Sandro Lanfranco
Department of Environmental Science, GF Abela Junior College, University of Malta, Msida

Introduction
Natural and urban environments are not always very safe places. Various factors, both natural as well as anthropogenic may threaten human populations with a whole range of diseases and debilitating conditions. Some such threats, including many diseases, are probably as old as the human species itself whilst others are a consequence of the recent growth and industrialisation of the human population. These environmental health hazards may be very generally classified into two major categories: infectious organisms and hazardous materials. Infectious organisms are an ancient threat and still represent the major health hazard in developing countries. The threat from toxic chemicals, on the other hand, is more characteristic of more developed countries.

Infectious organisms
An infectious organism (e.g. bacteria, viruses, protozoans) is one which carries out part of its life cycle within an organism (called the host organism) from a different species. In doing so, it usually causes undesirable side-effects which in several cases, may lead to the death of the host organism. Such infectious agents are simply using their host as a source of food and as a vehicle of transmission into other habitats. The principal conditions in this respect are diseases of the respiratory system (whooping cough, pneumonia, tuberculosis, influenza) and of the gastrointestinal system (diarrhoea, cholera, amoebic dysentery). Several of these conditions, may, directly or indirectly result from malnutrition. Lowered levels of food intake would generally dampen the immune response of the human body towards infectious agents, making individuals more susceptible to infection and disease. A person in a diseased condition would encounter more problems in acquiring food leading to further malnutrition and increased susceptibility to even more infection. Apart from this, inadequate hygiene would lead to an increased possibility of eating food contaminated with the spores or resting stages of pathogenic organisms, opening up fresh avenues for infection.
Other major diseases include Trachoma (a disease of the eye caused by a bacterium), Malaria (caused by Plasmodium, a parasitic protozoan which is transmitted by mosquitoes), Schistosomiasis (caused by blood flukes), Filariasis (caused by worms and transmitted by mosquitoes) and Onchocerciasis (caused by nematodes and transmitted by flies).

Hazardous materials
A hazardous material is one that is capable of presenting a long-term or short-term threat to humans. This term comprises chemicals that may ignite readily, chemicals that are corrosive, chemicals that are unstable and liable to explode and toxic chemicals that cause deleterious effects when ingested or inhaled. A significant percentage of the hazardous materials introduced into the environment are degraded by natural processes and therefore pose little threat since they would have been sufficiently diluted. Some hazardous materials, most notably heavy metals and nondegradable synthetic organics are incapable of undergoing sufficient dilution to be rendered harmless.
The most toxic heavy metals enter the environment as compounds of lead, mercury, arsenic, cadmium, tin chromium, zinc and copper. These metals are widely used in industry and in homes and routes into the environment are therefore available. Exposure to minute quantities of such metals may have severe physiological and neurological consequences. Lead poisoning may give rise to mental retardation while exposure to mercury can cause birth defects and insanity.
Synthetic organic compounds are the chemical basis for all plastic, synthetic fibres, synthetic rubber solvents, pesticides and other products. These products may be toxic because they are similar enough to natural organic chemicals to mimic substances required by the body. Once within the body they resist degradation and therefore cause interference with physiological processes. The consequences vary and include poisoning, death, liver and kidney dysfunction and sterility.
Other hazardous materials may be classified according to their effect. Mutagens are factors that give rise to changes, or mutations, in genetic material. If such changes occur during the developmental stage, birth defects are usually the result. When such changes occur in a juvenile or adult organism, the resulting changes may cause growth of tumours. Radiation is a potential mutagen, while various chemicals may also induce similar effects.
Teratogens are a heterogeneous class of chemicals that specifically cause defects during embryonic development. One of the most widespread teratogenic compounds is alcohol. Ingestion of alcohol during pregnancy may give rise to foetal alcohol syndrome, a suite of symptoms that include behavioural problems and mental abnormalities. A well-documented case of teratogenicity is that of a medicinal called Thalidomide. This compound was widely used as a sleeping pill in the 1960s. Its popularity was based on the observation that it had no apparent side-effects. However, when taken by pregnant women during the first trimester of pregnancy, it caused abnormal foetal development leading to a condition called phocomelia. A person afflicted by this condition would have hands and feet but no arms or legs.
Carcinogens are chemicals or other factors that can cause cancer; rapid, uncontrolled proliferation of cells within the body. Rates of incidence of cancer have been rising in industrialised countries and have been mostly attributed to exposure to toxic chemicals and to lifestyle. Cancer is a condition that develops over a long term and increased incidence of cancer may simply be a consequence of longer average lifetimes.

Mobilisation and distribution of toxic substances
The threat posed by a particular substance does not only depend upon its chemical structure or on its potential effects, but also no its mobility. The rate at which a chemical may be distributed throughout the various compartments of the natural environment usually exerts an important influence on it potential for causing damage. A highly toxic chemical that is relatively immobile may cause less harm than a less toxic chemical that is highly mobile and more widely distributed.
Solubility is an important aspect of the mobility of a chemical. Chemical that are soluble in water can move through the environment with relative ease since water is ubiquitous. Chemicals that are insoluble in water but soluble in oil or in alcohol are less mobile. However, once they enter the human body they can move around freely and usually accumulate in fat deposits where they may persist for a very long time.
The distribution of toxins within the environment is also dependent on bioaccumulation processes carried out by living organisms. The cells of living organisms generally accumulate essential chemicals in order to stockpile substances. Various toxins may mimic such essential chemicals and would therefore be retained within cells. In this way, toxins that are sparsely distributed (and therefore dilute) in the natural environment may attain high (and toxic) concentrations within cells. When organisms that contain high concentration of toxins are eaten by predators, these toxins would in turn be accumulated by the cells of the predator. Since the predator would be likely to eat several such organisms, each of which would have high levels of accumulated toxins, the level of these chemicals in the predator would be magnified. Biomagnification therefore refers to the increased concentration of toxins in the higher levels of food chains and food webs.
 

Measuring toxicity
The effects exerted by a particular substance are not absolute, but depend on the level of exposure experienced. One litre of alcohol would be lethal if ingested at once, but relatively less harmful if taken in small, adequately-spaced doses. Determination of the toxicity of different chemicals is problematic because individuals within the same species respond differently to the same toxin. In any population, some individuals would be highly sensitive to a particular toxin and would therefore be the first to experience its effects. Others would require high exposure to this toxin before registering any effects. One convenient way of comparing the toxicity of different substances on the same population would be to determine the dose at which half the members of a population would be affected. In the case of a lethal dose, this would be called the LD50; the dose which would kill half the members of a population.