Overweight and obesity in childhood result from the interaction of genes, lifestyle behaviours, physiological factors, and social factors. Environmental factors such as air pollution may also play a role in their development. However, it is not yet clear whether there is an association between air pollution and childhood obesity, and if so, whether it is causal (i.e., whether air pollution directly causes overweight or obesity). 

A good way to investigate whether the two are linked is to see what happens when a child is suddenly exposed to higher or lower levels of air pollution as a result of moving to a different home address using a so-called “natural experiment”. This natural experiment shows that moving to an area with more air pollution exposure was associated with a small increase in body mass index (BMI) in children. There was not, however, a decrease in BMI in children that moved to less polluted areas.  
   
The biological mechanisms that link air pollution to weight gain are not fully understood but may include oxidative stress, inflammation of adipose tissue, reduced glucose uptake, hormonal disruption, changes in metabolism, and/or reduced lung function. More traffic may also lead to changes in behaviour such as going outdoors less, which may increase weight. Since the current study relied on available data registered in primary health care centres in Catalonia, the researchers were not able to include these behavioural changes in their analysis. 

This study highlights that moving to areas with more air pollution was related to increases in childhood BMI. However, decreases in BMI were not observed after children moved to less polluted areas. This finding might be related to the children’s baseline air pollution exposure. Reductions in air pollution levels seem to be less relevant for those who had already been exposed to higher levels of air pollution, compared to those who had been exposed to lower levels. The dose-response levels seem to be steeper at lower levels, so the sudden increase for children who moved to a higher air pollution level may have a much larger effect than among children moving to less polluted environments. 

Additionally, children moving to similar air pollution areas showed decreases in BMI. In this study sample, the majority moved to similar air pollution areas, and a lower proportion moved to less/more polluted areas, as also seen in other European populations. Also, it might be that these areas with similar air pollution levels are greener, more walkable, and with greater availability of play areas for children. Lastly, the mere act of moving may be a stressful event and this might have affected children’s weight as well. There might be other weight-related factors (such as walkability or behaviour change) that might explain the effects of moving to similar air pollution areas on weight and that were not captured in the current study.

Children have a peak prevalence in overweight/obesity at school age, including the “adiposity rebound” period at ages 5-6, which might also explain the age effect observed in this study. Recently, age has been shown to play an important role in associations between road traffic noise, exposure to green spaces and urbanisation, and weight outcomes in children and adolescents. Other research has demonstrated that the strength of association between air pollution exposures and weight varies along the different types of pollutants and ages, so further in-depth studies are needed to better comprehend this effect.

Even though the magnitude of the observed associations is small, the impact on global public health could be high. Exposure to air pollution is widespread, and 56% of the world’s population (approximately 4.45 billion people in 2021) lives in urban areas, which tend to be more polluted. Moreover, childhood BMI is an important predictor of health outcomes later in life. Therefore, even small associations between air pollution and child weight gain may have important consequences for the population’s health.

The urban population is expected to double by 2050, meaning that more people will live in more polluted and less green environments, which in turn necessitates deepening our knowledge on how the urban environment affects health. In addition, the reversibility of the effects of air pollution on health is unclear, since health effects depend on several factors, including the duration and intensity of exposure, individual susceptibility, and the specific health effects involved. Acute health effects of air pollution may be reversible, such as throat irritation, cough, and nasal congestion. However, air pollution may have long-lasting or even permanent consequences.

To address obesity, multiple aspects of the environment must be considered. A key action is reshaping the urban environment, ensuring that children and adolescents have access to healthy lifestyles in their families, schools, neighbourhoods, towns, and cities. Strategies to control and prevent childhood overweight and obesity need to be implemented across settings and coordinated across  all levels of government, incorporating a variety of approaches within a perspective that is inclusive and equitable.

High air pollution levels are an important concern and require attention, especially in places where children and adolescents spend considerable time, such as in schools. Adopting the European Union air quality guidelines (which are aligned with the World Health Organization -WHO- air quality guidelines) is critical for children’s overall health and for the health of future generations.

Changes in individual behaviours, community structures, lifestyles, built environments, and exposure to pollutants are associated with childhood weight. This study on residential relocation suggests that increased levels of air pollution post-move may lead to weight gain in children. It provides evidence to support ongoing efforts to lower air pollution levels, as well as community-level strategies to prevent childhood overweight and obesity.

DE BONT, J., Y. DÍAZ, M. DE CASTRO, et al. (2021): «Ambient air pollution and the development of overweight and obesity in children: a large longitudinal study», International Journal of Obesity, 45 (5), 1124-1132. 

EDWARDS, L., P. WILKINSON, G. RUTTER, et al. (2022): «Health effects in people relocating between environments of differing ambient air pollution concentrations: A literature review», Environmental Pollution, 292, 118314. 

MALACARNE, D., E. HANDAKAS, O. ROBINSON, et al. (2022): «The built environment as determinant of childhood obesity: a systematic literature review», Obesity Reviews, 23, e13385. 

SAUCY, A., U. GEHRING, S. OLMOS, et al. (2023): «Effect of residential relocation on environmental exposures in European cohorts: An exposome-wide approach», Environment International, 173, 107849. 

BANCO MUNDIAL (2022): Urban Development