Hay fever sufferers could reach new heights of misery over the next century as pollen levels soar, new research suggest.
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.
US lead scientist Dr Christine Rogers, from the University of Massachusetts at Amherst, said: "The implications of increasing CO2 for human health are clear. Stimulation of grass pollen production by elevated CO2 will increase airborne concentrations and increase exposure and suffering in grass pollen-allergic individuals."
For the study, grass plants grown in special reactor chambers were exposed to different atmospheric gas concentrations.
Different ratios of CO2 and ozone were tested to show their effects on grass and pollen.
The scientists found an atmospheric CO2 concentration of 800 parts per million - possible by the end of the century, say forecasts - raised pollen production per grass flower by 53pc.
Greater plant growth stimulated by the elevated CO2 further incre ased pollen levels, resulting in an overall boost of 202pc.