[Image of a world map with most countries colored red, indicating poor air quality]

1900

[Image of a world map with some countries still colored red, but some countries now colored orange or yellow, indicating a slight improvement in air quality]

1950

[Image of a world map with more countries colored orange or yellow, and a few countries now colored green, indicating good air quality]

2000

[Image of a world map with most countries colored green, indicating good air quality, but a few countries still colored yellow or orange]

2020

[Image of a world map with all countries colored green, indicating good air quality]