Mexico City residents are choking on record levels of dirty air this week, after suspended-particle pollution hit an all-time high on Monday. Outdoor activities at schools and youth sports centers were prohibited, and parents were warned to keep kids indoors. Factories in the southeastern part of the city were ordered to cut back operations by 30 to 60 percent, and more than 150 brickyards were shut down. Last year, the World Resources Institute named Mexico City the most dangerous city in the world for children in terms of air pollution. Air quality did improve in 1999, making it the city’s best year in the last decade, but that respite now seems to have been short-lived. This week’s problems were blamed on shifting winds that carried smoke and particles from outlying industrial plants and agricultural fires into the city.