Aflatoxins, potent cancer-causing chemicals created by funguses occurring naturally in some foods, were produced more abundantly than normal on irradiated foods in several studies. The exact reasons and overall health effects are unknown but aflatoxins are 1,000 times more potent carcinogens than the banned pesticide EDB, for which irradiation is a possible substitute. This could mean a decline in the nutritional quality of our food supply overall. Bacteria and viruses can develop resistance to radiation, just as insects do to pesticides. So the effectiveness of irradiation as a preservative over the long term is not known.
Radiation can also cause dangerous mutations and the development of new strains of pest organisms.
Irradiation does not protect food from contamination that may occur after the treatment, unless it is tightly sealed at the time of irradiation. For fruits and vegetables, other long-term methods of preservation, such as canning, freezing, or refrigeration, will probably still be needed. Fruit treated with radiation may become brown or mushy or ripen abnormally. Irradiated citrus fruits will bruise easily and black spots may appear on the peel. Meats must be irradiated in a vacuum, since irradiation in the presence of oxygen causes rancidity in fat-containing foods.