PANNA: U.S. EPA Gives Rice Herbicide Molinate High Toxicity Rating


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U.S. EPA Gives Rice Herbicide Molinate High Toxicity Rating
May 21, 2002

The U.S. Environmental Protection Agency (EPA) recently released its Preliminary Risk Assessment for molinate, an herbicide used extensively on rice in California, Louisiana, Arkansas, Texas, Tennessee, Missouri and Mississippi, as well as in many countries around the world. Documents released with the risk assessment confirm earlier evaluations by California state agencies that molinate is a reproductive toxicant, a neurotoxicant and a suspected carcinogen. The documents also show that molinate poses serious fertility risks to workers who mix, load and apply the pesticide.

Equally disturbing is a summary of correspondence between EPA and the California Rice Commission (CRC) included in the risk assessment package. The documents indicate that the CRC had access to the risk assessment before it was released to the public and attempted to negotiate with high level EPA officials to eliminate language that identifies molinate as a reproductive toxicant. The CRC’s explicitly stated goal was to change the language to avoid a listing of the chemical as a reproductive toxicant under California’s Safe Drinking Water and Toxic Enforcement Act (Proposition 65). This could trigger warnings to individuals exposed to molinate in California, and would limit discharges of molinate into sources of drinking water. Fortunately, EPA stood by its analysis of the data, resulting in a strong risk assessment that provides solid backing to severely curtail or eliminate use of this pesticide.

Molinate is a thiocarbamate herbicide used almost exclusively in rice production for control of watergrass. Zeneca Ag Products (now Syngenta), the primary manufacturer, sells the herbicide under the trade names Ordram(r) and Arrosolo(r). Approximately 5.6 million pounds of the herbicide were used in the U.S. each year between 1995 and 1997, with around 40% of the total acres of rice treated. Rice fields are treated with molinate either by tilling granules into the soil before rice seedlings are planted or by applications to flooded fields during the growing season. Water from these fields is released into nearby water bodies in late spring in the U.S., resulting in significant concentrations of molinate in local drinking water sources.

In the risk assessment, EPA makes a clear statement that molinate is a reproductive toxicant. Studies with rats show abnormal sperm, decreased sperm numbers, decreased litter size, decreased percent of live births, decreased pup viability, increased incidence of microscopic lesions in the ovary, testes and adrenal glands, delayed vaginal opening, reproductive organ weight effects and decreased brain weight. While rats appear to be the most sensitive species, rabbits, mice and dogs show similar effects when exposed to molinate. While molinate is not highly acutely toxic, it is a cholinesterase inhibitor and causes neurotoxicity in rats, mice, and dogs from both chronic and one-time exposures. Delayed developmental neurotoxicity was demonstrated in the hen. Increased incidence of kidney tumors in mice from cancer studies resulted in EPA listing the chemical as “suggestive of carcinogenicity in humans.” The results of these animal studies have important implications for human health and are consistent with results obtained in the toxicological tests carried out by California Office of Health Hazard Assessment and California Department of Pesticide Regulation.

Potential routes of exposure to molinate include inhalation (for mixers, applicators, field workers and residents of rice-growing regions), dermal (for mixers, applicators, field workers and anyone exposed to drift of spray droplets or residues on plants), and dietary (from drinking water sources contaminated with molinate and from residues on rice and rice products). Molinate has been detected frequently in both ground and surface waters in areas close to rice cultivation. Many of the water sources tested are used as drinking water supplies, including the Sacramento River in California (water supply for over 400,000 people) and the Mississippi River in the New Orleans area (water supply for over 1.21 million people).

Exposures experienced by workers who mix, load and apply the pesticide were found to be particularly high. Even when workers were wearing full-face respirators and special coveralls over activated carbon suits, data showed that worker exposure exceeded EPA’s acceptable levels.

Inhalation exposure for people living near rice fields was not evaluated by EPA, even though air monitoring studies conducted by the California Air Resources Board demonstrate substantial amounts of molinate present both near application sites and even in ambient air during the application season.

The California Rice Commission’s efforts to undermine Proposition 65 continue. They insist that the reproductive toxicity outcomes seen in laboratory animals do not translate to humans. Yet the data do not support CRC’s conclusions and scientists at EPA evaluating the studies remain unconvinced.

With all the problems associated with molinate, it seems possible that Syngenta may voluntarily cancel registration of this chemical in the U.S. rather than spend additional money to prove that exposures can be kept under “acceptable” levels. Unfortunately, such an action will not necessarily affect sales in other regions of the world–mostly developing countries–resulting in continuing high exposure potentials for molinate use outside the United States.

Sources: US EPA Preliminary Risk Assessment for Molinate:; California Department of Pesticide Regulation, Evaluation of Molinate as a Toxic Air Contaminant, March 2000:; Molinate; Availability of Risk Assessment, Federal Register: April 2, 2002 (Volume 67, Number 63):

Contact: PANNA.

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