Thousands of Americans have been told to stay indoors due to hazardous air quality sweeping across several US states. 

Parts of Tennessee, Georgia and Alabama have been deemed 'Unhealthy' due to fine particle pollution (PM2.5), microscopic particles small enough to penetrate deep into the lungs. 

The particles can also trigger inflammation, breathing difficulties and other health issues when inhaled, and are considered life-threatening and are a major cause of premature death worldwide, responsible for an estimated 4.2 million deaths annually.

The advisories are focused on where the three states meet just west of Chattanooga, Tennessee, north of Huntsville, Alabama and Rome, Georgia. 

Chattanooga’s air quality issues are largely caused by its valley geography, which traps pollution under temperature inversions and prevents dirty air from dispersing. 

Prescribed burns and controlled fires in northwest Georgia and the surrounding area frequently send smoke plumes into the valley. 

Hazardous air is also being reported across southern Arizona, focused on the Sonoran Desert region.

Health officials urged locals to stay indoors if possible, adding that windows should be shut during peak smoke hours and closely track real-time air quality alerts.

The hazardous levels in Arizona are likely tied to extreme events like massive dust storms or heavy wildfire smoke.

The Sonoran Desert is naturally dusty, with conditions, wind, construction, agriculture, and vehicle traffic on unpaved or disturbed areas kicking up coarse dust.

Recent weeks have seen reports of blowing dust events across Arizona due to strong winds and dry ground, which can create localized spikes. 

Data on AirNow.gov suggests elevated but not necessarily emergency-level pollution, with hotspots indicating localized worsening. 

However, the Tennessee hotspot is much more localized and heavily influenced by mountain-valley trapping. 

The Cumberland Plateau acts like a wall, concentrating pollution right over Chattanooga.

The dominant pollutants are PM2.5 and ozone, with PM10 also elevated. PM10 is particulate matter with a diameter of 10 micrometers or less, consisting of dust, soot, smoke, and liquid droplets suspended in the air. 

Late March is peak season for prescribed burning across Georgia, Alabama and the Southeast for forest management, wildfire prevention and land clearing.

Multiple burns are occurring right now, sending smoke plumes that concentrate in localized spots like Macon, Georgia, which is under an unhealthy air quality warning.

High pollen counts common in March can also add to the particle load and make the air feel hazier, but the primary driver of the red shading is smoke from the burns.