Background Motorised travel and associated carbon dioxide (CO₂) emissions generate substantial health costs, many of which disproportionately affect socio-economically disadvantaged groups. These health costs may include contributing to rising obesity levels. Obesity has in turn been hypothesised to increase motorised travel and/or CO₂ emissions, both because heavier people may use motorised travel more and because heavier people may choose larger and less fuel-efficient cars. These hypothesised associations have not been examined empirically, however, nor has previous research examined associations with other health characteristics. Recent years have, however, seen increasing research and policy attention to the potential ‘co-benefits’ of pursuing policies which simultaneously enhance public health and promote environmental sustainability. We therefore aimed to examine how and why weight status, health, and physical activity are associated with transport CO₂ emissions.

Methods 3463 adults (18–91 years, 45% male) completed questionnaires in the baseline iConnect survey at three study sites in the UK, self-reporting their health, weight, height and past-week physical activity. Seven-day recall instruments were used to assess travel behaviour and, together with data on car characteristics, were used to estimate CO₂ emissions. We used path analysis to examine how far active travel, motor travel and car engine size mediated associations between health characteristics and CO₂ emissions.

Results CO₂ emissions were higher in overweight or obese participants (multivariable standardized probit coefficients 0.16, 95% CI 0.08, 0.24 for overweight vs. normal; 0.16, 95% CI 0.04, 0.28 for obese vs. normal). Lower active travel and, particularly for obesity, larger car engine size explained 19–31% of this effect, but most of the effect was directly mediated by greater motorised travel distance. Walking for recreation and leisure-time physical activity predicted higher motorised travel distance and therefore higher CO₂ emissions, while active travel predicted lower CO₂ emissions. Poor health and illness did not independently predict CO₂ emissions.

Conclusion Establishing the direction of causality between weight status and travel behaviour requires longitudinal data, but the engine size association suggests at least a potential causal effect of obesity on CO₂ emissions. More generally, transport CO₂ emissions are differently associated with different health characteristics, including associations between a health good and an environmental harm (recreational physical activity and high emissions). Thus health-environmental ‘co-benefits’ cannot be assumed. Instead, attention should also be paid to identifying and mitigating potential areas of tension, for example promoting low-carbon recreational activity.