An important notion in toxicology is that any chemical can poison any organism, as long as a sufficiently large dose is experienced. In other words, all chemicals are potentially toxic, even water, carbon dioxide, sucrose (table sugar), sodium chloride (table salt), and other substances that are routinely encountered during the course of the day. However, exposures to these chemicals, or to much more toxic substances, do not necessarily result in a measurable poisonous response, if the dose is small enough. Toxicity is only caused if the exposure exceeds physiological thresholds of tolerance. According to this interpretation of toxicology, it is best to refer to "potentially toxic chemicals" in any context in which the actual environmental exposure to chemicals is unclear, or when the effects of small doses of particular chemicals are not known.
However, it is important to understand that there is scientific controversy about this topic. Some scientists believe that even exposures to single molecules of certain chemicals could be of toxicological significance, and that dose-response relationships can therefore be extrapolated in a linear fashion to a zero dosage. This might be especially relevant to some types of cancers, which could theoretically be induced by genetic damage occurring in a single cell, and potentially caused by a single molecule of a carcinogen. This is a very different view from that expressed above, which suggests that there are thresholds of physiological tolerance that must be exceeded if toxicity is to be caused.
The notion of thresholds of tolerance is supported by several lines of scientific evidence. It is known, for example, that cells have some capability of repairing damage caused to nuclear materials such as DNA (deoxyribonucleic acid), suggesting that minor damage caused by toxic chemicals might be tolerated because they could be repaired. However, major damage could overwhelm the physiological repair function, so that there would be a threshold of tolerance.
In addition, organisms have physiological mechanisms for detoxifying many types of poisonous chemicals. Mixed-function oxidases (MFOs), for example, are a class of enzymes that are especially abundant in the liver of vertebrate animals, and to a lesser degree in the bloodstream. Within limits, these enzymes can detoxify certain potentially toxic chemicals, such as chlorinated hydrocarbons, by rendering them into simpler, less toxic substances. Mixed-function oxidases are inducible enzymes, meaning that they are synthesized in relatively large quantities when there is an increased demand for their metabolic services, as would occur when an organism is exposed to a large concentration of toxic chemicals. However, the ability of the mixed-function oxidase system to deal with toxic chemicals can be overwhelmed if the exposure is too intense, a characteristic that would be represented as a toxicological threshold.
Organisms also have some ability to deal with limited exposures to potentially toxic chemicals by partitioning them within tissues that are not vulnerable to their poisonous influence. For example, chlorinated hydrocarbons such as the insecticides DDT and dieldrin, the industrial fluids known as polychlorinated biphenyls (PCBs), and the dioxin TCDD are all very soluble in fats, and therefore are mostly found in the fatty tissues of animals. Within limits, organisms can tolerate exposures to these chemicals by immobilizing them in fatty tissues. However, toxicity may still result if the exposure is too great, or if the fat reserves must be mobilized in order to deal with large metabolic demands, as might occur during migration or breeding. Similarly, plants have some ability to deal with limited exposures to toxic metals, by synthesizing certain proteins, organic acids, or other biochemicals that bind with the ionic forms of metals, rendering them much less toxic.
Moreover, all of the chemicals required by organisms as essential nutrients are toxic at larger exposures. For example, the metals copper, iron, molybdenum, and zinc are required by plants and animals as micronutrients. However, exposures that exceed the therapeutic levels of these metals are poisonous to these same organisms. The smaller, sub-toxic exposures would represent a type of contamination.