Take care when air quality gets dodgy, particularly on hot summer days.
Health risks of air pollution include cardiovascular and respiratory disease, cancer, impaired cognition and osteoporosis.
Years-long look at military veterans finds link between air pollution and kidney damage, with tens of thousands of new cases every year.
Older people in cities with lots of wood-burning devices might run heart attack risk.
Traffic pollution is also a big contributor.
Research proves that it's five times more dangerous for your heart than same-sized particles from other sources.
Some cities showed improvements in air quality, while others had their worst record ever.
