About 300 million children — roughly 1 in 7 worldwide — live in areas with “toxic” levels of air pollution, according to new research from the United Nations Children’s Fund.

The findings offer the latest reminder that some of the planet’s most vulnerable inhabitants face the worst environmental health threats. Their release Monday comes as international leaders prepare to convene next month in Morocco to continue their efforts to cut carbon dioxide emissions and stave off the worst effects of global warming.

UNICEF used satellite imagery to detail how an estimated 2 billion children live in areas where outdoor pollution exceeds minimum air-quality guidelines set by the World Health Organization. Of those, about 300 million children live in areas where outdoor pollution has reached particularly dangerous levels — often six or seven times beyond international guidelines. The result: air pollution has been linked to more than a half-million deaths each year of children younger than 5, nearly twice the death toll caused by malaria. Poor air quality also contributes to an array of respiratory conditions, including pneumonia, asthma and bronchitis.

“Many of these children are already disadvantaged by poverty and deprivation. Some are already at heightened risk from conflicts, crises and the intensifying effects of climate change,” Anthony Lake, UNICEF’s executive director, wrote in a foreword to the report. “Air pollution is yet another threat to their health and well being — and yet another way in which the world is letting them down.”

And while millions of adults around the world also suffer the effects of air pollution, children are particularly susceptible, study author Nicholas Rees said. Children breathe twice as quickly as adults, meaning they can take in much more polluted air relative to their body weight. Their respiratory tracks are more susceptible to infection, their immune systems weaker than an average adult’s, and their brains are still developing. The harm can endure over a lifetime.

“That physiological difference essentially makes them even more vulnerable,” said Rees, a policy specialist for UNICEF. “It’s also that the poorest children are the most affected. If we look at the burden of mortality associated with air pollution, we find that the vast majority — about 90 percent of outdoor air pollution deaths — are in low- and middle-income countries.”

Last month, the WHO released an analysis underscoring the extent of the risks of indoor and outdoor air pollution, which the agency has said accounts for millions of deaths globally each year. The report found that more than 90 percent of people face pollution levels that fall short of WHO standards.

With children and adults alike, the greatest concern is a form of pollution called PM2.5, which refers to particles smaller than 2.5 micrometers — so small that they can be inhaled, travel into the lungs and enter the bloodstream. The WHO considers a concentration greater than 10 micrograms per cubic meter of these fine particles to be dangerous.

The sources of air pollution are among the main factors fueling the warming of the planet, namely fossil-fuel emissions. Particulate air pollution in cities can come from many sources, from smokestacks to automotive tailpipes, from manufacturing facilities to exhaust from planes and ships. As urban air quality declines, researchers say, people living in cities face increased risks of heart disease, stroke, lung cancer and acute respiratory illnesses.

And in many places, the burden of pollution is only growing worse, not better. Another WHO report, released earlier this year, found that air pollution continues to worsen in urban areas in many parts of the world, hitting the poorest city dwellers hardest and contributing to a wide range of potentially life-shortening health problems. The agency found that in “low- and middle-income countries,” 98 percent of urban areas with populations of more than 100,000 fall shy of its air-quality standards.

Asia accounts for a large number of deaths attributable to air pollution. But increasing industrialization and urbanization are contributing to growing air-quality risks in parts of Africa.

George D. Thurston, a professor of environmental medicine and population health at the New York University School of Medicine, said many children in developing countries face a “triple whammy” when it comes to air pollution. They already lack proper nutrition and other needs that put them more at risk. Beyond significant levels of outdoor pollution, they often encounter additional air pollution at home, where unvented cooking with fuels such as wood, dried leaves, dung or even coal is common.

“You’re really talking about a susceptible population,” Thurston said. “There would be a big return on investment, in terms of public health, by cleaning up that pollution.”

The clearest way to do that, he said, is to ramp up the use of clean, alternative energy sources, which will be essential in coming decades for countries needing to provide energy to expanding populations.

“The things you have to do to prevent climate change are also good for public health,” said Thurston, who is part of a group of U.S. and Indian researchers meeting soon to discuss new ways to deal with pollution in the developing world. “You’re going to save lives, and by the way, you’re also going to save the planet.”

UNICEF on Monday urged world leaders to do just that by moving faster to cut fossil-fuel combustion and accelerate clean-energy development. The group also said countries must act more aggressively to monitor air pollution, provide poor children better access to health care and minimize exposure to polluted air by taking measures such as ensuring factories are not near schools and playgrounds.

“Protecting children from air pollution is not only in their best interest, it is also in the best interests of their societies,” Lake wrote, noting that the benefits would lower health costs, increase productivity and lead to more sustainable development. “We can make the air safer for our children. And because we can, we must.”

Chris Mooney contributed to this report.

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