While pesticide labels must identify all active ingredients by name, inerts routinely are listed only as a percentage of the contents. According to pesticide law, active ingredients 'kill, repel or mitigate' pests, while inert ingredients simply help the product work better. A strategically inserted inert can enhance a pesticide's toxicity by facilitating absorption of chemical ingredients through the leaves of a plant, the shell of a beetle - or the skin of a human. They are biologically, chemically and toxicologically active. Many inerts are more toxic than the active ingredients.
Most pesticides contain significantly more inert than active ingredients.
Experts speculate that the combined effect derived from mixing inert and active ingredients is frequently what makes pesticide products so lethal. Recent studies on the popular Monsanto Corp. herbicide, Roundup, for example, indicate that addition of the inert ingredient polyoxethylene to the active ingredient, glyphosate, substantially increases Roundup's toxicity. Under US law, pesticide labels need identify only the small list of inerts that the EPA believes pose a 'toxicological concern'. The labels omit the names of inert ingredients that are merely 'potentially toxic'. Workers in frequent contact with xylenes suffer a greater frequency of leukaemia than the population at large.