April 29, 2004 -- Rising greenhouse gas levels may be contributing to expanding rates of asthma in U.S. cities and worsening allergies in millions of urban and suburban people, a new Harvard Medical School report shows.
Researchers say that carbon dioxide emissions from cars, trucks, and industry are causing plants and molds to boost pollen and spore production. More pollen in the air is likely worsening allergic diseases such as asthma and may be to blame for the rise in cases among children, they conclude.
Soot from diesel engines may also contribute to the problem by irritating the lungs of asthma suffers, in effect weakening their defenses to the pollen, according to the report, which was funded by the Civil Society Institute, a nonprofit organization funding research in environmental and health issues.
"We are seeing some very troubling new evidence that may be exacerbating the allergies and allergens and the assaults on our respiratory systems," says Paul R. Epstein, MD, the report's co-author.
"Frankly it is a new problem that we do not know how to solve," says Epstein, who is associate director of the Center for Health and Global Environment at Harvard Medical School.
Asthma on the Rise
Approximately 14 million U.S. adults and 9 million children have asthma, according to 2001 CDC figures. Childhood asthma rates have more than doubled in the U.S, since 1984 in a trend that has largely baffled scientists. Some cite improved diagnosis for the increased numbers, while others blame worsening pollution and crowded living conditions that expose children to asthma-causing cockroach dander and dust mites.
Researchers say that atmospheric carbon dioxide (CO2) may be a main culprit. Recent studies identified CO2 levels in large U.S. cities including Phoenix and Baltimore, Md., that are at times up to 60% higher than in rural areas. Burning fossil fuel -- coal, oil and natural gases -- is the greatest contributor to the continuing increase in atmospheric carbon dioxide.
A 2003 study published in the Journal of Allergy and Clinical Immunology suggested that ragweed, which produces one of the most common allergens, is responding to the higher CO2 levels in the atmosphere by producing more pollen.
"Ragweed grew faster, flowered earlier, and produced significantly greater above-ground biomass and ragweed pollen at urban locations than at rural locations," an abstract of the study concludes.
Not Just Asthma, Not Just Cities?
Experts warned that the pollen problem may spread beyond inner cities. Global atmospheric CO2 levels have risen from 280 parts per million (ppm) at the start of the industrial revolution to 379 ppm today, according to the report.
It means that ragweed could thrive in suburban and rural areas as well, says Christine Rogers, PhD, a Harvard researcher and the study's other co-author. Up to 40 million Americans suffer from ragweed allergies, also known as hay fever.
"All Americans certainly are at risk for this," says Georges Benjamin, MD, executive director of the American Public Health Association.
But not everyone is convinced of the connection between CO2, pollen, and asthma. While CO2 has been shown to boost growth and pollen production in plants, no studies, including this Harvard report, have been able to find a hard link between increased pollen and asthma, says Bill O'Keefe, president of the Marshall Institute, a Washington think tank that regularly questions the extent humans' impact on CO2 levels and climate change.
"No one debates that CO2 levels are going up," says O'Keefe, who adds that his group receives funding from petroleum producers and industry groups. But of the link between those levels and rising rates of asthma, he says, "I think it's a stretch."