COVID-19 lockdowns brought bluer skies to some of the world's smoggy cities.
But not everyone's air got cleaner. And even with the improvements, air in some cities remained unhealthy.
After initial reports of dramatic drops in air pollution, scientists looking more deeply at the data are finding the improvements were smaller than they expected. Not all pollutants declined. And the benefits varied widely from place to place.
Their findings have implications for efforts to make the world's air healthier.