Carbon monoxide (CO) is a colourless, odourless gas that poses a threat to life at concentrations of just a few hundred ppm. The developing foetus is particularly vulnerable to CO exposure, and maternal exposure to much lower levels of the gas is associated with adverse outcomes such as low birth weight. This study aimed to quantify CO exposure in pregnant women's homes and assess whether breath CO levels could be linked to home-based CO exposure and sociodemographic factors. CO levels were monitored continuously over two weeks in 161 households selected for indicators of lower socio-economic status and proximity to gas appliances, a risk factor for environmental CO exposure. Exhaled breath CO measurements were taken before and after the monitoring period. Of the households monitored, 57.8% had detectable CO levels, with 31.7% experiencing levels above 4ppm and 14.3% above 10ppm. CO exposure varied significantly across households, with both chronic low-level and intermittent high-level exposures observed. Six households included in the study exceeded current World Health Organisation recommended limits of 3.5ppm for ≥24 hours, and three exceeded the limit of 9ppm for ≥8 hours. Higher CO levels in the household were associated with the use of gas for cooking. Higher exhaled CO levels were associated with number of smokers in the household and eligibility for the UK government NHS Healthy Start scheme. Following the monitoring period, exhaled CO levels were only associated with number of smokers in the household, suggesting an intervention effect. This study indicates that a high proportion of pregnant women are exposed to CO within the home, albeit predominantly within current recommended limits, and that exposure may be linked to socio-economic factors in addition to smoking. This study highlights the need for improved CO monitoring and mitigation strategies, particularly in vulnerable populations, to protect maternal and foetal health.

The authors have declared no competing interest.

This study was funded by the CO Research Trust.

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The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

The Integrated Research Application System (IRAS) gave ethical approval for this work (IRAS ID 301261; East Midlands - Nottingham 1 Research Ethics Committee).

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All data produced in the present study are available upon reasonable request to the authors