The Copernicus Atmosphere Monitoring Service (CAMS) observed a seasonal rise in grass and olive pollen release and transport across southern Europe and "extreme levels" of birch pollen in northeastern regions.

Finland in particular experienced "extreme daily means of birch pollen" in May "that led to symptoms even among individuals without known allergies", the EU agency said in its latest air quality update.

Scientists say that climate change is altering the production and distribution of pollen and spores, as more and more people have reported developing allergy symptoms.

As winter frost thaws earlier and spring weather gets warmer, plants and trees flower earlier, extending the pollen season and misery for allergy sufferers.

Around a quarter of adults in Europe suffer from airborne allergies, including severe asthma, while the proportion among children is 30 to 40 percent.

That figure is expected to rise to half of Europeans by 2050, according to the World Health Organization.

CAMS director Laurence Rouil said bouts of extremely high pollen levels were not unusual in spring but noted the particular "severity and extent" of this year's episode.

Air pollution can also increase people's sensitivity to allergens, while invasive species are spreading into new regions and causing fresh waves of allergies.

Between March and May, air quality across Europe was also affected by wildfires.

April fire emissions in the UK were the second highest since 2003, while the Netherlands recorded unprecedented wildfire emissions levels in this period of the year, CAMS said.

Further afield, large wildfires in eastern Russia sent "significant" smoke into China and Japan while plumes from major blazes in Canada were observed over Europe in late May, it added.