Western United States has the worst air quality in the world, data indicates

Wildfire smoke has choked the West Coast as firefighters continue to battle massive wildfires spanning several states. According to IQAir, an air quality monitoring system, many U.S. cities had the worst air quality of any major cities around the world as of Monday.

Several major cities were affected, including Seattle, Los Angeles, San Francisco and Portland.

Portland took the number-one spot, reaching hazardous air quality levels. The hazardous category indicates high concentrations of hazardous particles in the air, including smoke, dust and soot.

“In recent days, the air quality in many areas along the West Coast – including San Francisco, Portland, and Seattle, which normally have good air quality – has been worse than other parts of the world because of the dense smoke from numerous large wildfires,” Patrick H. Zahn, lead forecaster at Sonoma Technology, an environmental consulting firm in Northern California, said.

The Air Quality Index (AQI) is a measurement that runs from 0 to 500. “The higher the AQI value, the greater the level of air pollution and the greater the health concern,” AQI stated. “For example, an AQI value of 50 or below represents good air quality, while an AQI value over 300 represents hazardous air quality.”

At 3 p.m. PDT Monday afternoon, the AQI reached 409 in Portland.

The Environmental Protection Agency warned that wildfire smoke can cause poor air quality and irritate an individual’s eyes, nose, throat and lungs, making it difficult to breathe.

 "People with asthma and other respiratory conditions may exhibit symptoms from such exposure and if so, should remain in close contact with their health care providers," the EPA told Fox TV stations. "If the levels are unhealthy, they should stay indoors, keep doors and windows closed when temperatures permit, operate air purifiers or HVAC systems with air filters, and stop or minimize outdoor activity. We also recommend that everyone near the fires pay close attention to local authorities for fire and evacuation warnings and prepare a plan for a quick departure, if necessary," the EPA said. 

The EPA said children are also especially at risk for health effects from exposure to wildfire smoke. “It can travel hundreds of miles so it’s important to pay attention to local air quality reports even if no wildfires are nearby,” the EPA wrote on Twitter.

Residents in Portland were asked to stay indoors and reduce activity levels.

Zahn said that the smoke has lingered and is now entrained behind an approaching weak cold front.

“Although we usually expect a clean-out behind a front, this entrained smoke may actually cause AQI levels to remain high after it moves inland on Thursday. However, sustained onshore winds should gradually bring a cleaner air mass into northern California on Friday and into the weekend," Zahn added.

Wildfires continue to scorch millions of acres in the Western United States. Authorities said over the weekend that wildfires killed at least 35 people from California to Washington state, according to the Associated Press.

Sonoma Technology said that current satellite imagery shows direct smoke transport from numerous large wildfires along the West Coast intro California's Central Valley, the San Francisco Bay Area, the Lake Tahoe region and father north in Oregon, Washington, Idaho and Montana.

RELATED: 3 more die in California wildfire as state passes 3 million acres burned

According to Q13 FOX in Seattle, just under 300 firefighting crew members were able to contain the Cold Springs Canyon Fire in Washington State by 45% on Sunday. The fire spread through 188,852 acres in the area.

Much of California has also been impacted by wildfires. The North Complex Fire has burned over 261,000 acres and is 26% contained, while the August Complex Fire in Mendocino and Humboldt counties is 30% contained. The North Complex Fire claimed three more lives, bringing the total death count in recent California wildfires to 22, according to KTVU FOX 2.

So far, over 3.3 million acres have been scorched across the state of California.

“We are over 27 times the amount of acreage burned. In fact, just to put acres burned in perspective. Over the size of the state of Connecticut has burned in these fires this year,” said Daniel Berlant, Cal Fire deputy director.

RELATED: Satellite video shows smoke from West Coast wildfires being pulled into Pacific cyclone

According to local Oregon reports, more than 20 people remain missing from the wildfires burning across Oregon.

Gov. Kate Brown said Monday that 10 people were confirmed dead and that the number would likely rise as more confirmations come in from local law enforcement and medical authorities, Fox 12 Oregon reported.

Brown urged Oregon residents to heed the warnings and advice of local officials.

The Riverside Fire in Oregon has burned over 134,000 acres and remained at 0% containment.

President Donald Trump and Democratic nominee Joe Biden focused their presidential battle on the wildfires on Monday.

Trump met with fire officials in California. "We need good, strong forest management, which I've been talking about for three years," Trump told reporters. "We have the best people in the world looking at this."

Biden declared the fires were a display of an urgent need to address climate change. “This is another crisis, another crisis he [Trump] won't take responsibility for,” Biden said. “If you give a climate denier four more years in the White House, why would we be surprised that we have more America ablaze?”

Forecasters say the air quality will likely remain at unhealthy levels as the smoke persists from the fires.

“As a weak low pressure approaches the West Coast later this week and into the weekend, slightly stronger onshore winds may help to disperse smoke somewhat, and rain showers in the Pacific Northwest may reduce fire activity slightly,” Zahn said. “However, the long-range air quality outlook depends heavily on the fire behavior of numerous large fires along the West Coast. If the fires continue to burn and produce significant smoke, air quality impacts could be felt into next week or longer.”