Childhood behavioural problems linked to exposure to tobacco smoke and traffic density before birth
The exposure of the pregnant mother to tobacco smoke and road traffic pollution can influence the development of behaviour in early childhood. This is the conclusion of a study recently published in Environmental International and led by a team from the Barcelona Institute for Global Health (ISGlobal), a centre supported by the "la Caixa" Foundation. This is the first study to investigate the impact that the exposome - that is, the set of environmental exposures (chemical and non-chemical) - during the prenatal and postnatal stages can have on child behaviour. Until now, these environmental exposures have been studied separately and not in multiple ways.
Childhood is a critical time for people's mental health and well-being, as it is when brain development accelerates. The causes of behavioural problems are not yet well understood, but it is known that the limited genetic component involved in these disorders interacts with multiple social and physical exposures that are particularly sensitive in the prenatal and infancy periods.
The study is part of the large European Human Early-Life Exposome (HELIX) project, from which data were used. The research was based on six longitudinal birth cohorts from six European countries. A total of 1,287 children aged 6-11 years were followed to characterise their exposures and assess their potential behavioural problems. Eighty-eight prenatal and 123 school-age children's environmental factors were measured, including outdoor, indoor, chemical, lifestyle and social exposures.
During pregnancy, smoking and traffic were the factors that showed the strongest associations with behavioural problems.
"We found that maternal exposure to tobacco smoke during pregnancy was the most important prenatal exposure associated with emotional and behavioural problems in children," explains Léa Maitre, first author of the study and postdoctoral researcher at ISGlobal. It should be recalled that this maternal exposure to smoking "is closely related to other co-exposures, such as parental psychopathology symptoms, socioeconomic factors, the father's smoking habits and the home environment (quality of attachment, support and stimulation to which a child is exposed at home), which may explain a large part of the effect of maternal smoking during pregnancy on child behaviour".
The study also found that increased residential vehicle traffic density on the nearest road during pregnancy was associated with higher scores of externalising problems (aggressive and rule-breaking behavioural syndromes) and ADHD. The biological explanation is plausible, although the exact mechanisms by which this occurs remain elusive.
Postnatal exposure to tobacco and car traffic density were not as strongly associated with children's behaviour as during pregnancy, which may indicate that the period of pregnancy is more sensitive to the harmful effects of these exposures. This would occur partly because of the rapid development of the nervous system at that stage, but also because exposure takes place in utero, among other hypotheses.
The study also found that children aged 6-12 who enjoyed longer sleep, ate a healthy (Mediterranean) diet and whose parents had strong family and social ties had fewer problems with internalisation, i.e. withdrawal (e.g. not talking), somatisation (headaches) and anxiety or depression (nerves).
In contrast, greater exposure to lead and copper, indoor air pollution and an unhealthy diet were associated with greater behavioural problems.
In particular, a diet of prepared foods, sweets and caffeinated beverages was associated with increased risk of ADHD symptoms, although impulsivity traits in children with ADHD can also lead to poor dietary choices and emotional eating.
One of the strongest associations with ADHD was observed with parental (mainly maternal) social and family ties: those parents who had contact with relatives or friends less than once a week were 31% more likely than average to have children with ADHD symptoms.
Indoor air pollution and levels of copper and lead in the blood were associated with an increase in childhood behavioural problems.
"Our results confirm the harmful role of maternal smoking and exposure to traffic during pregnancy in childhood behavioural disorders, but also highlight the potential protective role of a healthy family lifestyle during childhood (diet, sleep, regular social contact)," concludes Martine Vrijheid, last author of the study and head of ISGlobal's Children and Environment programme. "Promoting healthy family habits early on and regulating air quality and lead exposure could help prevent the future development of mental health disorders".
For Jordi Julvez, second author of the study, neuropsychologist and researcher at the Institut d'Investigació Sanitària Pere Virgili (IISPV-CERCA), the importance of the research lies in the fact that "for the first time studies of human behaviour are being conducted taking into account a wide variety of environmental determinants and lifestyles in the same analysis, from the scope of the psychological development of the child: this is the closest we have come so far in adjusting our studies to the multifactorial reality of human psychological development".
