In November 2019, thousands of campaign fliers appeared overnight in New Delhi, announcing the success of the city’s government in bringing about a 25 percent reduction in air pollution. Anticipating residents’ incredulity—concentrations of fine particles known as PM 2.5 had the day before risen to more than 100 times World Health Organization guidelines—the posters offered an explanation for the apparent mismatch: Though pollution had fallen, smoke from burning crops in neighboring states had only just arrived.
A world away in San Francisco, November usually marks the end of fire season, rather than its start. Just like their counterparts in Delhi, San Franciscans spend large chunks of the year in a haze of smoke that pours in from nearby fires.
