OREGON • Wildfire smoke from the western United States stretched across the country this week, covering skies in a thick haze and triggering health alerts.
Air quality remained in the unhealthy range across much of the east coast as the haze pushed southwards on Wednesday.
In recent weeks, a series of near-relentless heatwaves and a deepening drought linked to climate change has helped to fuel exploding wildfires.
In southern Oregon, the Bootleg Fire grew so large and hot that it created its own weather, triggering lightning and releasing enormous amounts of smoke.
More than 80 large fires are burning across 13 American states.
Now, the effects are being felt thousands of kilometres from the flames. As the smoke moved eastwards across New York and Philadelphia on Tuesday, concentrations of dangerous microscopic air pollution reached highs in the "unhealthy" range for most of the day.
New York's reported high of 137 on the city's air quality index was firmly in that unhealthy range, although still bearable for the general population, according to its Department of Environmental Conservation. The city's median reading last year was 47, in the healthy range.
"What we're seeing here today is the convergence of several smoke plumes," said Dr Nancy French, a wildfire scientist at Michigan Technological University, noting that much of the US was experiencing some haze, even as the highest surface pollution swept across the Midwest and the north-east.
On Tuesday, eerie orange sunsets were coupled with scratchy throats and watering eyes for many across the two regions.
Fine particulate matter, released during wildfires and also through the burning of fossil fuels, is dangerous to human health. Breathing high concentrations of PM2.5 can increase the risk of asthma attacks, heart attacks and strokes.
The microscopic air pollution is known as PM2.5 because the particles are less than 2.5 microns in diameter, or one-thirtieth the diameter of a strand of human hair.
Singapore uses the Pollutant Standards Index (PSI), computed based on the 24-hour average of PM2.5 concentration levels, to measure air quality.
It is not unprecedented to see smoke travel long distances, said Assistant Professor Roisin Commane, an atmospheric scientist at Columbia University, but it does not always descend to the surface.
Prof Commane said people should avoid going outdoors in high-pollution conditions, and especially avoid strenuous exercise. She suggested that filtered masks can provide protection for those who cannot avoid the outdoors.
"A lot of the masks people have been wearing for Covid-19 are designed to capture PM2.5," she said, referring to N95-style masks. "That's the right size to be very useful for air quality."