Satellite images have picked up an area of smoke lying across the UK which can be traced back to wildfires in North America, according to the Met Office.
It said the cloud of smoke has “over the last day or so given us some vivid sunrises and sunsets”.
Forecasters in the US warned nearly a third of Americans will experience poor air quality on Thursday as smoke from the ongoing Canadian wildfires fills the skies over the mid-west and east, causing unhealthy and, in some spots, dangerous conditions.
The National Weather Service in the US said air quality alerts have been issued until midnight – from Wisconsin and northern Illinois to Michigan, and stretching to New York and the East Coast.
It has led to the cancellation of sports and cultural events – with British actress Jodie Comer halting her Broadway show after experiencing breathing difficulties.
Flights have also been cancelled.
More than 100 million Americans have been warned not to spend long outdoors and to wear a mask if necessary to avoid breathing problems, and children and the elderly have been urged to minimise or avoid strenuous activities.
“Take precautions on Thursday. If you have health conditions, including respiratory conditions such as asthma, reduce your time outdoors,” New York City Mayor Eric Adams tweeted.
On Thursday morning, a dull sky hung over Chicago for the third consecutive day.
The air quality was “unhealthy” – and according to IQAir.com which tracks pollution, it had the poorest air of any major city on the planet.
Canadian officials have said this is shaping up to be the nation’s worst wildfire season ever.
The season has begun early on ground that is drier than normal and accelerated quickly – with smoke from the wildfires pouring towards the US east coast.
New York landmarks were shrouded in a yellowy-brown haze earlier this month as smoke from the fires drifted south.