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 Testing of pesticides focuses on the active ingredient. But many other “inert” ingredients are added to the pesticide formulation that is actually sold. A new study suggests that these additives can make pesticides more dangerous to cells than current safety testing reveals.

A team of French scientists has concluded that studies focused solely on the active ingredients of commercially sold pesticides substantially underestimate their potential hazards.

The study suggests that inert ingredients in pesticides can magnify the effects of active ingredients, sometimes as much as 1,000-fold. Eight commercial products out of nine tested were hundreds of times more toxic than their active ingredient alone.

Normally, tests of pesticide safety are carried out only on the active ingredient, the chemical that targets the pest. But pesticides sold to consumers and farmers are complex mixtures of other chemicals deemed “inert,” which implies these additives don’t have biological effects.

The scientists from the University of Caen exposed three human cell lines to the active ingredients of three herbicides, three insecticides and three fungicides. Then they exposed the cell lines to the commercial formulations, which contained “inerts,” and compared the results.

The new finding, if confirmed, has significant implications for pesticide safety because if inert ingredients commonly amplify pesticide effects, then safety standards may not be protective of human health.

In the study, fungicides were on average the most toxic, followed by herbicides and then insecticides.

Roundup, a commercial herbicide sold by Monsanto that uses the active ingredient glyphosate, was by far the most toxic of the herbicides and insecticides tested, according to the study, which was published in the journal BioMed Research International. Used to kill weeds on lawns, gardens and crops including soybeans and corn, glyphosate is one of the most widely used herbicides in the United States.