Effects of pesticide mixtures at the acceptable daily intake levels on rat carcinogenesis.

Possible modifying effects of pesticide mixtures on tumorigenesis were investigated with medium-term carcinogenesis protocols for rapid detection of carcinogenic agents using male F344 rats. In the 8-wk liver model, administration of 20 pesticides (19 organophosphorus compounds and one organochlorine), added to the diet each at acceptable daily intake (ADI) levels, did not enhance rat liver preneoplastic lesion development initiated by diethylnitrosamine. In contrast, a mixture of these 20 pesticides at 100 times the ADI significantly increased the number and area of liver lesions. In the second experiment using a multi-organ carcinogenicity protocol of 28 wk, mixtures of 40 pesticides (high production examples) or 20 pesticides (suspected carcinogens) added to the diet at their respective ADI levels did not modulate carcinogenesis in any organ initiated by five known potent carcinogens in combination. These results thus provide direct support for the safety factor (usually 100) approach using ADI values for the quantitative risk evaluation of pesticides.