The UN's World Meteorological Organization (WMO) has found that a shifting climate has already begun altering the production and distribution of pollen and spores.

As winter frost thaws earlier and spring weather gets warmer, plants and trees flower earlier, extending the pollen season, numerous studies have shown.

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

More and more people, particularly in industrialised nations, have reported developing allergy symptoms in recent decades.