Global Warming May Be Spurring Allergy, Asthma
By Gautam Niak (The Wall Street Journal) - May 3, 2007
There’s growing scientific evidence that global climate change is linked to the dramatic rise in allergies and asthma in the Western world.
Studies have found that a higher level of carbon dioxide turbocharges the growth of plants whose pollen triggers allergies. In 2001 Lewis Ziska planted ragweed — the main cause of hay fever in the fall — at urban, suburban and rural sites near Baltimore. The plots had the same seeds and soil and were watered in the same way. Yet the downtown plants soon exploded in size, flowering earlier and producing five times the pollen of rural plants. The city pollen was a lot more toxic, too. The likely cause? The city plants experienced warmer temperatures and 20% more carbon dioxide, the effect of more cars and pollution.
“We can see the changes now, and they already have implications for public health,” says Dr. Ziska, a plant physiologist at the U.S. Department of Agriculture. Allergies and asthma are closely linked; more than 70% of asthma sufferers also have allergies.
The Intergovernmental Panel on Climate Change, a group of the world’s leading climate researchers, will address the issue in its August report. According to Bettina Menne, a doctor at the World Health Organization and a lead author of the chapter on climate change and health, the report will say that higher temperatures and carbon-dioxide levels have increased the abundance of pollen, known to trigger allergies and worsen asthma. It will also conclude that spring, when allergy-causing tree pollen is at peak levels, has been arriving 10 to 15 days earlier over the past three decades, a trend expected to continue in coming years.
“We can assume that allergic disorders are starting earlier because the pollen season starts earlier,” Dr. Menne says. “Climate change contributes to the problem, but we don’t exactly know by what amount.” The upcoming IPCC report remains inconclusive on whether pollen is becoming more toxic.
For years, scientists have tried to explain the big increase in allergies and asthma in Western countries. Today, roughly 35 million Americans have seasonal hay-fever allergies and about 20 million suffer from asthma. Even though the air in many cities is much cleaner than in the past, the prevalence of hay fever has increased in the U.S. over the past few decades. In 2004, asthma affected more than 6% of the American population, up from a little over 3% in 1980, according to the U.S. Centers for Disease Control and Prevention in Atlanta.
Childhood asthma is increasing at an even-faster rate. The percentage of children with asthma jumped to 9% in 2005 from 3.6% in 1980, according to the CDC.
In 2004, a Harvard Medical School study linked the childhood asthma “epidemic” among inner-city youths to climate change. Stating that higher carbon-dioxide levels in cities promote pollen production in plants, fungal growth, and opportunistic weeds, the study noted that asthma among preschool children grew 160% between 1980 and 1994, more than double the increase for the overall U.S. population.
Much research remains to be done. No one has measured populations of allergy-inducing plants nationwide to establish how pollen levels have changed over the years.
The climate-change connection is “an interesting hypothesis that deserves further research,” says Darryl Zeldin, a senior scientist at the National Institute of Environmental Health Sciences, part of the National Institutes of Health. “All the data isn’t in.”
Some scientists blame dust mites, mold, and other indoor allergens. Others point to specific pollutants in the air, changed diets, genetic susceptibility and higher obesity rates. Another idea, called the “hygiene hypothesis,” argues that people today suffer from more allergies because they faced less exposure to infectious agents in childhood.
“I do think that climate change contributes” to the rise in allergies and asthma, “but it’s not the only answer,” says Christopher Randolph, an allergist and clinical professor at Yale University.
The links between climate change, allergies and asthma are multilayered. Higher carbon-dioxide levels, the result of human activity, are believed to warm the atmosphere, which in turn affects the geographic location and growing season of allergy-producing plants. A higher level of carbon dioxide separately spurs the growth of ragweed and other allergy culprits. Studies indicate it may make pollen more potent as well. Over the longer term, other potential impacts of global climate change — changes in precipitation, more intense hurricanes — could affect the growth of molds, which also cause allergies, or affect how far plant pollens are dispersed.
In recent years, pollen counts have hit record levels in many urban areas. A pollen count of 150 grains per cubic meter or more is very high; many U.S. and European cities sometimes show counts in the thousands. The season for birch — a major hay-fever nuisance in Western Europe — has arrived five days earlier each decade over the past 30 years.
It isn’t surprising that rising carbon-dioxide levels and warming climes will alter the biology, chemistry and geographical distribution of plants. After all, many plants thrive when there’s more carbon dioxide in the air, or when it’s warmer.
But the latest research has found that such shifts especially boost invasive plants, including allergy culprits like ragweed. “No one had thought in the 1990s that higher carbon-dioxide levels would have a disproportionate effect on weeds,” says Paul Epstein, associate director of the Center for Health and the Global Environment at Harvard Medical School. “It came as a surprise.”
In the U.S., about three-quarters of the 35 million people who suffer nasal allergies are primarily allergic to the pollen of ragweed, whose season extends from mid-August to October. Common ragweed is a gray-green plant that can attain a height of about 3 feet. A North American native, it often erupts along roadsides, river banks and in urban lots disturbed by construction or digging.
