Paraquat and glyphosate are two widely used herbicides designed to control unwanted plant growth. While both serve the same general purpose in agriculture and other settings, they differ significantly in their chemical makeup, how they affect plants, and the human health implications associated with their use. Understanding these distinctions is important for comprehending their varied applications and regulatory statuses around the world.
Distinct Chemical Identities
Paraquat belongs to the bipyridylium herbicide family. It is a synthetic compound in commercial use since the early 1960s. Its chemical structure involves two linked pyridine rings.
Glyphosate is an organic acid classified as a phosphonate, a type of organophosphorus compound. Unlike some other organophosphorus compounds, glyphosate is not an insecticide. It was first recognized for its herbicidal potential in 1971.
Different Modes of Action
Paraquat functions as a “contact herbicide,” damaging plant tissue directly where it is applied. Once absorbed by leaves and stems, it disrupts photosynthesis by accepting electrons from Photosystem I within plant cells. This generates reactive oxygen species, or free radicals, which rapidly destroy cell membranes, leading to desiccation and plant death, often within hours or days. Incomplete spray coverage can result in partial plant kill, allowing undamaged parts to regrow.
Glyphosate is a “systemic herbicide,” absorbed through plant foliage and transported throughout the plant’s vascular system. It inhibits the EPSPS enzyme, part of the shikimate pathway. This pathway is unique to plants, fungi, and some bacteria, and is essential for synthesizing aromatic amino acids. By blocking this enzyme, glyphosate prevents plants from producing these necessary building blocks, leading to a slower, more complete plant death.
Health and Safety Implications
Paraquat is known for its high acute toxicity to humans. Even a small amount, such as a single sip, can be fatal if ingested, and there is no specific antidote. Exposure can lead to immediate and severe symptoms, including pain, swelling, and damage to the mouth, throat, and intestines. Once absorbed, it causes toxic reactions in organs throughout the body, particularly the lungs, kidneys, and liver, potentially resulting in lung scarring, kidney failure, liver damage, and heart failure.
Studies have linked paraquat exposure to an increased risk of Parkinson’s disease. People living near farms where paraquat is applied may face a higher risk. This association is attributed to paraquat’s interference with dopamine production and regulation in the brain, a process disrupted in Parkinson’s disease.
Glyphosate exhibits lower acute toxicity compared to paraquat. However, concerns regarding chronic exposure have led to ongoing scientific and legal debate. In 2015, the International Agency for Research on Cancer (IARC) classified glyphosate as “probably carcinogenic to humans” (Group 2A). This classification was based on “limited” evidence of cancer in humans, specifically linking it to non-Hodgkin’s lymphoma, and “sufficient” evidence of carcinogenicity in experimental animals.
Other potential health concerns associated with glyphosate exposure include disruption of the gut microbiome. While some regulatory agencies, such as the U.S. Environmental Protection Agency (EPA) and the European Food Safety Authority (EFSA), conclude that glyphosate is not likely to be carcinogenic to humans when used as directed, the IARC’s classification and subsequent lawsuits highlight differing interpretations of scientific evidence.
Usage and Regulatory Status
Paraquat’s high toxicity has led to significant global use restrictions. It is banned in over 50 countries, including all European Union member states, China, and Brazil. In the United States, the EPA classifies paraquat as a “restricted use pesticide.” This means it is not available for public or residential use and can only be bought and applied by certified pesticide applicators. The EPA has also implemented measures like special packaging with a blue dye and an emetic to deter accidental ingestion.
Glyphosate is one of the most widely used herbicides worldwide, commonly found in products like Roundup. It is extensively applied in agriculture, especially with genetically modified crops designed to resist its effects, and also in landscaping, forestry, and non-crop areas like roadsides and railways. Its widespread use is due to its effectiveness in controlling a broad spectrum of weeds and its ability to reduce the need for tilling, which helps minimize soil erosion. Many regulatory bodies, including those in the United States, Canada, and the European Union, continue to approve glyphosate for use, often with conditions and restrictions. Despite these approvals, legal challenges and public debate regarding its safety persist in various regions.