Towns dotted around Popocatepetl in central Mexico have been coated with layers of volcanic ash in recent days after the country's most active volcano began erupting more vigorously again, causing disruptions to air travel and classes to be suspended.

Meaning “Smoking Mountain” in the Aztec language Nahuatl, Popocatepetl has been spewing water vapor, carbon dioxide and other gases including sulfur dioxide — along with huge amounts of volcanic ash.

“It was a lot quieter last year,” said Ana Lillian Martin del Pozzo, a volcanologist at the National Autonomous University of Mexico (UNAM). “Then, in March we saw it was getting more active and there are signs activity will continue to increase.”

Activity remains well below levels recorded in 2000 when some 50,000 people were evacuated, she said. Still, on May 20, flights from Mexico City's main airport were suspended for hours due to ash billowing from the volcano.

In 2019, smoke columns reached as high as 9 km (5.6 miles).

Mexican officials have warned some 25 million people living in a 100 km radius of the crater will need to protect themselves and their homes from ash rains. Evacuation routes have been readied.

Volcanic ash is a mixture of volcanic glass, minerals or crystals and other rock fragments.

Popocatepetl, which straddles the border of the central state of Puebla and the State of Mexico, had lain dormant for seven decades before erupting in December 1994.

In previous eruptions, volcanic ash has mostly fallen on these two states. On occasion, it has traveled as far as the state of Queretaro, over 100 miles (161 kilometers) northwest of Popocatepetl, and Veracruz state on the Gulf Coast.

Where volcanic ash ends up depends mostly on the wind but also on other weather conditions. This time around, its effects on overall pollution are already being felt by many Mexicans.

So far, wind has been carrying volcanic ash and gases, including toxic sulfur dioxide, away from the densely populated Valle de Mexico capital area that is home to some 22 million people.

Most volcanic ash has settled in Puebla. About 6.6 million people live in Puebla, many of them in the state capital of the same name.

While much of the impact has been felt in Puebla, Victor Hugo Paramo, head of the Environmental Commission of the Megalopolis - which oversees the capital and several surrounding states - said winds are set to change in June, as is common in the local rainy season, pushing Popocatepetl's impact deeper into the Valle de Mexico.

Last weekend, some volcanic ash traveled as far as Chalco, a municipality on the southeastern rim of Mexico City, he said.

Scientists said the most accurate way to measure the impact of volcanic ash on human health is by looking at how airborne particle matter (PM) surges after eruptions.

The particles vary in size, shape and chemical composition. Scientists have long been worried about the effects because smaller ones such as PM2.5 and PM10 are inhalable and have been linked to a broad range of heart and lung defects.

PM10 have a diameter of just 10 microns or less - a human hair has a diameter of between 50 and 70 microns, and fine beach sand 90 microns - while PM2.5 are even smaller.

Burning gas, gasoline, diesel or wood produce much of the PM2.5 that make up airborne pollution; PM10 includes PM2.5 but takes in more pollutants from sources such as dust, pollen and some bacteria.

Mexico's index for air and health shows an upward trend in pollution in Puebla since Popocatepetl's eruptions have increased, according to a Reuters analysis of the data compiled from sensors at different points in each state.

That pollution surge was particularly visible on May 19, 21 and 22, the data showed. On May 20, Chalco showed some impact.

“Larger, heavier (particles) will likely fall around the volcano. Smaller ones can travel much further,” said Paramo.

Paramo said it was hard to gauge Popocatepetl's environmental and health impact because of many different factors at play - but that some will likely be felt by millions of Mexicans.

On clear days, Popocatepetl is visible from the densely populated capital 72 km away.

There, pollution levels are already notoriously high, with nearby industrial plants and millions of cars pumping waste gases into the air daily.