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Independent study looks at chemicals commonly used to combat noxious weeds

Health risks associated with the use of chemical herbicides to control noxious weeds are "essentially nil," according to an independent scientific report prepared for an organization of resource management agencies in the Missoula area.

The report was completed on April 7 by Allan S. Felsot, a professor and specialist in entomology and environmental toxicology at Washington State University's Food and Environmental Quality Lab in Richland, Wash.

On Monday, Felsot visited Missoula to discuss the report with local government representatives, members of the public and the group that commissioned it - the Missoula Valley Weed Managers.

Weed managers in the group include the University of Montana, Missoula County Extension Office, Missoula County Weed District, Bureau of Land Management, Forest Service, Montana Department of Fish, Wildlife and Parks, Montana Department of Natural Resources and Conservation, Missoula Conservation District and Plum Creek Timber Co.

Because of concerns about the health effects of herbicides, the organization wanted an independent, scientific assessment of the health risks, according to Jerry Marks, Missoula County Extension agent.

The weed-management group agreed to pay $8,000 for Felsot to prepare the report, which is written with a minimum of technical language, and was peer-reviewed by scientists from several other universities. The report focuses on health effects of picloram, 2,4-D and clopyralid, the herbicides most commonly used to combat noxious weeds in Montana.

As a preface to his discussion of the report Monday, Felsot said uncertainty is inevitable in assessing risks of exposure to any chemical compound.

"Absolute safety can't be guaranteed," he said.

However, Felsot added, a logical process can be used to determine risk, using science as a guide. The process looks at potential hazards and their probability under certain conditions.

The dose, or exposure level, is the key to determining health risk, he said.

Every day, he said, people are exposed to compounds found in nature that have toxic effects at high doses. Examples include some vitamins and table salt.

The U.S. Environmental Protection Agency has established a dose for herbicides that is believed to be safe, Felsot said. Safe is defined by EPA as a reasonable certainty of no harm following acute or chronic exposure. That "reference dose" is determined in a laboratory by finding the exposure level at which there are no observable effects and then dividing that level by 100.

The reference dose also assumes a daily exposure during a 70-year period.

"That's a very large safety factor," Felsot said.

Herbicides addressed in his report, he said, "have been used for 30 or 40 years, or longer. We have a lot of human experience with them. They have been well studied by the manufacturers and universities. The EPA has conducted risk assessment on them, using the most sensitive end point (or health effect), and they have concluded there is low probability of them causing ill effects when used in the method they were intended. The probability of causing harm is essentially nil."

Felsot's report, which is available on the Internet and at the Missoula County Extension Office, is prepared in a question-and-answer format.

It addresses many health concerns about herbicides, including the following:

Are children uniquely vulnerable to the toxic mechanisms of chemicals?

Do chemicals accumulate and build up in body fat and breast milk?

Do chemicals that mimic hormones disrupt the endocrine system at doses similar to environmental exposures?

Are exposures to mixtures of chemicals more dangerous than exposure to a single chemical?

What is known about the safety of herbicides proposed for use in the Missoula Valley?

What precautions should be taken during the spray operations to ensure that bystanders are safe?

"This kind of information," said Marks, "is useful when a land manager is putting together a vegetative management plan. They have to assess the effects of whatever tools they use, whether it's bio-controls, revegetation, herbicides or whatever."

Land managers are bound by state and federal laws to control noxious weeds, Marks said.

Felsot's report will be valuable as a communication tool in explaining vegetation management and associated risks to the public, said Andy Kulla, weed specialist for the Lolo National Forest.

"You need to balance the risks and ecological effects of the problem, with the risks and ecological effects of the solutions," he said. "Most ecologists consider invasive species one of the greatest threats to ecosystem health that we face today."

Kate Supplee, manager of Missoula's Open Space Program, said Felsot's report reinforces the city's recently adopted vegetation management plan for Mount Sentinel, Mount Jumbo, the North Hills, Kim Williams Trail and John Toole Park. The plan incorporates ground application of herbicides as one of several ways to control weeds and restore native vegetation on the city-owned lands.

"I think it provided some good, common-sense information," Supplee said. "It's useful because it offers us some reassurance that we're taking the right approach in using all the tools in our city open space weed management plan. Of course, we're not just using herbicides. But it is an important tool."

Reporter Daryl Gadbow can be reached at 523-5264 or at

If you're interested

A report assessing the safety of herbicides for vegetation management, prepared for the Missoula Valley Weed Managers by Allan S. Felsot, environmental toxicologist for Washington State University, is available at the Missoula County Extension Office, 126 W. Spruce St., and on the Internet at

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