SAN FRANCISCO (NYTIMES) - Ian MacDonald, a 14-year-old in Portland, Oregon, got a treat from his parents the other day. He was allowed to go outside.

It was for less than five minutes. "I let him take out the garbage. We wore N95 masks," said his father, Dr Kelvin MacDonald, who has been insisting his family stay in the house as wildfires flare nearby.

Dr MacDonald is a paediatric pulmonologist, and he is concerned about the health risks to children from the ashen-thick air.

"It is unethical to expose them and find out what happens 50 years from now," said Dr MacDonald, who is also an associate professor of paediatrics at the Oregon Health & Science University.

While the science is still emerging, he said, the thrust is already clear: "This does not look good for children."

The fire that is breathing down the Pacific Northwest and parts of California poses particular peril to young lungs and is especially acute for children with medical conditions like asthma, which afflicts 1 in 12 children. Their lungs can become so inflamed by the microparticles that airways may close, creating - in extreme cases - mortal threat.

Healthy children exposed to wildfire smoke at the current levels, even for just a few weeks, can become vulnerable to infection, too, in some cases triggering latent asthma.

Such exposures can heighten existing asthma symptoms and increase hospital visits, according to the Centres for Disease Control and Prevention and research from the Environmental Protection Agency.

In the short term, smoke can kill immune system cells that protect the lungs from pneumonia and other disease. In the long run, doctors fear, such toxin exposure could prompt asthma in children with a genetic predisposition but who might otherwise not have contracted it.

Much of 2020 has been spent focusing on imperiled lungs of the older and the infirm, who are at greater risk of Covid-19, but these fires prey on developing lungs, experts said.

Dr John McQueston, a paediatric pulmonologist who works for Randall Children's Hospital in Portland and serves satellite clinics in some of the hardest-hit regions in the state, predicted a spike in asthma-related illness.

"I would not be surprised if six months from now, when children have problems with asthma, I'll ask their parents, 'When did it start?' and they'll answer, 'It happened right after the fires,'" he said.

The intensive care unit at the hospital where he works admitted a boy under the age of 5 who was having a severe asthma attack apparently made so by the smoke. Dr McQueston said the boy recovered, in no small part by getting into the hospital's clean air.

In recent days, Dr McQueston has dealt with a handful of other cases involving asthmatic children living just downwind from fires; they had intensified symptoms but had been treated at home with medication.

Dr McQueston said there might well be more acute issues in the region if not for the coronavirus pandemic, which was already keeping so many children at home and less exposed to infection.

In Oroville, California, near some of the state's worst fires, Dr Sidharth Bagga, a pulmonary critical care specialist for children and adults, said he and his colleagues were receiving "drastically more phone calls" about lung health for people of all ages. He's particularly concerned about the children, though, because many seem to be playing outside still.

Long-standing evidence from more polluted nations shows the risks of unhealthy air to developing lungs, with one estimate showing that, worldwide, 237,000 children under age 5 died in 2015 from asthma and other conditions associated with air pollution, mostly in Asia and Africa.

The research on wildfires is more recent, reflecting the fact that industrial pollution has been considered more common. But researchers say the differences might not be so pronounced, in that the wildfire air carries tiny particulates that threaten lungs from scorched trees, cars, homes and businesses and industrial areas, Dr MacDonald said. "It could be pretty bad if man-made objects are being burned."

If the science remains murky, the bottom line seems plain, experts said.

"In the absence of a compelling reason, it's best to keep the kids at home," said Dr David Cornfield, chief of pulmonary, asthma, and science medicine at Stanford Children's Health. He was involved in cases of two children with severe asthma who died after exposure to smoke from wildfires, one in 2016 and one in 2013.

Air quality is measured by the density of pollutants; when a widely used index of air quality developed by the EPA reads below 50, conditions are considered safe. That index has well exceeded 400 in the areas around Portland, causing them to be labeled "hazardous" and an "emergency situation," while the levels in the San Francisco Bay Area, hovering between 200 and 300, are "very unhealthy."

Dr Cornfield said he didn't want to be too prescriptive about what precise level should keep children inside but said a decent guideline is to cease outdoor activity at 100. Above 150, he said, "you don't really want to have people sitting outside."

That puts a major crimp in school reopening plans, which are predicated on keeping windows open, having ventilation, even holding outdoor classes to stanch the spread of the coronavirus. That, too, preys on the lungs, so administrators are now caught between wildfires that would call for tightly insulated schools and the threat of the virus, which argues for open air.

Children have been less well studied than adults, but experts said the challenges of youth are very likely intensified by at least three key differences: children's lungs remain in development, which means their immune systems may be altered; their airways are narrower; and they simply breathe more often and faster. That leads to more toxic intake for their body mass. "Infant respiration may be double or triple adults," said Cornfield of Stanford.

At the particularly high levels of smoke in places like Portland, Cornfield said, at the least, families should consider getting an air filter or using air conditioning to clear the air inside. In a perfect world, he would urge stronger measures, at least for the short term.

"I honestly think that if there's the potential to relocate to a less challenged environment, one should really do that," he said. "Most people are not so blessed with a plethora of options."