Hormone disruptors such as heavy metals have a u-shaped dose-response curve. These substances can be quite toxic at very trace levels and are actually less toxic at moderate levels. This is because at trace amounts, the body sees these substances as potential messengers in systems such as the endocrine (hormone) system. There is a lot of literature about this topic, which has evolved over the last 40 years or so. Google Scholar is a good way to access this and other scientific literature related to toxic substances and chronic illness.