This may be lost on you, but, Paracelsus, the father of toxicology made the observation in the 15th century, that the dose makes the poison. In short, background concentrations of poisons saturate the environment we live in. Examples: carbon monoxide in the air we breath, aflatoxin in the grain we eat and the milk we drink, cyanide containing compounds in many plants we consume, and so goes the endless list. We live in a toxic world. CO is found your blood, so is cyanide. See how it works? There is a threshold that must be reached to induce adverse effects from poisons all around us.