Glyphosate is a widely used herbicide that targets and kills most plants, including broadleaf weeds and grasses. It functions by inhibiting a specific enzyme pathway, the shikimic acid pathway, which is necessary for plant growth and is also present in some microorganisms. First registered for use in the U.S. in 1974, glyphosate is now one of the most frequently applied herbicides globally, used in agriculture, forestry, and for weed control in residential and industrial areas. Its widespread adoption significantly increased after the introduction of glyphosate-resistant genetically modified crops, allowing farmers to control weeds without harming their crops. This chemical is often an active ingredient in many herbicide products, such as Roundup, and its concentrations can vary from 1% for home use up to 40% for commercial applications.
Understanding Glyphosate Toxicity Measurements
Toxicity is quantified using scientific metrics. One common measure is the LD50 (Lethal Dose 50%), which represents the amount of a substance that causes death in 50% of a test population, typically animals, when administered in a single dose. For glyphosate, the acute oral LD50 in rats is reported to be 5600 mg/kg body weight, indicating low acute toxicity. Similarly, the acute dermal LD50 in rats is greater than 2000 mg/kg body weight, and acute inhalation toxicity in rats (LC50) is greater than 5.48 mg/L.
Beyond acute exposure, chronic toxicity examines the health impacts from repeated or long-term exposure. To assess this, scientists determine the No-Observed-Adverse-Effect Level (NOAEL), which is the highest exposure level at which no observable adverse effects occur in a study. For glyphosate, NOAELs in short-term mouse studies have been identified at 3125 ppm, equivalent to 507 mg/kg body weight per day, with effects on salivary glands observed at higher doses. An Acceptable Daily Intake (ADI) is then derived from the NOAEL, representing the amount of a substance that can be ingested daily over a lifetime without posing an appreciable health risk. The ADI for glyphosate for consumers has been set at 0.5 mg/kg body weight per day by some regulatory bodies.
Human Health Impacts
Glyphosate’s potential effects on human health are widely discussed. One significant area of concern is its possible link to cancer, particularly non-Hodgkin lymphoma. In 2015, the International Agency for Research on Cancer (IARC), a specialized agency of the World Health Organization (WHO), classified glyphosate as “probably carcinogenic to humans” (Group 2A). This classification was based on “limited” evidence of cancer in humans and “sufficient” evidence in laboratory animals.
Glyphosate has also been investigated for its potential to act as an endocrine disruptor. Studies suggest that glyphosate and its commercial formulations may interfere with hormonal systems, potentially leading to reproductive issues and developmental problems. Some research indicates that these effects could occur at levels lower than those typically used in agricultural settings, raising concerns about vulnerable populations like pregnant women and developing fetuses.
The impact of glyphosate on the human gut microbiome is another area of emerging research. Glyphosate targets the shikimic acid pathway, which is present in many gut bacteria, even though humans do not possess this pathway. Disruption of this pathway in gut bacteria could lead to imbalances in the gut microbiota, potentially contributing to conditions such as inflammatory bowel disease (IBD) and celiac disease.
Environmental Effects
Glyphosate’s widespread application has led to concerns regarding its environmental impact. While glyphosate binds tightly to soil and is broken down by bacteria, its persistence can vary, potentially lasting up to six months depending on climate and soil type. This binding generally limits its leaching into groundwater.
However, glyphosate can affect water quality through runoff and direct application, particularly formulations designed for aquatic weed control. Measurable quantities of glyphosate and its associated surfactants have been detected in surface waters. These components have the potential to alter the physiology of aquatic organisms, with amphibians showing increased sensitivity compared to other vertebrates.
The herbicide’s use can also affect biodiversity, particularly non-target organisms. Indirect impacts on birds and other animals can occur due to the reduction of weeds and wildflowers, which serve as habitats and food sources. Studies have indicated that glyphosate can reduce soil invertebrate biodiversity, with some research showing a reduction in mean species richness by up to 21% in treated plots. Furthermore, there is evidence that glyphosate use may favor exotic soil fauna species over native ones, contributing to biodiversity loss.
Regulatory Classifications and Guidelines
Regulatory bodies worldwide have evaluated glyphosate, leading to varying classifications and guidelines for its use and residue levels. The U.S. Environmental Protection Agency (EPA) has concluded that glyphosate is “not likely to be carcinogenic to humans.” The EPA’s assessment considered a more extensive dataset than the International Agency for Research on Cancer (IARC), including 15 animal carcinogenicity studies compared to IARC’s eight. The EPA also found no risks of concern from ingesting food with glyphosate residues, assuming highly conservative exposure levels.
In contrast, the International Agency for Research on Cancer (IARC), a World Health Organization (WHO) agency, classified glyphosate as “probably carcinogenic to humans” (Group 2A) in 2015. This difference in classification stems partly from IARC’s consideration of both glyphosate and glyphosate-based formulations, while the European Food Safety Authority (EFSA) focused solely on glyphosate as an active substance in its assessment.
The European Food Safety Authority (EFSA) and EU Member States have concluded that glyphosate is “unlikely to be genotoxic or to pose a carcinogenic threat to humans.” EFSA established an Acceptable Daily Intake (ADI) for consumers at 0.5 mg/kg body weight per day and an acute reference dose (ARfD) at the same level. Despite differing conclusions on carcinogenicity, regulatory bodies like the EPA, EFSA, and WHO have established guidelines and maximum residue limits to manage potential exposures.