Government orders for people to stay home to halt the spread of the coronavirus have resulted in a sharp decrease of vehicle traffic around the world. Cities such as Delhi, India, Los Angeles in the United States and Seoul in South Korea all have reported better air quality. But that is not the case with Mexico’s capital, although its traffic has decreased by 60 percent. The official website of Mexico City says that air quality there is “poor.” It warns that exercising at certain times of the day presents a “high” to “very high” health risk. Photographs taken by Reuters last week show a thick haze over the city.