Why is it that in almost every laundry the bottles of bleach and disinfectant are on the highest shelf? The answer is simple: because these substances are potentially toxic. We don't want children – who may not know to avoid such hazards – getting their hands on them. But if these substances are so dangerous, why are they in the house at all? Again, the answer is simple: because they are useful. When talking about toxic substances, we must always bear this in mind. Such substances are potentially dangerous, but many also perform useful roles in the modern world. If we are to live safely with them, we need to minimise the risks they present – and we can only do this if we understand them. What is a toxic substance? A toxic substance can be defined as one with an inherent ability to cause systemic damage to living organisms – another word for it is 'poison'. Toxic substances occur in the air, the soil, the water and in other living things, and they can enter the body in various ways:
Another important term is 'risk'. While the bleach on the top shelf of the laundry is certainly toxic, there is no particular risk as long as it stays there. We need to be informed about potential risks in order to make sensible decisions (Box 1: Chances and risks). The importance of dosage The concept of dosage, or concentration in the organism, is also important. Even everyday substances such as water or oxygen would be toxic if we consumed enough of them. But the dosage required would be so ludicrously high that the risk of poisoning from such substances is very low. Many substances may be essential for the proper functioning of an organism at low doses but can be dangerous at higher doses. For example, a deficiency in manganese during pregnancy has been linked to high infant mortality and reduced growth and an irreversible loss of muscle coordination in surviving offspring. On the other hand, workers exposed to high levels of manganese (such as in manganese mines) may incur brain damage that causes memory impairment, disorientation, hallucinations, speech disturbances, compulsive behaviour and acute anxiety. Since it is the concentration of the substance that is important, the same dose of a poison may affect a small individual of a species but pass through a larger individual unnoticed. This is the reason that doses for most pharmaceutical drugs for children are prescribed on the basis of the child's weight. (In addition, children may be more sensitive to some substances because their detoxifying mechanisms are not fully developed.) Acute and chronic toxicity Poisons can be divided on the basis of whether they cause acute or chronic toxicity. Acute toxicity occurs when a single dose produces immediate symptoms of poisoning – think of the movie in which a murder victim clutches the throat shortly after swallowing a tainted drink. The usual way of assessing acute toxicity is the LD50 value, which is the amount of a substance per kilogram of body weight that is lethal to 50 per cent of test animals (usually rats). For example, the LD50 value for aspirin is 1.7 grams per kilogram, which means that 1.7 grams of aspirin per kilogram administered to a rat population will kill half of them. Chronic toxicity occurs as a result of exposure to repeated, non-lethal doses, causing damage over a long period of time. Alcohol can have chronic toxic effects: repeated heavy drinking can damage organs such as the brain, liver and kidneys. Many industrial chemicals can cause long-term adverse effects. Toxicity studies No amount of understanding of toxicity can predict absolutely an individual's response to a specific substance in all situations. Toxicity studies can, however, provide information that can be used to significantly reduce the risk of any adverse effect for the population or groups within it. Data from toxicity studies, together with information on chemical properties, are used when preparing warning statements and safety directions related to the use of the substance. These statements are included on the label of the container to inform users of precautions they should take to minimise exposure. The biochemistry of toxicity The toxicity of a substance can vary. For example, while mercuric ion is highly toxic, some compounds of mercury – such as calomel – are insoluble in bodily fluids and will pass through the human body with little harmful effect. When a toxic chemical enters an organism such as a human, it becomes one of countless different chemicals moving around the body. Often, the toxic effect occurs when a toxic chemical replaces a chemical normally present as part of the structure of proteins and enzymes, thereby rendering them incapable of performing their normal functions. The poisons cyanide and arsenic both work in this way (Box 2: Cyanide and arsenic). Living with toxic substances We live in a world awash with toxic substances. We want the benefits they bring, but we also want to safeguard ourselves and our environment from their deleterious effects. There are several ways we can do this. For a start, we must assess the hazard posed by toxic substances and ask whether they are likely to be used in ways that create a risk. All toxic substances must be handled, stored and disposed of as safely as possible. Exposure standards in the workplace must be set and maintained. In addition, we should substitute toxic substances in industrial processes (and around the home) with less toxic substances wherever possible. Many industries are already doing this as part of a move towards cleaner production. Until recently, for example, a company in Victoria used molten baths of potentially toxic substances such as nitrates, nitrites, carbonates, cyanides, chlorides and caustic salts to provide heat treatment for metal components. Under a cleaner technology initiative, the company replaced these molten baths with what is known as a fluidised bed treatment technology, which uses less hazardous aluminium oxide and gases such as liquid petroleum gas, natural gas, ammonia and nitrogen. Improving our knowledge is essential Many toxic substances remain in use. Inevitably, too, more will be discovered by industrial chemists and applied to the workplace. The toxicity of many substances – and their effects on human health and the environment – is still unknown, so more research is needed. If we understand toxic substances we can minimise the hazard they pose. By doing this, we will continue to enjoy the benefits they bring – without the risk of poisoning ourselves and our planet (Box 3: DDT and biological concentration). Boxes 3. DDT and biological concentration
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Posted February 1999.
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