October 20, 2014

expert reaction to new study on traffic pollution and unborn children’s lungs

A study published in the journal Thorax has examined the effect of exposure to traffic-related pollution during pregnancy, and development of the lungs in infants. They suggest that exposure to pollutants during, but not after, pregnancy negatively impacted on lung function of children before the age of 5.

Dr Anna Hansell, Assistant Director of the Small Area Health Statistics Unit, Imperial College London, said:

“This is a well-conducted although relatively small study of 620 pre-school children, suggesting that air pollution during pregnancy may affect lung development up to age 4.5 years.

“Lung development primarily takes place in the middle and end of pregnancy and in the first two years of life.  If air pollution is an important influence on lung growth, one might expect to see effects of air pollution at each of these stages.  However, Morales and colleagues found statistically significant associations only for air pollution exposure in mid-pregnancy.

“There are several possible explanations – the finding may be due to chance,  mid-pregnancy may be a particularly sensitive period of development for the lung or issues in the study design may have meant that an effect was there but just couldn’t be clearly shown for the exposures in late pregnancy and the first year of life.

“In contrast, a recently published large study of nearly 6,000 European children in the ESCAPE study (European Study of Cohorts and Air Pollution Effects) found small adverse effects of current air pollution on lung function at age 6-8 years, but no effect of air pollution at the address at birth.  More studies are needed specifically looking at different time windows in the pregnancy period to see whether the findings of Morales and colleagues can be confirmed.”

Prof. Andrew Bush, Consultant Paediatric Chest Physician at the Royal Brompton & Harefield NHS Foundation Trust, said:

“It has long been known that the foetus is affected by what is going on in the environment, the most obvious example being maternal smoking, which causes lifelong damage to the baby’s lungs.

“In this prospective study, Morales and colleagues studied exposure of pregnant women to traffic-related air pollution, specifically benzene and nitrogen dioxide, and measured lung function in their offspring nearly five years later. They showed clinically significant reduction in lung function in those most heavily exposed in utero, but no effect of post-natal exposure.

“This finding is important for many reasons. Firstly, we know from many birth cohort studies that lung damage age 4-5 years will never recover, and these children will be at higher risk of COPD as adults. Secondly, we know that in mid-childhood, traffic pollution prevents the lungs growing normally, making the situation worse. Thirdly, this study also showed that the worst effects were on the already disadvantaged, namely allergic children and those from economically deprived families.

“This study shows that it is a political imperative to cut traffic pollution especially in populated areas, to preserve the lung health of babies into old age.”

Prof. Jon Ayres, Professor of Environmental & Respiratory Medicine at the Institute of Occupational and Environmental Medicine, University of Birmingham, said:

“This paper shows an association between exposure during pregnancy to known air pollutants – benzene and nitrogen dioxide – and lung function in the children at age 5.

“There are some problems with the study in terms of exposure assessment and it is curious that the findings were reasonably strong for the second trimester but less for the whole pregnancy.  This could mean an association by chance as it would be unusual for exposures to vary importantly during pregnancy such that associations differ between the second compared to the first and third trimesters.

“However, if true – and this needs to be replicated with larger studies with better exposure assessment – then this raises the question of what component of air pollution might be responsible.  It is highly unlikely that the toxic component will be either of those measured here and ultra fine particles are likely the important component.  Such a study would also need to take great care to deal with maternal diet as differences in maternal Vitamin E and d have been shown to be associated with respiratory morbidity in the children.”

Prof. Seif Shaheen, Clinical Professor of Respiratory Epidemiology at Barts and The London School of Medicine and Dentistry, Queen Mary University of London, said:

“There is an accumulating body of epidemiological research suggesting that the lung health of children is influenced by the environment before birth. For example, the damaging effects of smoking in pregnancy are well known. This latest study adds to this evidence base, and suggests that exposure to traffic-related pollutants (NO2 and benzene) in the middle of pregnancy may have detrimental effects on the developing lungs of offspring.

“The study has been carefully undertaken, with rigorous control for potential confounding factors in order to tease out possible independent effects of air pollution, before and after birth, on lung function in early childhood.  One weakness is that pollution exposure levels were estimated from samplers placed in different places in the study area; more accurate estimation of exposure during pregnancy and infancy could have been obtained from measurements in participants’ homes. However, the cruder methods used might have led to an underestimation of effects of pollution on lung function. Another weakness is that trying to obtain reliable lung function measurements of this kind is a challenge in children this young (mean age 4.5 years); unsurprisingly, usable lung function data were only obtained for a subset, which could have biased the results. That said, the conclusions of the study are very plausible.

“This is an important research study. The results suggest that more needs to be done to reduce air pollution in order to improve public health, and in particular the lung health of the next generation. These findings should be taken very seriously by policy makers.”

‘Intrauterine and early postnatal exposure to outdoor air pollution and lung function at preschool age’ by Eva Morales et al. published in Thorax on Monday 20 October 2014. 

Declared interests

Dr Hansell was an investigator in the ESCAPE study, working on respiratory effects of air pollution in adults, but was not involved in the studies in children. She declares a Greenpeace membership but has not received any money from the organization nor been involved in campaigns.

Prof Bush is a journal editor for Thorax.