Worse, such greenhouse gases as tropospheric ozone and diesel fumes have simultaneously increased allergy sufferers' sensitivity to ragweed pollen by as much as seven times, the report released this week said. And the outlook is for another doubling of pollen levels this century. Lewis Ziska, the USDA plant physiologist who conducted the study, found that excessive amounts of carbon dioxide in the air cause ragweed plants to produce abnormal amounts of pollen. The gas also appears to make the plants start producing pollen earlier in the season. These carbon dioxide concentrations are a direct result of industrialization and expanding automobile use.
Scientists believe they are the principal cause of the trend to global warming.
Ziska says carbon dioxide levels have risen from 280 parts per million in the early 20th Century to 370 parts per million. If trends continue, carbon dioxide levels will rise to 600 parts per million within decades. Most of the increase to the current high levels occurred over the last 40 years or so, he said. Working in cooperation with Johns Hopkins University, Ziska performed his study in three areas of Maryland that typified air quality conditions in the rest of the country. One was within the city of Baltimore, another was in the surrounding suburban area and the third was in open farm country.