Thanks for asking about the toxicity of glyphosate versus that of surfactants used with glyphosate. For herbicides like glyphosate to be most effective at controlling unwanted plants, they need to be applied with a surfactant. Surfactants (short for “surface-acting agents”) are soapy substances that help to reduce surface tension of the water so the drop of spray solution can spread over the surface of a leaf and help to penetrate the waxy layer (the cuticle) of the plant. Herbicidal soaps are often used in organic gardening to help penetrate the waxy layer of plants and cause the plant to dehydrate and die.
The surfactants used with glyphosate are similar to those used in personal-care and household cleaning products that we are exposed to every day when we wash our hands, hair and dishes. The surfactants in these products perform the same function as they do when mixed with an herbicide like glyphosate. For example, surfactants found in shampoos reduce the surface tension of water to help it spread and move around our hair and help remove the oily layer with dirt from our hair.
You are correct that glyphosate for acute oral toxicity is placed in the US Environmental Protection Agency Toxicity Category III. The surfactants used with glyphosate are also in Toxicity Category III for acute oral toxicity, as are many of the surfactants used in personal and household cleaning products. The surfactants mixed with glyphosate in Roundup-brand products do not increase this acute toxicity level. For example, Roundup-brand products (containing primarily glyphosate, surfactants and water) are in Toxicity Category IV for acute oral toxicity. The reason for the change from Category III to Category IV is the result of the formulated product being diluted with water.
You may have read claims on the Internet that when glyphosate is mixed with surfactants, the formulated Roundup-brand products are more toxic. These claims relate to the results of petri dish experiments. Glyphosate and Roundup-brand products were poured over unprotected cells in a petri dish. This direct exposure to high concentrations used in these studies intentionally bypasses normal processes and limits exposure. While glyphosate had very little effect on cell function, the Roundup formulations because of the surfactant component did alter cell function. This is not a surprise, given that the surfactants in the Roundup-brand product in the petri dish were doing what any surfactant would do: they were disrupting the biological membrane of the unprotected cell. In fact, surfactants are routinely used in cell biology to disrupt cell membranes to isolate membrane proteins. Petri dish experiments with surfactants from personal and home care products, as well as caffeine and citric acid (normal components in coffee and orange juice, respectively), have shown they, too, can disrupt cell function.
Glyphosate, the surfactants used with glyphosate and Roundup-brand products, when used according to label directions, all have a long history of safe use and will not pose any unreasonable risk to human health.