Monosodium methyl arsonate

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Monosodium methyl arsonate (MSMA) is an arsenic-based herbicide. It is an organo-arsenate, meaning it contains organic arsenic, and it is less toxic than inorganic arsenic, but it is still hazardous.

All forms of arsenic pose health risks. The EPA and the Agency for Toxic Substances and Disease Registry consider arsenic a major health hazard, and arsenic is classified as a Group A carcinogen (cancer-causing).

In soils, arsenic changes form depending on conditions. In well-oxygenated soils, arsenate (AsV) is common; in chemically reduced (low-oxygen) soils, arsenite (AsIII) prevails. Arsenite is more toxic than arsenate, but arsenate can still harm humans, plants, and microorganisms.

Arsenic-contaminated soils pose serious health risks, and arsenic can move from soil into water, where it is even more dangerous.