Have the harmful effects of Environmental Chemicals upon Man and Ecosystems lessened since Rachel Carson published her book, Silent Spring in 1962?
Almost fifty years after the publication of Silent Spring (1962), the legacy of this book in relation to environmental toxicology still lingers on. Described as one of the most important contributions to Western literature and as one of the ‘books that changed America’ (Lutts R H, 1985), Rachel Carson’s Silent Spring (1962) is also considered the ‘effective beginning’ of ‘toxic discourse’ (Buell L. 2001). Rachel Carson caught the world’s attention when she documented the detrimental effects of pesticides particularly DDT on the environment, especially its effects on birds. As Carson pointed out, the synthetic insecticide industry was ‘a child of the Second World War’ (Carson 1962). According to the United States Environmental Protection Agency (USEPA), there are over 80,000 existing chemicals on the Toxic Substances Control Act (TSCA) inventory and each year an additional 2,000 chemicals are added. Release of these chemicals into the environment through agricultural and nonagricultural application and other means poses serious risks to both human health and ecosystems (e.g., plant and wildlife). The effects of industrialization, intensive agriculture and coastal engineering (including tourism) have seriously begun to threaten marine life (His et al., 1999).
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With the above as a backdrop I argue the fact the harmful effects of environmental chemicals upon man and ecosystems have not lessened since Rachel Carson published her book, Silent Spring in 1962, but have rather increased. I also point out the fact that there has been an increased level awareness and research into the potential effects of environmental chemicals since Rachel Carson, and this awareness and research, mostly affirms this argument.
Effect of Environmental Chemicals on Environment
The approach taken here is a broad look at the major groups of known environmental chemicals (contaminants), and discuss their historical and current use and application, sources of entry into the environment, their pathways, their fate and behavior, and their known effects effects on all levels of the biological hierarchy, from cells to organs to organisms to populations to entire ecosystems. However, a major limitation to this approach is that most research on effect of toxicants in the environment i.e. risk assessment models express their results in terms of effects on individual organisms, without corresponding information on how populations, groups of species, or whole ecosystems may respond to chemical stressors. This discussion has been limited to only 2 of major groups of environmental chemicals namely Persistent Organic Pollutants (POPs) and Metals.
Persistent Organic Pollutants
Persistent Organic Pollutants (POPs) refers to a large grouping of organic compounds which are noted for their persistence and bioaccumulative characteristics. POPs include many of the first generation organochlorine insecticides such as dieldrin, DDT, toxaphene and chlordane and several industrial chemical products or byproducts including polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (dioxins) and dibenzo-p-furans (furans). Many of these compounds have been or continue to be used in large quantities and, due to their environmental persistence, have the ability to bioaccumulate and biomagnify. Some of these compounds such as PCBs, may persist in the environment for periods of years and may bioconcentrate by factors of up to 70,000 fold (L. Ritter, K.R. Solomon, J. Forget).
Behaviour of Persistent Organic Pollutants in the environment
The behaviour and fate of chemicals in the environment is determined by their chemical and physical properties and by the nature of the environment. The chemical and physical properties are determined by the structure of the molecule and the nature of the atoms present in the molecule. Depending on the structure of the molecule, these physical and chemical properties span a large range of values. Compounds may be of very low persistence, of low toxicity and be immobile. These compounds are unlikely to present a risk to the environment or to human health. The physico-chemical characteristics of POPs such low water solubility, high lipid solubility, semi volatility and other characteristics account for their behaviour in the environment. POPs exhibit a number of physico-chemical properties that account for their behaviour and effect on the environment.
Table 1 Physico-chemical properties of POPs accounting for the behaviour and effect on the environment
Description and Behaviour
Allows for mobility through the atmosphere that is sufficient to allow relatively great amounts to enter the atmosphere and be transported over long distances.
Long Half life
Since POPs are highly halogenated, carbon-chlorine bond is very stable towards hydrolysis and offers greater resistance to biological and photolytic degradation. As result of this POPs are very persistent in the environment.
Low water Solubility and High Lipid Solubility
POPs have very low water solubility and high lipid solubility leading to their propensity to pass readily through the phospholipid structure of biological membranes and accumulate in fat deposits. Because of this, and combined with their resistance biological degradation, POPs tend to biomagnify through food chains. Biomagnification results in much greater exposures in organisms at the top of the food chain.
