More than 20 million people could suffer hay fever this summer as unusually high levels of pollen sweep across the UK.
Grass pollen, which is the worst culprit for causing the symptoms and affects 95% of sufferers, is set to reach its peak.
Unusually high pollen levels have affected up to 20 million hay fever and asthma sufferers in recent weeks, but the Met Office has warned the worst is still to come.
A heatwave in April left behind a "pollen bomb" caused by releases from birch, plane and oak trees.
Millions of sufferers in southern England were affected before it spread northwards.
The Met Office has teamed up with the NHS to identify which of the 150 different grass pollen cause the most allergies, with research finding more than half of sufferers (57%) do not know what is causing their symptoms.
A poll of 2,000 British hay fever sufferers also revealed two-fifths (41%) of them experience the condition so badly it ruins their summer.
Experts have said many people may be causing their symptoms to worsen unnecessarily.
More than a third of respondents (35%) said they frequently hang washing out to dry in the summer, seemingly unaware that pollen sticks to their clothes.
A further 40% said they leave their windows open to cool their home in the summer, allowing pollen inside.
And only 49% of sufferers said they take antihistamines to relieve their hay fever.
The Met Office's Yolanda Clewlow, manager of the UK pollen network, said: "We know how seriously hay fever can impact people's lives in the UK, particularly as a result of grass pollen.
"This has led to our involvement in a dedicated research programme to identify the most significant of the 150 different species of grass pollen in the UK.
"We aim to help inform hay fever and asthma sufferers and empower them in managing their symptoms more effectively."
Tips on easing the symptoms of hay fever include:
- Putting Vaseline around the nostrils to trap pollen
- Wear wraparound sunglasses to prevent pollen getting into the eyes
- Staying indoors whenever possible
- Get advice from pharmacists on treatments including antihistamine drops, tablets or nasal sprays.
A spokeswoman for the Met Office said: "The reason we have such high pollen levels this year is because we have had perfect weather for pollen.
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"It's been lovely and warm, followed by rain, and then dry days and breezes that lift the pollen off the grass, plants and trees.
"It is unlikely that this will be a record-breaking year, although it is more severe than average and the worst in over a decade."