Over the past few years, ragweed has begun spreading through Europe. Barely seen on the Continent a decade ago, ragweed can now be found in Eastern Europe, especially in Hungary, and also in France, Italy, the Netherlands and Scandinavia.
“It’s a good example of a plant changing its distribution because of climate change,” says Jean Emberlin, director of the National Pollen and Aerobiology Research Unit in Britain.
A host of independent studies draw a link between higher carbon-dioxide levels and weed growth. A paper published in 2002 by Harvard’s Dr. Epstein and his colleagues showed that a single ragweed plant, which normally produces one billion pollen grains per season, produces 61% more when its exposure to carbon dioxide is doubled in a lab setting.
At the Jasper Ridge Biological Preserve in the eastern foothills of California’s Santa Cruz mountains, scientists from Stanford University mimic future global-warming conditions by using artificial heaters and bombarding plants with carbon-dioxide emissions. Their key finding: Higher amounts of the gas fuel weed growth, and drive out native flora.
Some of the most dramatic results have been obtained by Dr. Ziska. A researcher at the USDA’s Crop Systems and Global Change lab in Beltsville, Md., Dr. Ziska spent several years trying to draw attention to how changing concentrations of carbon dioxide were likely to affect crop and plant life. No one seemed interested. In early 1999 Dr. Ziska, who suffers from ragweed allergies and asthma himself, had an idea: Would people pay more attention if he could show that rising carbon-dioxide levels affected their health?
Wearing a face mask to protect himself from the pollen, Dr. Ziska began to cultivate ragweed in large aluminum chambers at three different levels of carbon-dioxide concentrations. The first was 280 parts per million of air, the level that existed before industrialization. For the present era, he set the level to 370 parts per million. He also experimented with 600 parts per million, a conservative estimate for the carbon-dioxide concentration expected by the end of the century.
Over the course of a year, Dr. Ziska found that each ragweed plant produced 21 grams of pollen at the end-of-century level — almost twice that produced at the present-day level and more than four times that produced at the “pre-industrial” setting. Also, the higher the dose of carbon dioxide, the bigger the plant.
Dr. Ziska then decided he needed a “real world” setting. Driving around one day, he noticed large clumps of ragweed growing in empty lots and along the sides of the road. He realized that he had a ready-made setting for his experiment: Baltimore itself.
Cities are subject to the “heat island” effect — a dome of higher temperatures caused by the warmth absorbed, then reflected, by structures and pavement. Moreover, the greater number of cars means urban areas have more carbon dioxide than outlying ones. Dr. Ziska figured that if he could measure how ragweed and about 35 other plants grew in downtown Baltimore versus a suburban and rural plot, it would provide clues about how they might compete in a warmer, carbon dioxide-richer world.
Through his field experiment, Dr. Ziska found that ragweed seed emerged three or four days earlier at the urban site than at the rural one, which is 40 miles away. Plus, the plants were huge, weighing nearly 190% more at the city location compared with the rural spot. A lot more pollen was produced in Baltimore as well.
In 2005, two years after that study was published, Dr. Ziska and other scientists published research about a pollen protein that triggers an allergic reaction to ragweed. Chemical analysis showed that there was far more of the protein on the pollen of plants bathed in higher levels of carbon dioxide — thus making it a lot more potent — compared with pollen of plants exposed to lower amounts of the gas.
One recent afternoon, Dr. Ziska visited the rural site, near Buckeystown, Md. He strode over to a 12-foot-tall metal tripod, hung with a solar panel and electronic instruments, and checked several measurements. The carbon dioxide level was 395 parts per million and the temperature was 47 degrees Fahrenheit.
In the experimental plots of land, each about 5 square yards, several plants were starting to come up. But some of the most prominent were 3-foot-tall saplings of oak, astor and Norway maple, each of which can also trigger severe nasal allergies.
An hour later, Dr. Ziska was at the urban site, a fenced-off area in Baltimore’s downtown science museum. Here, the instruments told a different story: The temperature was 53 degrees Fahrenheit and the carbon-dioxide level was a much higher 477 parts per million — not far from the 500-level in the overall atmosphere expected by 2050, based on a conservative estimate.
Resilient as ragweed is, other plants can be tougher in the long run. After five years of running the Baltimore field experiments, Dr. Ziska found that the amount of ragweed growing in the urban location declined as hardier plants took over. Eventually, pollen-bearing trees — the main source of spring allergies — emerged dominant.
These trees are also flourishing. The saplings here were at least twice the size of the plants at the rural site, and already sprouting large leaves. Last year, one of the plants at the urban site, a weed called ailanthus, had become a 20-foot giant and had to be chopped down. Even now, the plants at the museum site are at least five years ahead in their growth compared with their counterparts at the rural location.
Then Dr. Ziska bent down and pointed to a handful of half-inch seedlings sprouting in one of the beds. “That’s where ragweed came up last year,” he said. “Looks like it’s coming up again.”