Investigations are presently under way, and these could eventually be reflected in changes in the practice of water treatment, just as dental recommendations have, sometimes, led to the adjustment of fluoride levels. Similarly, research on heart and blood vessel diseases has shown that they seem to be more prevalent in soft water regions. Of far more concern to most engineers is the need to control toxic materials in water effluents. Radioactive isotopes are obviously a danger, although not immediately toxic in any likely doses.
The pathological effects depend on the chemistry of the isotope: Strontium 90, for example, is dealt with by the body as if it were calcium and becomes localized in the bones, while phosphorus 32 is treated as if it were the stable isotope phosphorus 31 and may get incorporated into the chromosomes of dividing cells, possibly causing mutation or cancer. Toxic ions are currently receiving much attention, since cyanide, lead, cadmium, arsenic; mercury, manganese and chromium are all among the industrial wastes which may be released. Although heavy metal may be discharged in highly diluted form, they can come back to man in a concentrated form if they are absorbed and retained by plants or animals and these creatures serve as food for other animals. The metals, thus, pass up the food chain, becoming progressively more concentrated as each predator eats much prey. That is why heavy metals, dumped in the sea, are especially likely to be found in the largest amounts in large predator fish like the tuna. The other animals, responsible for concentrating toxic metals, are shellfish which filter large volumes of sea water and may retain some constituents. The dangers to health from, heavy metals depend, to some extent, on the form in which they occur. Mercury, for example, is not accumulated by animals as a metal or ion, but in organometallic form, such as methyl or dimethyl mercury.
Methylation of mercury by microorganisms occurs readily in the environment, especially at the bottom of lakes and seas, and, thus, there is always a danger that wastes discharges, as relatively inert, may reappear in a more alarming form. Another type of water pollution occurs when chemicals persist in the environment. An immense number of synthetic compounds are now made and used; many of these, if discharged, are readily metabolised by micro-organisms, and cause no health danger unless present in high concentrations. Some, however, are not degraded and persist with the potential to build up in concentration if their discharge is continued.
Examples of these include some plastics, the halogenated hydrocarbon, pesticides and some types of detergents. The latter can become an indirect public health danger, if allowed to build up to levels, that impair the efficiency of water purification plants. At one time or another, humans have been poisoned fatally by each of the common pesticides, though this has resulted from the deliberate or accidental ingestion of large amounts. Through pollution, people, now, contain a few parts per million of chlorinated hydrocarbons in their fat tissue, but no adverse effects on health have been detected at this level. If only medical aspects were considered, it would be concluded, in general, that pesticides have greatly increased health throughout the world through control of disease bearing pests and augmentation of food production.