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MEXICO CITY (Reuters) - Mexico’s government ordered schools in and around Mexico City to be closed on Thursday in an extraordinary step taken due to elevated levels of pollution in the smog-wreathed capital.

The education ministry said in a statement on Wednesday that the measure applies to public and private schools in the Mexico City metropolitan area, which is home to well over 20 million people. It recommended that children avoid exercise, remain indoors and avoid using contact lenses.

Two of the city’s principal seats of higher learning, the National Autonomous University of Mexico (UNAM) and the National Polytechnic Institute, also said they would suspend classes in the metropolitan area on Thursday due to the pollution.

On Tuesday the city’s authorities declared an environmental emergency. They have come under pressure to act due to reduced visibility caused by smoke and ash in the air during an extended dry spell.

President Andres Manuel Lopez Obrador addressed the problem in a news conference on Wednesday, calling the pollution in the capital “highly regrettable.”

Smoke from nearby wildfires has pushed pollution to levels deemed potentially harmful to human health.