A giant plume of toxic air particles has engulfed a major southwestern city, triggering warnings to stay indoors and close all windows.

The US Environmental Protection Agency (EPA) revealed a roughly 80-mile-wide stretch of Arizona and California showing high levels of dangerous toxins known as fine particulate matter on Friday.

These are microscopic particles of toxic compounds or heavy metals often produced by factories and car exhaust small enough to penetrate deep into the lungs. They cause inflammation, breathing difficulties and other health issues when inhaled.

More than 200,000 people in the US live within the affected zone, which stretches north for about 90 miles, from the US southern border to Blythe, California.

The largest city in the zone deemed 'hazardous' by the EPA was Yuma, Arizona, which has a population of nearly 100,000 in the city itself and another 110,000 living within the region's greater metro area.

Melissa Zaremba from NBC News 11 said: 'It's been a gusty and dusty day so far! That's why our air quality is reading unhealthy for Yuma County.'

The station's chief weather forecaster added that an air quality alert would remain in effect until 9pm local time (12am ET).

'It would be a good idea to stay indoors,' Zaremba wrote on social media.

As Zaremba noted, dust storms at the US-Mexico border are a common problem, and the EPA warned that Friday's air quality levels in the region had deteriorated to 'hazardous' levels, the worst rating for air quality measurements the agency gives.

Along with high levels of smaller pieces of particulate matter, known as PM2.5, the dust blowing through the Arizona air has also sent levels of larger particles called PM10 skyrocketing as well.

PM10 is a type of particulate matter 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. 

This form of air pollution is noticeably larger than PM2.5, however, both can damage the lungs, worsen respiratory issues such as asthma and contribute to heart attacks and strokes that cause premature death.

AccuWeather's air quality tracking system registered that levels of both PM2.5 and PM10 had reached the 'unhealthy' range.

The weather agency warned: 'Healthy individuals may experience difficulty breathing and throat irritation with prolonged exposure. Limit outdoor activity.'

According to the World Air Quality Index project, a nonprofit providing real-time air pollution data for hundreds of countries, the air quality index (AQI) in Yuma reached 400 by 3pm 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.

When outdoor air quality reaches the 300 to 500 hazardous zone, the EPA warns that everyone should avoid all physical activity outside.

Anyone with heart or lung disease, older adults, children and pregnant people could be especially at risk to this concentrated air pollution and should remain indoors and keep their activity low.

Health officials also urge everyone within these zones to keep windows and doors closed to limit outdoor pollution from leaking inside. Using air conditioners or air purifiers can help as well.