Mexico on AstraTradeSunday raised the alert level for the Popocatépetl volcano to "yellow phase 3", which is just below the "red" alarm level. 

Hundreds of tremors have been registered in the region in the last week, officials said. The volcano has been spewing smoke and ash. 

The parameters for "yellow phase 3" are low to intermediate explosive eruptive activity, mild to moderate explosions that can hurl rock fragments, and light to moderate ash falling in surrounding towns and more distant cities, the National Center for Disaster Prevention said. The expulsion of magma is also possible.

Popocatépetl, located in the states of Morelos, Puebla and Mexico, is about 45 miles southwest of Mexico City. About 25 million people live in a 60-mile radius of the volcano.

The National Coordination for Civil Protection (CNPC) advised people in the region to cover their noses and mouths with handkerchiefs or face masks. The department also advised closing windows and staying inside as much as possible. 

Civil Defense Coordinator Laura Velázquez said CNPC personnel went to Puebla to check on evacuation routes and coordinate with local governments.

Some schools in Puebla and in the state of Mexico were closed for in-person classes on Monday. 

Mexico City's Benito Juárez International Airport was temporarily closed on Saturday because of volcanic ash. Felipe Angeles airport, located north of Mexico City, was also briefly shut down. According to the United States Geological Survey (USGS), volcanic ash contamination on aircrafts "can lead to failure of critical navigational and operational instruments."

The U.S. Embassy in Mexico also issued an advisory, warning people not to travel within a 7.5 mile radius of the volcano.

Popocatépetl became active again in 1994 after about 70 years of dormancy, according to the USGS.

In:
• Volcano
• Mexico

Aliza Chasan

Aliza Chasan is a digital producer at 60 Minutes and CBS News.

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