Wildfires raging across the western United States and Canada, including a "monster" two-week-old blaze in Oregon, have belched smoke and soot that gusted eastward and caused harmful air pollution as far away as New York City.
In 13 western states, more than 80 large active wildfires have charred almost 526,000 hectares of drought-parched vegetation in recent weeks.
Several hundred additional fires have burned in western and central Canada. They included 86 classified as out of control on Tuesday in British Columbia alone, leading officials there to declare a state of emergency.
The jet stream and other cross-continental air currents have carried smoke and ash thousands of kilometres. People in distant cities were feeling the air contamination in their eyes, noses and lungs.
In the US West, parts of Idaho and Montana suffered from unhealthy levels of air pollution from 40 large blazes nearby and smoke from southern Oregon's Bootleg fire, currently the largest in the United States.
The wildfires themselves posed a more direct risk to life and property.
The Bootleg blaze has blackened 157,260 hectares of desiccated brush and timber in and around the Fremont-Winema National Forest, south of Portland, since erupting July 6. Only three other Oregon wildfires over the past century have burned more territory.
As of Tuesday, an army of some 2200 personnel had managed to carve containment lines around 30 per cent of the fire's periphery, while the blaze expanded farther to the east and north.
At least 67 homes have been destroyed and another 3400 were listed as threatened, with an estimated 2100 people under orders to evacuate or be ready to flee at a moment's notice.
The western conflagrations, marking a heavier-than-normal start of the wildfire season, have coincided with record-shattering heat that has baked much of the region in recent weeks and caused hundreds of deaths.
Scientists have said the growing frequency and intensity of wildfires are largely attributable to prolonged drought and increasing bouts of excessive heat that are symptomatic of climate change.
The Bootleg fire is so large that it has at times generated its own weather - towering pyrocumulus clouds of condensed moisture sucked up through the fire's smoke column from burned vegetation and the surrounding air.
These clouds can spawn lightning storms and high winds capable igniting new fires and spreading the flames.