5.6 Land and water pollution
In this section we will just take a couple of examples that show how easy it is to expose ourselves to long-term damage inadvertently. Pesticides, developed to control insects and other vermin, can increase agricultural productivity. Although pesticides were originally hailed as one of the wonders of modern technology, it was quite quickly discovered that there was a downside to their widespread use. One problem was that of bioaccumulation. Pesticides tended to be stable chemicals and through the workings of the food chain they became concentrated in the tissues of carnivores. This was first noticed when birds like the eagle showed poor hatching success. Eggs that had failed to hatch were analysed and shown to have high levels of pesticides such as the organochloride substance dichlorodiphenyltrichloroethane (DDT). There are 129 substances (including DDT) that are so toxic that there is agreement within the EU that pollution by them must be stopped completely. Unfortunately, many of them continue to be used in ELDCs. The regulations on the application and handling of pesticides hold no sway in these areas and there is evidence of a considerable level of direct poisoning. One study suggested that in parts of Indonesia about 20% of farmers observed spraying had neurological, respiratory or intestinal symptoms of poisoning (Kishi, 1995). Organophosphates are now the more widely used class of pesticides. They are far more toxic to insects than to mammals and they are broken down more rapidly, into biologically inert products, so they are classified as safer. This is very much a relative term! The organophosphates inhibit the activity of acetylcholinesterase (AChE), which is the enzyme that breaks down the neurotransmitter acetylcholine (ACh).
Name a major site of ACh release.
ACh is the neurotransmitter used at the neuromuscular junction.
What is its action there?
It triggers muscular activity.
What will be the outcome of inhibiting AChE?
In the absence of AChE, the muscle remains active.
The amount of ACh will increase in quantity as each new impulse arrives at the neuromuscular junction. ACh binds to receptors at the muscle cell membrane and is then released into the synapse. In the absence of AChE, molecules of ACh will re-attach to receptors and repeatedly stimulate the muscle. This results in muscles going into spasm. Muscle spasms at the larynx will prevent air from entering the lungs and a person poisoned by organophosphates will in effect die by suffocation.
But many incidents of poisoning are sub-lethal and not seen by any health workers. The more severe cases are often misdiagnosed and assumed to be stroke, respiratory or even cardiovascular disease. We have no idea whether there will be any longer-term effects of these chemicals but there are suspicions that they are carcinogenic. There is also a range of studies that provide evidence of immunosuppression by pesticides. Unfortunately, the application of pesticides isn't very efficient. Only about 15% of the pesticide actually reaches the target pest; the rest is dispersed into the environment, especially on windy days. Amongst other organisms, fish seem to be particularly susceptible to pesticide poisoning. Fish are a significant part of many human diets and high levels of pesticides are found in individuals who live in areas far removed from those where the pesticides were used.
DDT exhibits another unfortunate characteristic (which it shares with a range of unrelated chemicals) in that it mimics the effects of oestrogen.
What kind of substance is oestrogen?
Oestrogen is a steroid and also a sex hormone.
When DDT or other oestrogen agonists bind to oestrogen receptors they will inappropriately initiate activity in several quite different tissues. In general this activity relates to reproductive events. For example, overactivity of oestrogen receptors will have a feminizing effect on males. This might explain why there have been reports of human sperm counts falling in recent years, the outcome of which could be lower fertility. However, oestrogen is also required for successful implantation. A widespread class of organic pollutants, the polychlorinated biphenyls (PCBs), are also oestrogen mimics, and there is evidence of animals with high tissue concentrations of PCBs showing abnormal reproductive behaviour, reduced fertility and defective reproductive systems (Colborn, Dumanoski and Myers, 1996). Whilst the evidence for interference with human reproduction is as yet scant, the outlook is not very encouraging. Concern has been expressed that the habit of covering food in cling film and microwaving food in plastic containers increases our exposure to PCBs and thereby the possibility of their contaminating our foodstuffs and being ingested.
These are two examples of materials designed to make life more hygienic that seem instead to pose serious threats to our well-being. It is very hard when developing new technologies and new materials to be sure where they are leading us. This is true in another sense, for the more successful we are in promoting health and longevity, the closer we come to provoking a different kind of catastrophe, that of over-population.