The equivalent of 11,000 new cases of asthma occur every day worldwide due to toxic air from traffic, researchers say.
The research, published in the journal Lancet Planetary Health, is the first global assessment of the impact of traffic fumes on childhood asthma based on high-resolution pollution data.
Most of the new cases occur in places where pollution levels are already below the World Health Organization limit, suggesting toxic air is even more harmful than thought.
Canada has the third highest rate of new traffic-related asthma cases among the 194 nations analyzed, while Los Angeles and New York City are in the top 10 worst cities out of the 125 assessed. Children are especially vulnerable to toxic air and exposure is also known to leave them with stunted lungs.