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Professor David Mortensen tells Vermont committee paraquat’s toxicity, volatility prompt concern; offers alternatives
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Summary
Professor David Mortensen testified to a Vermont legislative committee considering H.739 that paraquat has unusually high acute toxicity and newly reported volatility, argued those factors raise exposure risks for applicators and nearby residents, and outlined alternative herbicides and integrated approaches.
Professor David Mortensen, a retired weed-science researcher who chaired the Department of Agriculture, Nutrition and Food Systems at the University of New Hampshire, told a Vermont committee on April 9 that the herbicide paraquat presents acute risks for applicators and potential exposure for nearby residents and urged the panel to weigh those risks as it considers H.739, a bill to prohibit paraquat’s sale and use.
Mortensen described decades of field experience applying paraquat by backpack and said the compound produced rapid, noticeable effects on workers he called “knockdown” reactions. He cited toxicology metrics to explain his concern: “The LD50 is 150 milligrams per kilogram of body weight,” he said, noting that most other herbicides fall in the 2,000–10,000 mg/kg range. He argued that the herbicide’s acute toxicity, combined with real-world difficulty following label guidance on buffers and personal protective equipment, makes routine use hazardous.
The witness also cited a recent Environmental Protection Agency analysis that he said revised paraquat’s volatility coefficients. “In a rather remarkable report from the Environmental Protection Agency published 10/30/2025,” Mortensen said, “the herbicide is some 4,000 fold more volatile,” and EPA models indicate potential movement “up to 2.7 miles away from the sprayed area.” He told the committee that this mechanistic change increases the likelihood that ground applications can still result in off-target movement.
Committee members pressed Mortensen on Vermont’s local context: members noted most Vermont uses are small-volume, close-ground, directed hooded sprayer applications for crops such as strawberries, and asked whether those application methods materially reduce risk. Mortensen acknowledged the local practices but said volatility and application drift mean small, directed uses still carry exposure potential, and that bans in some jurisdictions reflect evaluation of those trade-offs.
On human-health studies, Mortensen summarized longitudinal epidemiological work linking paraquat exposure and Parkinson’s disease, including what he described as a 2024 International Journal of Epidemiology study showing elevated odds ratios for applicators (about 2.0) and nearby residents (about 1.7) over long follow-up periods. He underscored that epidemiological studies demonstrate correlation rather than proof of causation, but said the totality of animal toxicology and longitudinal human data is concerning.
Mortensen told the committee he reviewed authorship and affiliations in some industry-related papers and raised conflict-of-interest concerns where an author had acted as an expert witness for the manufacturer; he offered to provide PDFs and references to committee members for further review.
A member asked about available alternatives for strawberry interrow weed control; Mortensen pointed to the New England Small Fruit Guide and listed post-emergent options such as carfentrazone, flazifop, cethoxetim, aciflurofen and bicyclopyrone, and noted growers sometimes pair preemergence residual herbicides with lower-dose burn-down treatments. He said some alternatives can be more expensive but are viable in the settings discussed.
The committee did not take a vote. Chair opened the session for additional testimony and said the panel will hear apple growers and other users at a follow-up meeting scheduled for the next day. Mortensen volunteered to share citations and data with committee staff.
The hearing transcript records sharp questioning about causation and local application practices, and several members said they were still weighing evidence; the committee did not reach a consensus and will continue testimony before considering next steps on H.739.
