Climate change poses profound threats to human health, including via extreme weather events, infectious diseases, food insecurity and broader social disruptions
. A major driver of climate change is global energy use and the associated production of greenhouse gases, particularly carbon dioxide (CO2)
. The transport sector generates around a quarter of these greenhouse gas emissions (23% globally
, 27% in the United Kingdom (UK)
), and this contribution is rising in both absolute and relative terms
Motorised travel also adversely affects current health by increasing urban air pollution, noise pollution and road traffic crashes
. Motorised travel — particularly car travel — may also contribute to rising obesity levels, both because it is a sedentary behaviour itself and because it may displace physically active modes such as walking or cycling. In recent years, studies from around the world have documented a positive cross-sectional association between car use and overweight or obesity
[5–8] and one Chinese study has also demonstrated a prospective association
Yet while a causal effect linking car use to obesity is highly plausible, a recent paper hypothesised that obesity may also increase motorised travel and/or CO2 emissions
. First, heavier people may use motorised travel more because, particularly at higher weights, walking requires greater physical effort. Secondly, heavier people may choose larger cars and so increase their fuel consumption. In mathematical models, these pathways both contributed approximately a 12% increase in transport CO2 emissions from a population with an ‘overweight’ versus a ‘normal’ weight distribution. This led the authors to conclude that the obesity epidemic is an important environmental problem as well as a public health priority
. To our knowledge, however, the modelling assumption that heavier individuals choose larger cars has never been tested empirically, nor has any study examined whether weight status is in fact associated with transport CO2 emissions. Empirical examination is particularly warranted because overweight and obesity are also major contributors to ill health
. This poorer health status could plausibly have effects in either direction, potentially strengthening the association by further diminishing the ability to walk or cycle, or alternatively decreasing the association by diminishing the ability to travel at all.
As for associations with physical activity, it is widely assumed that walking or cycling for transport (‘active travel’) substitutes for at least some motorised travel and thereby reduces CO2 emissions [e.g.
. This assumption is supported by the finding that energy expenditure from walking is negatively correlated with fossil fuel use from car driving
 and that individuals in ‘walkable’ neighbourhoods make more walking trips and travel fewer vehicle miles
. One study has also reported that lower car use is associated with increased likelihood of meeting recommendations for total physical activity
. For these reasons, promoting active travel has been discussed as one area in which measures undertaken to reduce greenhouse gas emissions may also produce health ‘co-benefits’
[15, 16]. However, we know of no previous study which has disaggregated total physical activity and examined the carbon implications of engaging in recreational walking, cycling or other physical activity.
The lack of empirical evidence on these issues partly reflects the fact that studies estimating CO2 emissions do not usually collect ‘health’ data, and vice versa. This paper therefore capitalises upon a unique interdisciplinary study to examine how and why health, physical activity and weight status are associated with transport CO2 emissions. In doing so, we test the general hypothesis that any associations may in part be explained by differences in levels of active and/or motorised travel. We also test the more specific hypothesis that obese individuals generate more CO2 emissions because they use larger cars.