Image copyright Facebook Image caption The air is so heavy the snow has settled on Capitol Hill
Rising temperatures on the West Coast of the US over the past week have encouraged wildfires to engulf thousands of acres, with Californians in the grips of a state of emergency.
So why is the nation so seriously affected? And how will it end?
In a conversation with BBC Radio 4’s PM programme in Washington DC, Eric Stockton, Director of the Center for Air Quality and Climate Change, explains how the thick smog is nothing new.
To access the MP3 version of this interview, please click here .
Metro air quality is an assessment of the impact on human health of air pollution across the United States.
Data from sensors monitor air quality levels across 300 different US metropolitan areas, including the EPA’s smog index.
Image copyright EPA Image caption Pews and cots provided at a shelter in the town of Blanchard, north of San Diego, California
As winter began last week in the US, wildfires encroached upon vast swathes of land.
Average temperatures on the west coast began to rise by 1C to 4C above normal and those warmer conditions led to “particularly severe” air quality problems and wildfires due to increased fuel and smoke in the air.
Temperatures are expected to fall this week, allowing the air quality to improve and the storm systems to move towards the east coast.
Officials have been taking measures to prevent the heavy air pollution from continuing to worsen, with efforts to send people home earlier and workers in the Washington DC area to go home earlier, plus air-clearing efforts by private companies.
Urban residents and their cars have been burning more to stay warm, with less outside work in sub-zero temperatures.
But the higher temperatures expected to return in the region won’t bring relief.
Eric Stockton says it will take a lot more science to understand how climate change is causing local and regional air quality to be exacerbated by warmer temperatures.
“But it does make sense that as we get hotter the amount of time that is spent outside has increased,” he explains.
“So it could be a lot hotter in the hot, desert valleys and hotter in the mountains and the high desert, and that can lead to more smoke if there’s more fuels.”
Eric says it is not clear if the increase in air pollution is linked to climate change, but that there is some evidence.
During the hottest summer on record in Phoenix, a study showed the number of deaths that occurred were more than double compared to other years.
“It showed that there was a lot of extra particulate matter in the air. There’s also a lot of thermal warming that they associate with the heat.
“We might not know the whole story but the big story is one of a lot of deaths that occur in the summertime in some of these large cities like Phoenix.”
Image copyright AFP Image caption Esteban Duarte, right, at a community center in San Diego, which has been making space for displaced residents
So this odd combination of dry, cold weather and dangerous air quality with wildfires is the worst in the US since 1997, when total annual air pollution exceeded levels in 2016.
Since then, Eric says there has been some improvement.
“When we do have bad air days, it’s now 12-18 days a year on average, versus 33 in ’97, and that’s down fairly significantly.”
We’ll continue our series with discussions in major cities across the US.