Health warnings have been issued across parts of the US on Thursday, with officials urging residents in affected areas to stay indoors if possible.
Federal data shows pockets of unhealthy air quality emerging in Oregon, Arizona and Georgia, alongside widespread moderate pollution across much of the country.
Air quality maps show elevated levels of PM2.5, microscopic particles small enough to penetrate deep into the lungs, creating hazardous conditions, particularly for people with respiratory conditions such as asthma.
These particles are typically produced by vehicle emissions, industrial activity and burning wood, and can trigger inflammation, breathing difficulties and other health issues when inhaled.
In southern Oregon, near the town of Chiloquin, air quality has reached unhealthy levels due to smoke from controlled burns, with pollution trapped near the ground by a temperature inversion that prevents it from dispersing.
In southern Arizona, including areas near Tucson, blowing dust from dry desert soils is driving similar spikes, as strong winds lift fine particles into the air and combine with emissions from traffic and construction.
Although major metro areas like Phoenix remain mostly in the moderate range, localized surges are pushing some neighborhoods into unhealthy territory.
In the Southeast, particularly around Atlanta, pollution is being driven primarily by ozone, which forms when emissions from vehicles, power plants and industry react in sunlight as temperatures rise.
Across parts of Kansas, Missouri and Oklahoma, smoke from seasonal agricultural and prescribed burns has created another cluster of hot spots.
These fires are common in the spring and are used to manage land, but the resulting smoke can travel long distances and linger, especially under calm weather conditions.
Officials stressed that the situation is not a nationwide emergency, but rather a series of localized pollution events tied to seasonal conditions.
However, residents are advised to limit prolonged outdoor activity, especially strenuous exercise, as air quality fluctuates.
Sensitive groups, including children, older adults and those with heart or lung conditions, are urged to avoid extended time outside during peak pollution periods.
Experts also recommended keeping windows closed, using air conditioning on recirculate mode and running HEPA air purifiers where possible to reduce indoor exposure.
The poor air quality comes as a historic heatwave is sweeping over the Southwest, triggering alerts for millions to stay indoors until 8pm local time on Friday.
The extreme heat warning covers Southern California, as temperatures are expected to reach 104 degrees Fahrenheit in some areas.