Other key properties that account for the behaviour of organic pollutants in the environment include: Henry’s Law Constant (H), Octanol-Water Partition Coefificient (KOW), Organic Carbon-water Partition Coefficient (KOC)
Historic and current sources of POPs into the environment
During the second half of previous century a considerable global environmental pollution developed due to the tremendous use of several polychlorinated organic compounds, some of them being used as pesticides (hexachlorobenzene, hexachlorocyclohexane etc.), insecticides (DDT), defoliants (e.g. dioxin containing Agent Orange in Vietnam war), multipurpose industrial chemicals (polychlorinated biphenyls) and some of them originating as byproducts of large scale chemical industrial production (polychlorinated dibenzodioxines and dibenzofurans). Recent comprehensive surveys on global contamination by persistent organochlorines have revealed that large quantities of these substances used in tropical regions are released into the atmosphere and redistributed via long-range atmospheric transport on a global scale. Related to this, it was also indicated that open ocean may serve as a final sink for such chemicals, and, therefore, toxic threat by persistent organochlorines to marine organisms is a matter of great concern (Langer, 1995).
Sources and Entry into the Environment
Applied to soils to kill termites, grasshoppers, corn rootworm, and other insect pests
A broad-spectrum insecticide on a range of agricultural crops.
Perhaps the best known of the POPs, DDT was widely used during World War II to protect soldiers and civilians from malaria, typhus, and other diseases spread by insects. It continues to be applied against mosquitoes in several countries to control malaria.
Used principally to control termites and textile pests, dieldrin has also been used to control insect-borne diseases and insects living in agricultural soils.
These chemicals are produced unintentionally due to incomplete combustion, as well as during the manufacture of certain pesticides and other chemicals. In addition, certain kinds of metal recycling and pulp and paper bleaching can release dioxins. Dioxins have also been found in automobile exhaust, tobacco smoke and wood and coal smoke.
This insecticide is sprayed on the leaves of crops such as cotton and grains. It is also used to control mice, voles and other rodents.
These compounds are produced unintentionally from the same processes that release dioxins, and they are also found in commercial mixtures of PCBs.
Primarily employed to kill soil insects and termites, heptachlor has also been used more widely to kill cotton insects, grasshoppers, other crop pests, and malaria-carrying mosquitoes.
Kills fungi that affect food crops. It is also released as a byproduct during the manufacture of certain chemicals and as a result of the processes that give rise to dioxins and furans
Mainly used for against fire ants and other types of ants and termites. It has also been used as a fire retardant in plastics, rubber, and electrical goods
Polychlorinated Biphenyls (PCBs)
These compounds are employed in industry as heat exchange fluids, in electric transformers and capacitors, and as additives in paint, carbonless copy paper, sealants and plastics.
Applied to cotton, cereal grains, fruits, nuts, and vegetables. It has also been used to control ticks and mites in livestock.
Although the production and use of a number of the above organic pollutants has been banned in most of the developed western countries, they are still widely used in some developing and third world countries including China, India, South East Asia, Africa, and South American.
Effect of POPs on Man and the ecosystems
Since the early 1960s, the chronic adverse effects of POPs on human health and on ecosystems, due to prolonged periods of exposure have been recognised. Several of the POP pesticides are carcinogenic in experimental animals and therefore are possibly carcinogenic to humans. Several POPs are a listed and classified as probably carsonigenic (IARC). Some are also suspected to depress the immune system (Repetto and Baliga, 1996).
DDT and DDE
Taking DDT as an example, there is conclusive evidence that, populations of birds of prey declined already in the 1960s as a result of eggshell-thinning. This was caused by DDE, a very stable metabolite of DDT (Faber and Hickey, 1973). DDT is also known to disturb sexual development and behaviour in birds such as gulls (Fry and Toone, 1981). And there are strong indications that the capacity of the immune system is impaired by DDT, but also by certain synthetic pyrethroids – pesticides that have been promoted as DDT alternatives (Rehana and Rao, 1992). The nervous system can suffer permanent damage from exposure during the foetal stage or early in life (Eriksson, 1992, Hussain et al., 1997). Lactation in women can be impaired by DDT/DDE – providing a possible link with oestrogen mimicry (Gladen & Rogan, 1995, Rogan et al., 1987).
