When Bt toxin is incorporated into common foods such as corn, we are exposed every time we eat these engineered foods. And of course, a pesticide engineered into every cell of a food source cannot simply be washed off before a meal. Moreover, in the bacteria that produce it, the Bt toxin exists in an inactive form, which only becomes dangerous to insects after it is activated by the insect's digestive system. In contrast, some genetically engineered Bt crops produce the toxin in its activated form. Humans have seldom been exposed to this active form of the toxin.
Genetic engineering can also introduce toxins unintentionally.
For example, a plant that ordinarily produces high amounts of a toxin in its leaves and low amounts in its fruit could unexpectedly begin to concentrate the toxin in its fruit after addition of a new gene. Unpleasant surprises of this sort arise because our knowledge of how genes are incorporated into engineered cells is far from precise. These newly inserted genes may sometimes end up in the middle of existing genetic instructions, and may disrupt those instructions. A foreign gene could, for example, be inserted in the middle of an existing gene that instructs a plant to shut off production of a toxin in its fruit. The foreign gene could disrupt the functioning of this existing gene, causing the plant to produce abnormal levels of the toxin in its fruit.