If we consider the available information we see that there is a concern about the possible dangers of nitrates in the environment. This concern is being publicised, and demands are being made that money, effort and much of our limited national resource be utilised to ensure that the public is not exposed to nitrates in food or in water. There is a concomitant belief that the nitrates in water and foodstuffs are largely due to the misuse of chemical fertilisers and that the toxic dangers to the community would disappear if alternative methods of farming could be used. Opinions have already started to polarise and it is essential that the interested parties base their actions on accurate information and present to the public these facts in an understandable manner. The unearthing and evaluation of available data can be difficult, and even when this is done the picture presented can be far from sharp.
Certainly, examination of the available, hard toxicological data indicates that nitrates are relatively non-toxic by the acute exposure route and in those trials carried out over complete animal life spans to discover chronic effects, no significant disease processes have been reported. Human ingestion of nitrates is inevitable. The amount of nitrate consumed largely depends upon the dietary intake of leafy vegetables. So much so that vegetarians are likely to have dietary intake of nitrates well in excess of the levels of individuals consuming a "normal" range of foodstuffs.
It is arguable that due to this high dosage, any nitrate related ill-health problems such as high levels of gastric cancer would be quickly revealed by an epidemiological study of vegetarians. However, the risk assessment of nitrates must take into consideration the fact that nitrate can be reduced in vitro and in vivo to nitrite by bacterial action and nitrite is more toxic than nitrate.
This conversion mechanism has been clearly demonstrated in animal models. But experiments have shown that it is not likely to occur in the normal person, except in the saliva. The peculiar physiological conditions which exist in the newborn infant may predispose to the production of nitrite in the stomach provided substantial amounts of nitrate are supplied in the feed.
When an excess of nitrite is absorbed by the newborn infant, it will enter the bloodstream and methaemoglobin will be formed from normal haemoglobin and thus endanger the well-being of a susceptible baby. Walton, following a literature survey in 1951, considered that 45 mg/litre of nitrate was the maximum safe level in water supplies. The recommendation is not based on precise trial procedures but was considered a wise and safe precaution in the light of the then existing knowledge.
Therefore, a target level now exists which completely fails to explain the absence of methaemoglobinaemia cases in some areas of the world where high levels of nitrates are found in the water supplies.
Certainly nitrites/nitrates are still viewed with suspicion, and users are being urged to find alternative material and/or reduce dosages used.
In this paper I have tried to display the dilemma which faces experts when they have to give judgements and set standards where facts are unavailable or insufficient, and there must be a temptation on their parts to adopt fail safe levels.
The cause of cancer is unknown but a great many influencing or potentiating mechanisms have been identified. Although the epidemiological information swings between incriminating and not incriminating nitrate in water, gastric cancer does seem to be offering some clues as to its causation. Clues which, unfortunately, at this point in time we are unable to unravel.
There are many hurdles to overcome to evaluate the contribution, if any, n-nitroso compounds make towards the onset of gastric cancer. The compounds are ubiquitous.
Certainly it is difficult to understand if there is an association why the rate of gastric cancer is falling dramatically in most countries whilst the amounts of nitrates used on land and the amounts reported in water courses have risen dramatically over a similar period.
There is no easy way to determine how effective the human biological systems are in neutralising potential carcinogens. A dose/response relationship seems likely but there are some who believe that there is no relationship between the dose of a carcinogen and response.
Dr D D Bryson, MB, ChB, MFOM, DIH, DPH. Imperial Chemical Industries plc, UK.
15 pages, 2 figures, 5 tables, 33 references