The US and Canada saw dangerous smoke this week. It's a routine peril for many developing countries

Almost the entire world breathes air that exceeds the World Health Organization's air-quality limits at least occasionally. The danger grows worse when that bad air is more persistent than the nightmarish shroud that hit the U.S. — usually in developing or newly industrialized nations. That's where most of the 4.2 million deaths blamed on outdoor air pollution occurred in 2019, the UN's health agency reported.