Air pollution linked to a higher risk of young children developing asthma, study calls for reducing pollutants
Asthma is among the most prevalent diseases in children worldwide. Health experts have known that air pollution can make asthma symptoms worse. However, not much is known about whether pollution increases the risk that children will develop asthma in the first place. Researchers now find that air pollution is linked to a greater risk of young children developing asthma.
According to scientists, children exposed to higher levels of fine particles in the air (known as PM2.5) are more likely to develop asthma and persistent wheezing than children who are not exposed. The finding supports emerging evidence that exposure to air pollution might influence the development of asthma, and it needs to be substantiated in future studies, says the team from Aarhus University, Denmark.
Parents with asthma, having a mother who smoked during pregnancy, or having parents with low education and low income, were other risk factors associated with a higher risk of developing asthma and persistent wheezing. “The findings from this study strengthen the evidence that parental asthma, parental education, and maternal smoking during pregnancy are risk factors for asthma onset and persistent wheezing in children. Moreover, we found that children exposed to high levels of PM2.5 were more likely to develop asthma and to have persistent wheezing,” write authors in the study published by The BMJ.
According to scientists, PM2.5 can come from multiple sources, which include power plants, motor vehicles, and domestic heating. The particles -- about 3% or less of the diameter of a human hair -- can penetrate deep into the lungs and some may even enter the circulatory system. The researchers say while short term peak exposure to air pollution has been associated with worsening of asthma, the risks of long term exposure and the timing of exposure for the onset of asthma is less clear. Besides, the role of air pollution combined with other risk factors, such as socioeconomic status, on asthma is also not clear.
The investigators, therefore, decided to identify risk factors (air pollution and family-related) for the onset of asthma and persistent wheezing in children. “As children of parents with asthma are more prone to develop the disease, genetic susceptibility has been considered an important risk factor. The rapid increase in the prevalence of asthma and allergy globally suggests that besides genetic factors, environmental and social factors might also play a part,” the team explains.
The analysis is based on more than three million Danish children born from 1997 to 2014 and followed for asthma onset and persistent wheezing from the age of one year to 15 years. Of these, 122,842 children were identified as having asthma and persistent wheezing, at an average age of 1.9 years. This information was then linked to detailed air pollutant measurements at the children's home addresses, parental asthma, maternal smoking, parental education and income.
After taking account of other potentially influential factors, the research team found higher levels of asthma and persistent wheezing in children of parents with asthma and in children of mothers who smoked during pregnancy. In contrast, lower levels of asthma and persistent wheezing were found in children of parents with high educational attainment and high incomes.
Exposure to PM2.5 as well as larger particulate matter (PM10), nitrogen dioxide, and nitrate was associated with an increased risk of asthma and persistent wheezing. For every additional 5µg-per-cubic-meter increase in concentrations of those pollutants, there was about a 4-5% increase in asthma or persistent wheezing. However, only the positive association of PM2.5 with asthma and persistent wheezing remained robust across the different models, and after further (sensitivity) analyses, the authors emphasize.
“A higher incidence of asthma was found in children of parents with asthma (adjusted hazard ratio 2.29 and mothers who smoked during pregnancy (1.20), whereas a lower incidence was found in children of parents with high educational attainment (0.72) and high incomes (0.85). Exposure to particulate matter less than or equal to 2.5 µm (PM2.5) and less than or equal to 10 µm (PM10) and nitrogen dioxide was associated with an increased risk of asthma and persistent wheezing, with hazard ratios per 5 µg/m3 increase in pollutant concentrations 1.05 for PM2, 1.04 for PM10, and 1.04 for nitrogen dioxide,” reveals analysis.
The authors caution that this is an observational study, so cannot establish a cause. However, strengths over previous studies include the large unselected number of children across all social classes and in a broad age group, and the use of high-quality comprehensive data to minimize bias, they add. While the study findings need to be validated in future studies, these results suggest that further reductions in PM2.5 “might help to reduce the number of children who develop asthma and persistent wheezing in highly exposed populations,” the team concludes.