Children who are exposed to high levels of air pollution — specifically, the types of tiny particle pollution emitted by cars, trucks and other vehicles — are at increased risk of developing asthma and persistent wheezing, according to a Danish study published online this week in the journal BMJ.
Although the study linked other factors — such as a family history of asthma, a mother who smoked during pregnancy and a low household income to the risk of having asthma during childhood, it identified air pollution as a separate and significant contributor to the disease.
These findings “support emerging evidence that exposure to air pollution might influence the development of asthma,” the study’s authors conclude.
Asthma is one of the most common diseases among children worldwide. It causes airways to become inflamed and narrow, making it difficult to breathe. In the United States, about 6 million children (one in 12 young people under the age of 18) have asthma. Young Black Americans are at particular risk. About 16 percent of Black children in the U.S. have asthma, compared to 7 percent of white children.
For the study, a team of Danish researchers led by Torben Sigsgaard, a professor of environmental and occupational medicine at Aarhus University, analyzed medical data on more than 3 million children born in Denmark between 1997 and 2014. Before reaching the age of 15, almost 123,000 of those children had developed asthma or persistent wheezing. Most (83 percent) began showing symptoms before their third birthday.
The researchers then compared this information — which of the children developed asthma and which didn’t — to air pollution measurements taken at the children’s residential addresses, as well as to other factors associated with an increased risk of developing asthma: parental asthma, maternal smoking, parental education and household income.
They found that the strongest determinant of whether a child developed asthma or persistent wheezing was having a parent with the disease. “This was expected, as genetic disposition is recognized as a strong risk factor for the development of asthma and allergy,” Sigsgaard and his co-authors write.
Maternal smoking during pregnancy also increased the likelihood that a child would develop asthma, as did having parents with a low income and a low level of education. Children of parents with high incomes and high levels of education, on the other hand, were less likely to have the disease.
Again, these findings were not surprising. “Health behaviour related to socioeconomic status — physical activity, diet, alcohol consumption, smoking, and health care — are suggested to be among the underlying drivers for the development of asthma,” the researchers point out.
Sigsgaard and his co-authors then turned to the air pollution data. They found that children exposed to higher levels of air pollution were more likely to have asthma or persistent wheezing than those who weren’t exposed, even after accounting for the other risk factors.
That link was most robust for exposure to fine particulate matter (PM2.5). These pollutants are quite tiny, with a diameter of less than 2.5 micrometers — or about 3 percent of the diameter of a human hair. When inhaled, they can penetrate deep into the lungs, causing both short-term inflammation and long-term damage.
PM2.5 particles come from a variety of sources, including motor vehicles, power plants, airplanes, residential wood burning, forest fires and dust storms. The current study found that the association between air pollutants and asthma was more pronounced for local rather than distant sources of PM2.5 pollution.
Limitations and implications
The study is observational, so it can’t prove a direct link between air pollution and asthma in children. The data used by the researchers also lacked information about the lifestyle of the children and their families — factors that may influence the development of asthma.
Still, the study’s results are in line with those from other research on this topic, including a 2017 meta-analysis that identified exposure to traffic-related air pollution as a major risk factor for childhood asthma.
Together, these results “suggest that further reductions in PM might help to reduce the number of children who develop asthma and persistent wheezing in highly exposed populations,” write Sigsgaard and his co-authors.