Picloram is of moderate toxicity to the eyes and only mildly toxic on the skin. It does not adhere to soil, so it may leach into groundwater, and has in fact been detected there. It is degraded in soil and water mainly by microbes. Picloram has very little tendency to accumulate in aquatic life. Gardeners who use dung as fertilizer should check to make certain that the animal source has not grazed on picloram-treated hay, as the dung still has broadleaf-killing potency. In regards to occupational exposures, the U.S.
Occupational Safety and Health Administration has established a
permissible exposure limit of 15 mg/m3 total exposure and 5 mg/m3 for respiratory exposure, over an eight-hour workshift.
Carcinogenicity A 2026 study in
Nature Medicine reported an association between picloram exposure signatures and early-onset
colorectal cancer. The researchers used
DNA methylation scores as indirect markers of previous exposure to lifestyle and environmental factors, including
pesticides. A higher picloram-related methylation score was associated with colorectal cancer diagnosed before age 50, compared with colorectal cancer diagnosed at age 70 or older. This association was replicated in a meta-analysis of nine colorectal cancer cohorts. The same study also compared estimated pesticide use with cancer incidence in 94 United States counties over 21 years. Picloram use remained associated with early-onset colorectal cancer incidence after adjustment for socioeconomic factors and other pesticide use. The authors noted that picloram was first registered as a
herbicide in the United States in 1964, so people who could have been exposed to it from childhood would often still be younger than 70 in 2026. They suggested that this could partly explain why the association was mainly seen for early-onset disease. The study found an association, but did not perform research to (dis)prove that picloram causes colorectal cancer. Therefore, the authors stated that further longitudinal and experimental research is needed to test
causality, dose dependence and latency. ==References==