Over one million Americans have been warned to avoid outdoor activity and shut their windows as the air quality near the US southern border reaches dangerous levels.

The US Environmental Protection Agency (EPA) issued a widespread alert on Friday in Texas and New Mexico. The air over two massive swaths of the South, engulfing approximately 1.3million people, was declared 'unhealthy' or 'hazardous' to breathe.

Air pollution known as fine particulate matter (PM2.5) has been blamed for the widespread conditions. These are microscopic particles small enough to penetrate deep into the lungs, causing inflammation, breathing difficulties and other health issues when inhaled.

The hazardous clouds cover large cities in Texas, including El Paso, Lubbock, Midland and Odessa. In New Mexico, major communities including Hobbs, Carlsbad and Deming are also in the danger zones, according to the EPA's real-time data.

Air quality-tracking website IQAir also registered a major patch of dangerous air over the South and noted that multiple pollution-causing fires had been reported just outside Odessa, with local wind patterns pushing the hazardous particles north.

At one point on Friday, IQAir stations in Odessa reported that PM2.5 levels were over 18 times higher than the World Health Organization recommended safe limits.

However, by 2pm ET, an even more dangerous plume of polluted air had formed to the west of this cloud, stretching roughly 200 miles across Texas and New Mexico's border with Mexico.

The EPA warned that several areas within the zone had been deemed 'hazardous' to human health, the worst rating the agency gives to air quality measurements.

Anyone within the two massive clouds, which each stretch across 200 miles of the South, has been urged to close their windows to avoid dirty outdoor air, avoid outdoor exercise, wear a face mask when leaving the house and run an air purifier if they own one.

This type of particulate matter is small enough to enter and damage the lungs, worsen respiratory issues such as asthma, and even contribute to heart attacks and strokes that cause premature death if you breathe in large amounts.

According to the EPA, all of the air in the large swath to the east, stretching from Fort Stockton, Texas in the south to the small city of Friona in the northern part of the state, has become unsafe for the public to breathe.

That is a distance of nearly 300 miles and encompasses more than 600,000 residents in Texas and New Mexico.

The more dangerous pocket of unhealthy air that has formed to the west centers right along the US-Mexico border, near the city of El Paso, which has a population of nearly 700,000 people.

The Texas Commission on Environmental Quality has also warned that levels of PM10 have reached unhealthy levels in El Paso.

PM10 is a type of air pollution made up of tiny solid particles or liquid droplets floating in the air that are less than 10 micrometers in diameter, thinner than a human hair.

These inhalable particles can get deep into your lungs when you breathe them in and are typically created by dust from construction, pollen, mold, smoke, soot, industrial emissions and wind-blown dirt.

PM10 is noticeably larger than PM2.5, the microscopic particles composed of toxic compounds or heavy metals from car exhaust and factory emissions.

In nearby Sunland Park, New Mexico, levels of PM2.5 had soared to more than four times the recommended safe limits.

IQAir also warned that the local air quality index in the area near El Paso had reached 174 by 2pm ET. 

Air quality levels are measured on a scale from 0 to 500: good (0–50) carries little risk, moderate (51–100) may affect sensitive individuals, unhealthy for sensitive groups (101–150) poses increased risk and unhealthy (151–200) impacts everyone, limiting outdoor activity.

The severe conditions that have developed around El Paso have been largely blamed on the natural geography along the US southern border, including large dust storms being blown north from the Chihuahuan Desert.

Severe smog from cars, trucks and factories has also flowed across the border from the Mexican city of Juárez, which has a rapidly growing population of more than 1.6million people.

This has led the American Lung Association to give El Paso an 'F' grade for ozone pollution in 2025.