As Mexico City's smog alert extends on its fourth day,  more than 1 million cars were driven away from the roads on Wednesday. City officials claimed that the alert slightly improved the air pollution, but it was notable that some areas still exceeded 1 ½ times  the acceptable limits.

Mexico City's smog alert was the first in 11 years. Metropolitan officials have offered free subways and bus rides to convince people not to use their vehicles.  On Thursday, a different set of vehicles will be advised to stay off the roads.  City officials have also reminded residents to control their outdoor activities due to the high ozone levels.  The ozone level nearly doubled the acceptable limits in the metro, ABC News reports.  Mexico sprawls in a high altitude valley surrounded by volcanic mountains that trap smogs.

During the air pollution alert campaign, students were not allowed to stay out during break time. People wore scarves or masks as they walked through the streets. Environment secretary Alejandro Pacchian warned that if Mexico City's smog alert will not bring substantial result,  they will be forced to suspend industrial operations in the factories, according to Belfast Telegraph.  Mayor Miguel Angel Mancera claimed that nearly 1.1 million cars in Mexican Valley and 450,000 cars in the metro were ordered to stay off the roads, but only about 800,000 followed the measure.

Records at the National Statistics Institute showed that there were 4.7 million registered vehicles in Mexico City in 2014.  In the past, Mexico usually reached high smog levels. However, with intervention, the city's air quality has improved since the 1990s.  As part of Mexico City's smog alert in the past, car owners were obliged to have their smog check regularly. Older vehicles were also banned on the roads as they contribute to more pollution. The regulations have become lenient since last year that could perhaps lead to a bigger air pollution problem, The Guardian reported.

Mexico City's smog alert was in 2002.  The city had its last alert for air particles three years after.  The present ozone alert was anon Monday.