Mexico City declared its first pollution alert in almost three years yesterday, when ozone levels in the famously smoggy city reached about 250 percent of acceptable levels. The alert resulted in some 350,000 cars being ordered off the city streets. That’s a lot, but it’s far fewer than the nearly half of the city’s estimated 3 million vehicles that were forced off the streets by such alerts before many residents upgraded to newer, cleaner models, which are permitted on the roads even during pollution emergencies. Despite its reputation for terrible air quality, the Mexican capital has made some strides, and many scientists now believe it has cleaner air than several other major metropolitan regions, especially in Asia. But environmentalists say the air pollution is still a serious problem, and are urging the city to stiffen its standards for smog alerts.