Last month marked the first time in a decade that Mexico City has gone a full year without a smog alert. The city was once considered to have the dirtiest air in the world, but Beijing, New Delhi, and other metropolitan areas now seem to have it beat. The improvement in Mexico City’s air is due to tougher regulations for vehicles and factories, driving bans, and recent strong winds that have been blowing smog out of the valley. But because smog alerts aren’t called until pollution reaches 2.4 times government-accepted levels, a lack of alerts doesn’t mean clean air. In fact, a study released yesterday by the Massachusetts Institute of Technology indicates that Mexico City’s air pollution may contribute to thousands of deaths each year, particularly because of emissions from old, dirty diesel freight trucks. Also, a recent study by the Mexican federal government suggests that rapidly increasing numbers of private cars and declining use of subways and buses could worsen pollution problems.