Elevated concentrations of PCBs have been reported in marine mammals and birds and are being suspected to be linked to a series of mass mortalities of marine mammals that occurred during the last decades (Langer, 2005). Numerous reports on immunotoxic effects of organochlorines were recently published. Thus, Jung et al. (1998) observed impaired leucocyte function in workers formerly exposed to dioxins, (Langer, 2005). PCB pollution and its effects is of major concern in the world today. There are even several areas which are so heavily polluted that they are now called PCB reservoirs, among them the Baltic See, Hudson Bay and Great Lakes in North America, (Langer, 2005).
Like other Organochlorines (OCs) dieldrin has been proved to be toxic to wildlife. After routine monitoring of wildlife by the UK in the 1980s high concentrations of dieldrin were found in the eels caught in the rivers of southern England. Independently large numbers of herons were being reported dead in the area. Post mortem investigations on the herons reported high concentrations of dieldrin in the heron tissues (Shaw I and Chadwick J, 1998).
TBTO used as antifouling paint in shipping and boat industry has been determined to have serious effects on ecosystems in harbours where paint stripping occurs. Research studies have determined that TBTO concentrations in the range of 0.001 – 0.01 ug/dm3 can lead to imposex in dog whelk, and concentrations in range of 0.01 – 0.1 ug/dm3 could lead to growth inhibition in algaea and oysters. Concentrations of TBTO in some harbours in the UK were determined to be over 0.6 ug/dm3 (Shaw I and Chadwick J, 1998). This has resulted in the restriction of the use of TBTO in ship paints in the UK.
Unlike most organic pollutants, metals occur naturally in rock forming and ore minerals. However, metals become pollutants where human activity relocates them into situations where they can cause environmental damage. Metals are extensively used in commercial and industrial applications and, as a result, exposure can occur from direct and indirect pathways. These exposures may be associated with such processes as smelting, welding, grinding, soldering, printing, and many other product manufacturing operations.
Behaviour of Metals in the environment
Similar to organic pollutants the physic-chemical properties of metals account for their behaviour in the environment and their toxicological effect. From a toxicologic standpoint these properties affect the absorption, distribution, metabolism, and resulting biological effects of metals. In addition, the fact that metals are elements and do not degrade in the environment means that they have a very high persistence, resulting in a greater potential for exposure than other, less persistent, toxic chemicals. In addition to their uncomplexed or elemental state, metals may exist in the environment as complexes with other substances. These complexes and various metal species may differ dramatically in their chemical and toxicological properties. For instance Trivalent (3+) chromium compounds, for example, which are noncorrosive and noncarcinogenic, are less of a concern than hexavalent (6+) chromium compounds, which are quite corrosive and may cause cancer on sufficient exposure by the inhalation route only.
Effect of Metals on Man and the ecosystems
Metals such as beryllium, cadmium, hexavalent chromium, nickel, mercury, lead and others have been demonstrated to be toxic and carcinogenic in humans, experimental animals, and other aquatic organism. Several authors and researchers have documented these effects overtime in different parts of the world. For purposes of this discussion only Lead is highlighted, however, effects of other metals contaminants such as Arsenic, Selenium, Cadmium, Nickel and Mercury hasve been reported around the world.
Lead poisoning remains one of the most prevalent diseases of environmental origin in the world (Needleman and Gatsonis, 1990). Children are exposed to lead in ambient air, foods, drinking water, soil and dust (Cheng et al., 1991). Exposure to lead in dust can thus account for a large fraction of the10 ug/dl average PbB reported in children from some parts of Durban (Nriagu et al., 1998).
Although many agree that Rachel Carson’s book Silent Spring (1962), and legacy has had a tremendous impact on making us aware and more vigilant to curb the rate of production and use of toxic chemicals, a number of factors still contribute to the continued effect of toxic chemicals in our world today. Increased industrialisation and urbanisation of our world today, lack of consensus and agreement to stop the use of dangerous chemicals by some countries in Africa, Asia and South America, the very nature and behaviour of some of these chemicals, are some of the factors contributing to the continued increase of effects and impacts of environmental chemicals on man and ecosystems over the past few decades.
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