MONTREAL -- A new study from the U.S. finds that a baby whose mother was exposed to high levels of ultrafine particles during pregnancy could be more likely to develop asthma.

Researchers hypothesize that oxidative stress from exposure to high levels of air pollution could interfere with the normal development of a fetus' lungs.

"This piques my personal curiosity," said Dr. Jean-Claude Lavoie, with the CHU Sainte-Justine. "We are at the frontier of scientific knowledge between early events, such as fetal life and events that will occur later in life as a child or adult."

The U.S. researchers followed some 400 women in the Boston area during and after pregnancy. They found that babies born to women who were exposed to 30,000 particles per cubic centimetre of air during pregnancy quadrupled their risk of asthma, compared to babies whose mothers were exposed to half that amount.

A total of 18 per cent of the children were diagnosed with asthma. After taking several other factors into account, researchers conclude that there is an independent effect of ultrafine particles.

These particles measure less than 0.1 micrometres and are produced by motor vehicles, among other things. They are thought to be so fine that they can pass through the lung barrier and enter the mother's bloodstream.

American researchers believe that they can reach the fetus, but Lavoie strongly doubts it.

"When you look at the probability, with what we know now, I would bet absolutely none of the particles making it to the baby," he said. "They're going to be filtered through the lung, all the cells, all the blood and other cells are going to come in and remove them."

Instead, he believes that oxidative stress (which occurs when the body no longer produces enough antioxidants to neutralize circulating free radicals) in the mother affects her fetus.

"When the oxidant-antioxidant balance is disturbed, it will disrupt organ development," he said. "In this case, it can disrupt the development of the bronchioles around the alveoli in the lung. Oxidative stress could increase the musculature around the bronchioles, so if there's an irritant, they'll contract a lot harder and you have an asthma attack."

Ultrafine particles have been associated with several other health problems, such as neurodegenerative diseases and cancer. In babies, they have been associated with premature births and very low birth weight.

Beyond recommending that women breathe the cleanest air possible during pregnancy; "Everyone is for virtue, but it can be complicated to enforce," Lavoie acknowledged.

"One might think that taking antioxidants would be appropriate for pregnant women who are exposed to a lot of air pollution, but a trip to the local pharmacy is probably not necessary," said, Lavoie, who instead recommends "the best nutrition possible, but also, regular exercise."

"If these particles induce oxidative stress, if this lady exercises, if she maintains her ability to defend herself, she will counteract this oxidative stress from the particles and the baby will probably be safer that way," he said. "For the last 15 or 20 years, what's been coming out in the scientific literature is that the best antioxidant is moderate exercise."

Some studies have shown that people are more motivated to exercise if they incorporate small physical activities into their daily lives, such as using the stairs instead of the elevator or walking whenever possible.

The findings were published in the American Journal of Respiratory and Critical Care Medicine.

-- this report by the Canadian Press was first published in French on June 4, 2021.