A cloud of pollen has coated cars and homes across a huge stretch of the east of England in a film of yellow dust.
A yellow-green haze is visible thanks to new satellite technology
Weather experts at the Met Office have been inundated with calls from baffled residents in East Anglia, Lincolnshire and East and North Yorkshire.
But scientists believe the dust is largely made up of pollen being blown across the North Sea from Scandinavia.
Satellite images taken over the last two days show a large yellow-green plume sweeping up the coast.
Mrs Pam Woodliffe, who lives near Wroxham in Norfolk, told BBC News: "I got a call yesterday from my brother who lives in Goole in East Yorkshire saying his car was covered in this dust.
"He said you could even taste it in the air it was that strong.
The phenomenon is believed to have been caused by birch pollen
"I went outside and found there was a film of yellow dust covering my bins and then some of my neighbours said their cars were covered in it, so its obviously affecting a very big area.
"We were a bit concerned because my granddaughter suffers from asthma."
Tests on the yellow dust have been carried out by the National Pollen and Aerobiology Research Unit.
The unit's head, Professor Jean Emberlin, advised anyone suffering from breathing problems or itchy eyes to consult their GP or a pharmacist.
"Every year there is long-range transportation of birch pollen from Germany and Scandanavia," she said.
"But it sometimes goes unnoticed , perhaps because it is not so large an event."
She said about a quarter of hayfever sufferers were allergic to birch pollen.
A Met Office spokesman said a wet April followed by a warm, sunny early May had produced record levels of birch pollen in Denmark.
"The explosion of birch pollen in Denmark this year has been caused by near-perfect weather conditions over the last few weeks," he said.
"A late start to the pollen season, a wet April followed by a warm, sunny early May have combined to see birch catkins releasing an enormous amount of pollen grains over just a few days."
The problem had probably been exacerbated by local tree pollen and rape seed crops in eastern England, he added.
There had also been widespread arable burning in the fields of western Russia which could have blown across to the UK and contributed to the dust.