Attention-deficit/hyperactivity disorder (ADHD) is a common disorder influenced by genetics that usually begins in childhood. The symptoms include inattention and hyperactivity, both of which negatively affect educational attainment. However, environmental factors may increase genetic susceptibility. A new study has identified the influence of air pollution and nearby green space. The more green space a child experiences, the less the risk of developing ADHD.
Irritable parents often urge their children to go outside and play in the fresh air and perhaps there is a good reason for this. Studies have indicated that access to green space may be associated with children’s mental well-being and cognitive development. However, these studies have often been small, and the association has been attributed to the confounder that parents with high educational attainment more often live in residential areas with more green space. Nevertheless, a new major study in Denmark confirms this association.
“We linked registry data for more than 800,000 children born in Denmark between 1992 and 2007 with data on green space in residential surroundings and the level of air pollution around their childhood home. This enabled us to estimate the risk of being diagnosed with ADHD later in life. Even after adjusting for parental educational attainment, the risk of ADHD increases with less green space in childhood. The new results should provoke a discussion of the current limit values for air pollution but also about how we plan the urban space in residential areas,” explains co-author Malene Thygesen, National Centre for Register-based Research, Department of Economics and Business, Aarhus University.
Many theories
The new study is the latest of many to examine the association between residential green space in childhood and the development of ADHD. Measurement technologies and systems for processing large data sets have improved, however, enabling associations to be found that could not previously be found. The new study was also very ambitious and systematic, with the researchers analysing satellite images of small areas measuring 210 by 210 metres around each residential address of the more than 800,000 children to determine the amount of green space in the surroundings.
“The satellite images enabled us to place each individual residence on a scale measuring green space from –1 to 1 and thus link this with the risk of developing ADHD. The children who grew up with the least green space had an increased risk of 55%,” says Malene Thygesen.
When the researchers adjusted for parents’ socioeconomic status, including educational attainment, the difference in risk of 20% was still statistically significant. To identify the long-term effects of more or less green space in childhood, the researchers analysed the residential address when the children were 0–5 years old and linked this with health data about ADHD later in their lives.
“Based on this, the risk of ADHD increases if you grow up in an area with less green space, which is in accordance with previous studies,” explains Malene Thygesen.
Several theories attempt to explain this association between green space and ADHD. Some highlight that access to green space improves opportunities for reducing stress, and others highlight that green space alleviates noise exposure and air pollution.
Practical measures
The researchers examined air pollution previously and in the new study. To investigate whether the association between green space in childhood and ADHD is influenced by the level of air pollution, the researchers calculated whether the increased risk associated with less green space could be caused by air pollution, such as nitrogen dioxide emitted by cars.
“The result showed that the concentrations of nitrogen dioxide explained more than 20% of the risk, and the risk was higher even in some places with air pollution lower than the limit values set by the European Union and WHO,” says Malene Thygesen.
If this association turns out to be causal, lowering the limit values further or incorporating green spaces in urban planning could reduce the number of children developing ADHD.
