Also, Table 3 estimates the LD50 for a 175-pound person for a range of substances. From this information, several points can be made. As the LD50 rating becomes larger, the substances become lower in toxicity. Also, many herbicides have an acute toxicity rating less toxic than many frequently used household compounds. Many people wonder how a herbicide that is extremely effective at killing unwanted plants can have such little toxicity for humans.
Plants differ from humans in many ways.
Researchers rely on those differences to produce chemicals that interrupt a plant's function but have little effect on humans. For example, in a process called photosynthesis, plants produce their own food by using carbon dioxide from the air, water from the soil, and sunlight. Since we do not have this ability, a chemical that blocks this process has a lethal effect on a plant yet does not affect humans. Other plant processes, such as blocking the synthesis of a particular amino acid that only plants can make, can be altered in a similar way, resulting in the death of plants but having no influence on people. Although the acute LD50 rating may indicate that the compound is relatively low in toxicity, take precautions when mixing, handling, or applying herbicides.