I grew up in rural Colorado, deep in the mountains, and I can still remember the first time I visited Denver in the early 2000s. The city sits on the plain, skyscrapers rising and buildings extending far into the distance. Except, as we drove out of the mountains, I could barely see the city – the entire plain was covered in a brown, hazy cloud.
That brown, hazy cloud was mostly made of ozone, a lung-irritating gas that causes decreases in lung function, inflammation, respiratory symptoms like coughing, and can trigger asthma attacks.
Denver still has air pollution problems, due in part to its geography, which creates temperature inversions that can hold pollution near the ground. But since 1990, ozone has decreased 18% across the U.S., reducing the smog that choked many cities in the 1960s and 1970s. The concentration of tiny dustlike particles of air pollution called PM2.5 has also decreased, by 37% since 2000.
