Hay fever sufferers could reach new heights of misery over the next century as pollen levels soar, the latest study suggests.
Scientists predict that higher levels of carbon dioxide (CO2) in the atmosphere will lead to a significant boost in the amount of air-borne grass pollen.
Despite increases in ozone, which suppresses plant growth, pollen production is expected to more than treble in the next 100 years.
Grass plants grown in special reactor chambers were exposed to different atmospheric gas concentrations, with different ratios of carbon dioxide and ozone tested to show their effects on grass and pollen production.
The scientists found that an atmospheric CO2 concentrations of 800 parts per million - possible by the end of the century according to some forecasts - raised pollen production per grass flower by 53 per cent.
Greater plant growth stimulated by the elevated CO2 further increased pollen levels, resulting in an overall boost of up to 202 per cent. This was despite the dampening effect of higher ozone levels.
Dr Christine Rogers was the US lead scientist from the University of Massachusetts at Amherst.
"Stimulation of grass pollen production by elevated CO2 will increase airborne concentrations and increase exposure and suffering in grass pollen-allergic individuals," Rogers said.
The researchers, whose findings are reported in the online journal Public Library of Science ONE, focused on Timothy grass, a major cause of hay fever.
"This is the first evidence that pollen production is significantly stimulated by elevated carbon dioxide in a grass species and has worldwide implications due to the ubiquitous presence of grasses in all biomes (vegetation habitats) and high prevalence of grass pollen allergy," Rogers said.
"These results are similar to our other studies performed in other highly allergenic taxa such as ragweed but with more extreme outcomes and wider impacts."