for this variable was too small to statistically analyse 

  • ‡Missing data n=90.

  • NS, not significant.