Carbon dioxide emissions may be raising the pollen counts in European cities, a continent-wide study has indicated.
Researchers from 13 EU nations analysed pollen levels for more than 20 species of trees and plants.
They found that many, including several that cause allergies such as hay fever, correlated with rising CO2 levels.
"We thought the increase in the amount of pollen could be related to land use changes, but we don't observe this," BBC quoted Annette Menzel from the Technical University of Munich, as saying.
"We tried to link it to temperature, but that's not possible.
"So the only effect that's left would be a CO2 effect; and we know from experiments in the real world and in climate chambers that CO2 does promote the amount of pollen [that trees produce]," Menzel added.
The study was presented at the European Geosciences Union (EGU) annual meeting.