Authorities have banned more than 1 million cars from the roads and offered free subway and bus rides to coax people from their vehicles as Mexico City’s first air pollution alert in 11 years stretched into a third day.

Officials advised people on Wednesday to limit outdoor activity owing to high ozone levels that were nearly double acceptable limits in the sprawling capital, which lies in a high-altitude valley ringed by smog-trapping volcanic mountains.

Amid a muddy brown haze, some residents covered their mouths with scarves or paper masks. The environment secretary, Alejandro Pacchiano, said measures would be considered if conditions did not improve further, such as suspending industrial activity at factories.

Buildings stand shrouded in smog in Mexico City. Photograph: Edgard Garrido/Reuters

The mayor, Miguel Ángel Mancera, told Televisa news that about 1.1m cars in the Valley of Mexico, including nearly 450,000 in the capital, were ordered off the streets under the restrictions.

According to the national statistics institute, about 4.7m vehicles were registered in the capital in 2014.

Mexico City used to regularly reach high smog levels, but air quality has improved significantly since the 1990s. Among other measures, rules were put in place that aimed to limit the circulation of older, more polluting vehicles and obliged car owners to get regular smog checks. However, a court relaxed those restrictions last year.