Web edition: July 11, 2006
U.S. agriculture has developed a heavy reliance on chemicals to safeguard crops from yield-robbing weeds. However, many of those herbicides can pose substantial health risks to people, pets, and wildlife, which is why laws prescribe how some of these chemicals are handled in fields. A study now finds that trace quantities of such agricultural chemicals nonetheless find their way into consumers' homesnot on the fruits and vegetables they buy but probably by hitchhiking on dust.
The findings are disturbing for a number of reasons, not the least of which is the link between pesticide exposure and non-Hodgkin's lymphoma, a malignancy whose incidence has exploded during recent years. Indeed, the new study was as an offshoot of a larger non-Hodgkin's lymphoma study financed by the National Cancer Institute.
What the research shows is that home exposure to agricultural weed killers increases as the acreage of nearby croplands increases.
In their new study, Mary H. Ward of the National Cancer Institute and her colleagues collected dust vacuumed from the homes of 112 Iowa lymphoma patients or healthy, randomly selected volunteers of their age. Using satellite-generated maps of agricultural fields in the state, the team calculated the acreage of croplands near the home of each participant. Both farm and in-town homes were included in the study.
This being Iowa, much of the cropland had been historically planted with corn and soybeans, almost all of which had been treated repeatedly with protective herbicides. Ward's team probed homes for specific chemicals known to have been used on the fields.
Analyses showed that at least one of six primarily agricultural herbicides was present in house dust from 28 percent of sampled homes. These chemicals included acetochlor, alachlor, atrazine, bentazon, fluazifop-p-butyl, and metolachlor.
Atrazine and metolachlor were the agents most commonly used to protect corn and soybeans from weeds. The next most-popular weed killers used on the crops were trifluralin and dicamba. At least one of these four herbicides showed up in 43 percent of homes.
Although atrazine had been applied to nearly 70 percent of corn acreage, it showed up in the house dust of only 8 percent of homes. Where detected, however, its concentration in dust ranged from 60 to 4,700 parts per billion (ppb). Metolachlor was found in about 20 percent of homes; its concentration ranged from 27 to almost 3,200 ppb.
However, such herbicide contamination paled in comparison to the amount of dust containing 2,4-D, the third most widely used herbicide in the United States and Canada. It was present in 95 percent of homes, typically in concentrations exceeding 1,000 ppb. In one house, 2,4-D's values reached an astounding 125,000 ppb. That it was the most abundant of the chemicals might not be too surprising, Ward notes. Not only is this chemical commonly employed to protect corn and soy, but it's also used along roadsides, in forests, and on lawns to fight weeds. Luckily, toxicity studies suggest that this is also one of the least toxic herbicides to people and animals.
As these are some of the first measurements of pesticides in house dust, the researchers don't have much with which to compare them. Most previous correlations between non-Hodgkin's lymphoma and herbicides came from questionnaires where data indicated only whether individuals had been exposed to certain chemicals and for how long. Even in the current study, the measurements offer only a snapshot of exposure on the day of dust collection.
In the new study, farm workers' homes were generally the most contaminated with weed killers. Some herbicide concentrations in their dwellings were more than triple those present in the homes of people who had never worked in agriculture.
Nearly 60 percent of the study's participants lived within 550 yards of cropland. The chance of finding agricultural weed killers in house dust increased by 6 percent for every 10 acres of cropland found within a roughly 800-yard perimeter of the house. The result was that herbicide-laced dust showed up in three-quarters of homes having at least 300 acres of cropland within that 800-yard perimeter.
Ward's team published its findings in the June Environmental Health Perspectives.
Of nearly 120 studies that have investigated the risk of non-Hodgkin's lymphoma associated with pesticide contact, most showed increased riskespecially for weed killersaccording to the Lymphoma Foundation of America. Printed information from the foundation states that the pesticides "more frequently associated with increased lymphoma incidence and/or deaths" are the herbicides 2,4-D and the triazines, which includes atrazine. Such herbicides are typically used on corn.
Some of Ward's colleagues have examined whether residential use of weed killers might contribute to risk of the cancer, but they've found no evidence of that. In the April 2005 Cancer Epidemiology Biomarkers & Prevention, the scientists report that carpets in healthy people's homes were as likely to contain the pesticides as were carpets in the homes of people with non-Hodgkin's lymphoma. The researchers also found no elevation in the cancer's incidence among people who had used herbicides in or around the home during the preceding 3 decades.
What did emerge in the team's investigations was some suggestion that people whose homes had been treated for termites were at elevated risk of developing the cancer. This risk was restricted to people whose homes had been treated with chlordane before its residential use was banned in 1988. A report of those findings appeared in the February Cancer Epidemiology Biomarkers & Prevention.
Last November, Australian scientists linked non-Hodgkin's lymphoma with workplace exposure to herbicides and other agricultural chemicals. Overall, "substantial exposure to any pesticide trebled the risk of non-Hodgkin's lymphoma," these researchers noted in the American Journal of Epidemiology. The herbicide 2,4-D was among those linked to the cancer.
A year earlier, scientists from institutions throughout the United States described finding an increased risk of certain cancersincluding a doubling in lymphomasamong the children of men who worked as pesticide applicators.
Cancer, however, is far from the only health or environmental risk associated with agricultural pesticides. For instance, some herbicides used on corn have been shown to disrupt normal reproductive developmentalbeit in frogs, in studies so far (SN: 11/2/02, p. 275; 4/20/02, p. 243). Some biologists now suspect that such changes may explain declining amphibian populations.
Agricultural chemicals may also affect human fertility. Four years ago, epidemiologist Shanna H. Swan of the University of Missouri and her colleagues studied sperm in men from big cities and small towns. Sperm concentrations and quality in men from semirural Missouri communities were below those of men from Minneapolis, Los Angeles, and New York City (SN: 11/23/02, p. 333). This suggests, Swan told Science News Online, that "environmental exposure to current-use pesticides is associated with poorer semen quality."
In an extension of that study, the Centers for Disease Control and Prevention in Atlanta will soon measure agricultural pesticides in the urine of men who had participated, notes Swan, now at the University of Rochester.
Clearly, there are lots of advantages to living in the country: farm-fresh food, skies clear of urban pollution, and little traffic. The new herbicide study suggests, however, that there can also be at least one health drawback.
Kori B. Flower
University of North Carolina
Chapel Hill, NC 27599-7105
Lymphoma Foundation of America
1100 North Main Street
Ann Arbor, MI 48104
Web site: [Go to].
Industry Task Force II on 2,4-D Research Data
8318 West 154th Street
Overland Park, KS 66223-2759
Web site: [Go to].
Shanna H. Swan
Department of Obstetrics and Gynecology
University of Rochester
601 Elmwood Avenue, Box 668
Rochester, NY 14642-8668
Mary H. Ward
Occupational and Environmental Epidemiology Branch
Division of Cancer Epidemiology and Genetics
National Cancer Institute
DHHS, 6120 Executive Boulevard
Bethesda, MD 20892
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______. 2002. More frog trouble: Herbicides may emasculate wild males. Science News 162(Nov. 2):275. Available at [Go to].