Dish handwashing: an overlooked source of contamination

Daily dishwashing is a common household activity. Washing dishes in a sink is considered safe for the environment because it requires few water and human energy. There are concerns, however, that residues from the detergent used may impact health and the environment. Here, we studied the adsorption and toxicity of two detergent ingredients, the biocide 1,2-benzisothiazol-3(2H)-one and the surfactant sodium dodecyl sulphate, left on tableware, cups and plates, made of glass, stainless steel, ceramic, plastic and wood. Results show that levels of biocide and surfactant residues were much higher on wooden plates, of 8.4 ng biocide cm(-2) and 226.4 ng surfactant cm(-2), than on glass, of 0.9 ng biocide cm(-2) and 55.9 ng surfactant cm(-2). Residues levels increase with material roughness. Toxicity analysis of compounds in water using the luminiscence of Vibrio fischeri revealed that toxic inhibition was much higher (20%) when the biocide and the surfactant were together than when the surfactant occurred alone (1.9%) or the biocide occurred alone (11.5%). Overall, our findings imply that, depending on the dish material and roughness, contaminants will be transferred either to humans through eating using contaminated dishes, or to wastewater and then to humans indirectly through contamination of natural waters.