MEXICO CITY — Residents in Mexico's capital are walking around with surgical masks on as their eyes water and they struggle to breathe.

Tuesday was the fourth straight day that the capital, which sits on a valley, has been engulfed in a thick haze of air pollution as forest fires rage across the country.

Authorities declared emergencies in 11 municipalities in the southern Mexico state of Oaxaca due to fires on Saturday, but there were also reports of fires in Valle de Bravo, Tepoztlán and Jalisco, according to The Associated Press.

Mexico City’s fire department reported 23 forest fires in total on Sunday.

Though the fires are on the outskirts of the capital, they’ve caused a decline in air quality in the city by emitting dangerous particles. For more than 48 hours, residents were exposed to PM2.5 particles, which have a similar structure to aerosols and can diminish a person’s ability to breathe as well as curtail the development of the respiratory system in children.

The heightened levels of air pollution have prompted the Megalópolis Environmental Commission (CAME), a Mexican government agency, to issue a warning for residents to remain indoors.

⚠⚠ ATENCIÓN

Toma en cuenta estas acciones para hacer frente a la contaminación del aire ocasionada por los incendios 👉 https://t.co/Xv56XYYFjw

🔸 Evita actividades al aire libre.

🔸Se recomienda que grupos vulnerables permanezcan en hogares. — CAMegalópolis (@CAMegalopolis) May 13, 2019

“In recent days the rates of contamination by fine particles have increased in the Valley of Mexico due to the influence of fires that are active in this atmospheric basin and its surroundings,” read a press release on CAME’s website. “The Environmental Commission of the Megalopolis has decided to apply an environmental alert in order to reduce the probability of affecting the health of the population, mainly of the groups most vulnerable to the effects of air pollution (infants, the elderly and patients with respiratory and cardiovascular diseases).”

The warning instructed residents to avoid spending time outdoors, cooking with firewood and coal, smoking and lighting candles, as such actions could put them at risk and exacerbate overall air quality.

In light of alarmingly bad air quality, Mexico City officials issue warning: BREATHING IS BAD FOR YOUR HEALTH. pic.twitter.com/ezIk7V4exf — Laura Martínez (@miblogestublog) May 13, 2019

The agency also suggested placing wet towels under doors and by the windows of homes near fire zones to prevent the fires from spreading.

On Tuesday, the Mexican agency that monitors pollution levels recorded that the risk of the city’s air quality was a 10 — the most dangerous it can receive on its scale.

Mexico City has experienced 73 days of above average temperatures so far this year, and the heat and current conditions have contributed to more than 770 forest fires, according to Reforma, a Mexico City newspaper. The National Forest Commission says more than 100,000 acres of forest were destroyed by fire this year through March alone.

“The climate works against Mexico City and the area has a baseline of poor air quality from urban sources,” said Kevin Cromar, the director of the Air Quality Program and an associate professor at the NYU Marron Institute of Urban Management, which works with Mexican agencies on air quality and research.

“The particles travel deep into the lungs, which could cause respiratory difficulty and an increased risk of lung cancer,” Cromar told NBC News.

NASA satellite imagery that reveals dozens of hot spots with thick smoke across Mexico has been shared widely by social and local media. The images, which show the smoke billowing in Central America, confirm that the effects of the fire could be even more far-reaching.

Research published by AGU Journals in 2015 looked at how smoke from fires in Mexico and Central America in April 2011 helped influence a major tornado outbreak that killed 313 people in the U.S.

“Air pollution doesn’t recognize jurisdictional boundaries,” Cromar said. “There’s no question that these fires will impact Central America, and if the wind is blowing upward, the United States as well.”

Mary Murray reported from Mexico City, and Gwen Aviles from New York.

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