Denver's 'Brown Cloud' was a visible layer of air pollution that settled over the city starting in the 1950s, caused by a mix of industrial emissions, power plant smoke, and car exhaust. The city's geography, nestled against the Rocky Mountains, trapped pollutants during temperature inversions, leading to some of the highest carbon monoxide levels in the country by the 1970s. This spurred a public health crisis linked to respiratory illnesses like childhood asthma. In response, Colorado became a pioneer in air quality regulation, often exceeding federal standards. Key measures included the first-in-the-nation requirement for oxygenated gasoline and the development of a vast sensor network to monitor pollution. Over decades, these efforts significantly thinned the Brown Cloud. The article notes that while challenges remain, particularly from wildfire smoke, Denver's campaign transformed the city from a cautionary tale into an example of environmental accountability.
