Over half a million Americans have been warned to stay indoors and lock their windows as toxins deemed hazardous to human health fill the air.

The US Environmental Protection Agency (EPA) revealed large swaths of heavily polluted air in Arizona, California, Arizona and Oregon, including pockets showing high levels of dangerous toxins known as fine particulate matter or PM2.5.

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.

The worst pockets have been detected over several major cities in Arizona, including Gilbert and Chandler, which have a combined population of more than 580,000.

The EPA has warned that air quality levels in both cities registered as 'hazardous' Monday morning - their worst rating for air quality measurements. Both are less than 20 miles east of Phoenix, which has a population of more than 1.6million people.