A thick, black layer has been hovering over Mexico City for days.

On Thursday, the city's elementary schools were ordered closed and drivers told to stay off the roads in the Mexican capital due to poor air quality. Authorities have also recommended that residents avoid exercising outdoors.

That followed city officials' declaration on Tuesday of an environmental emergency, due to severe air pollution exacerbated by wildfires.

At time of writing, the US Environmental Protection Agency's AirNow index on air quality (AQI) described Mexico City's air as an "unhealthy" level of 166. The lower the AQI, the less polluted it is.

Mexico is a city of more than 21 million people, and it has been plagued by air pollution for decades.

Some residents say they believe officials have been too slow to react to the emergency. But Mayor Claudia Sheinbaum says her administration is working on a comprehensive plan to address not only the current emergency, but air pollution in the long term. She attributes the city's problem to cars, factories, higher temperatures and wildfires.

According to Mexico's Department of the Environment, there were up to 100 wildfires burning in 20 out of the country's 32 states this week.