VOA常速英语:Climate Change: Good for Weeds, Bad for Allergies

More than 50 million people in the United States suffer from allergies, and their numbers are growing. New studies suggest that climate change may be at least partly to blame. Scientists say rising average temperatures and elevated levels of carbon dioxide are spurring the growth of many weedy, allergenic plants - such as ragweed - and extending the season of suffering for pollen-sensitive people across the country.

At the U.S. Department of Agriculture's research center near Washington, D.C., plant physiologist Lewis Ziska said weeds - defined as any unwanted plant - appear to be thriving in today's atmosphere, which is richer in carbon dioxide, or CO2. And Ziska said agricultural research studies show that if climate trends continue, allergy sufferers will have to endure longer and more intense pollen seasons in the decades ahead.

"The average ragweed plant growing in the year 1900, for that CO2 concentration was producing about 5 grams of pollen," he said. "If you go forward in time to the 1990s and look at the CO2 concentration at that time, the average ragweed produced about 10 grams of pollen. If you go into the future 50 years from now, at the [expected] CO2 concentration, that ragweed will produce 20 grams of pollen."

Ziska began studying allergenic weeds and pollen 10 years ago. He said the more pollen a weed puts out, the more potential harm there is to public health and to agriculture, where weeds can steal water, soil nutrients and light from commercial crops. Ziska said one of the most common and most allergenic weeds is the fast-growing plant called ragweed.

“This is common ragweed coming up in the spring," he said. "It can get up anywhere from four to six feet (1.5 to 2 meters) tall.”

Dr. Darryl Zeldin is a senior investigator and acting clinical director at the National Institute for Environmental Health Science in North Carolina. He said common allergies have been on the rise for the past 10 to 15 years. And he said that if there's going to be more pollen from weeds in the years ahead, he's certain the number of allergy sufferers - now estimated at 50 million people - will continue to rise, as well.

Zeldin said asthma and allergies are closely related. Most people with asthma also suffer from allergies.

“Allergies are thought to cost the public somewhere in excess of $8 billion to $10 billion every year, in both direct and indirect costs," said Zeldin. "Asthma is also a major issue and costs billions of dollars a year, not just in doctor's office visits, in hospitalizations, and emergency department visits.”

Zeldin also said there are many causes for allergies, and people with allergies often are sensitive to more than one weed. His latest research on the subject has shown that environmental and genetic factors play a role as well. And there is evidence of a connection between a pre-disposition to asthma and allergies, and obesity and cardiovascular disease.

“Asthma and allergies are a major health problem in the U.S.," he said. "(They) cost a lot of money, major cost of symptoms and hospitalizations and doctor’s office visits and there are not real cures.” 

Among the thousands of known weed species, only eight or 10 are known to induce allergies. Many species of weeds, however, have become resistant to herbicides. And that means more money will be needed for health and agricultural research, including new and safer methods of controlling weeds.