Health effects of long term pollutant exposure – A known unknown?: Part II

In Part I of this blog, we looked at the possible health effects of environmental pollutants. In the conclusion we will look at their unexpected and unpredictable effects.

There is also no doubt some of the environmental chemicals we are exposed to in our every day lives are harmful if taken in large enough amounts. Certainly, many of the pesticides, metals such as arsenic, lead and mercury and a great number of the industrial waste products are considered toxic. Yet governments believe in the amounts we are exposed to, they do not pose a risk to our health. The main reason for this – and this applies to most environmental chemicals – is the amounts finding their way into our bodies are so minute it is difficult to see how they can be harmful. Unfortunately, it may not be this simple. There is plenty of evidence – from reputable medical and scientific sources – to show tiny amounts of enviromental chemicals and pollutants can produce quite unexpected effects. A good example is the well known herbicide, glyphosate – commonly called Roundup®. At the amounts suggested by the manufacturers, it is highly toxic to broad leaf plants but apparently safe to humans. But a surprisingly different effect has been observed at concentrations much lower than those required to kill weeds. At these tiny non-toxic doses, like those generated by the drift of glyphosate spray from neighbouring farms, the herbicide may actually stimulate growth of certain plants (7). Another example is the antioxidant resveratrol, present in food and abundant in red wine. Its presence in red wine is thought by some to explain the “French paradox” – ie the apparently low incidence of heart disease in the French despite a relatively high intake of butter and cream. At higher concentrations, resveratrol can kill a variety of different cancer cells. This is the good news. However, the bad news is at much lower doses, the antioxidant may actually stimulate tumour growth, at least in the test tube (8).

So, these examples show we cannot always predict what a chemical substance will do at very low concentrations. Some scientists refer to this phenomenon as “hormesis”(8). While there was initially a fair amount of skepticism amongst scientists over this, there is increasing acceptance there is a degree of unpredictability about how chemicals can affect biological systems at low concentrations. Indeed, this unpredictability is a good example of what Donald Rumsfeld, the former US Defence Secretary termed “a known unknowns” in his now infamous press briefing of February 12, 2002.

But wait: It’s even more complicated than this! Read the rest of this entry »