allergy sufferers, beware: global warming could bring more hay fever, according to us government research that shows ragweed produces significantly more pollen as carbon dioxide increases. The ubiquitous weed makes nearly twice as much pollen now as it did 100 years ago and is likely to double its production over the next century with predicted increases in carbon dioxide levels, suggests a study by the us department of agriculture ( usda ).

"This research may help us understand the troubling impact of high carbon dioxide levels on our environment and our health,' says agriculture secretary Dan Glickman. About 15-20 per cent of the population in usa has hay fever