The bitter-sweet taste of toxic substancesHousehold items such as bleach, disinfectant and detergent are an integral part of everyday life – and they are all potentially toxic. How can we minimise the risks they present?
Key text
Key textWhy 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).
Mathematics of chance – probability
To calculate a risk numerically uses probability – the mathematics of chance. The calculation of probability is simple. You first count the number of ways something can happen and then divide this number by the total number of things that can happen. An example of throwing a dice can illustrate how probability is calculated. When you throw a dice, it lands showing one of six numbers – there are six possible outcomes, and only one way each of them can happen. So the chance of throwing a ‘five’, for example, is 1 in 6 (one sixth or about 0.17). Probabilities are often expressed as percentages which describe how many times an event happens out of 100 times. One sixth is about 17 per cent, so if you tossed a dice 100 times, you would expect to roll a ‘five’ about 17 times.
Weighing up pluses and minuses
One of the simplest ways to weigh up risks and benefits is to add up the consequences multiplied by the probability that they will happen. This gives you an overall expected outcome, as shown in the following example. A lucky dip has 100 tickets, each of which costs $5. The prizes are allocated to the tickets as follows:
There is a 1 in 100 chance of choosing a particular ticket. The expected outcome is: $100×1/100 + $40×1/100 + $5×2/100 + $0×95/100 – $30×1/100 = $1.20 This means that if you played many new games of lucky dip, you would sometimes gain and sometimes lose, with an average gain of $1.20 each time. (You could never actually win $1.20, that’s just an average.) But the tickets cost $5 each, so the expectation is that you will lose an average of $3.80 each time you play. Games of chance are designed to enrich the person running the game!
The probability of more than one event happening
You can calculate the chances of more than one independent event happening by multiplying together the chances of each one happening. For example, there’s a party on Saturday night and the probability that your best friend can go is 0.6 (and that they can’t go is 0.4). The probability of your worst enemy going is 0.3 (and not going is 0.7). The probability that both will go is 0.6 × 0.3 = 0.18. The probability that neither will go is 0.4 × 0.7 = 0.28. The probability that just your enemy will go is 0.4 × 0.3 = 0.12. The probability that just your friend will go is 0.6 × 0.7= 0.42. (Notice that all these probabilities add up to 1 because they cover every possibility.) We have assumed that the two ‘events’ (your friend or your enemy attending) are ‘independent’. The calculation will be different if your friend and your enemy discuss the party by telephone before deciding whether or not to attend! While you may not use numbers, your brain usually does a quick calculation before making decisions (eg, when deciding how much to spend on a raffle ticket or whether to go to a party or not.) Although these are trivial examples, similar kinds of calculations about chances and risk can be used to help make more important decisions (eg, those involving health and safety).
Risk perception
Many factors influence how people perceive risks and make decisions. More people worry about travelling by plane than by car, even though statistics show that they are 10 times more likely to be killed in a car travelling between two cities than in a commercial airline flying the same journey. More accidents happen in kitchens than anywhere else, but you don’t think twice about walking into your kitchen. Our perception of risk is often based on how dreadful we think a risk is and how well we understand the risk, rather than on probabilities. Using mathematics to analyse risks and benefits and to analyse the probability of an event occurring can help us make decisions. But we still have to decide how to measure the benefits and risks, and how best to weigh up the options. Further reading
Cyanide Cyanide is very popular with crime writers, but it is also used in mining when extracting gold and silver. (This is one reason why gold and silver mining can be controversial – the leakage of cyanide from processing sites can affect the health of humans and other animals.) Cyanide can be absorbed via the lungs, skin and stomach, from where it is distributed throughout the body. It inhibits certain enzymes within cells, preventing oxygen use by the cell and causing cell death. At lethal doses for humans, death can occur within 15 minutes. Arsenic Another substance popular with crime writers is arsenic, the twentieth most common element in the Earth's crust. Arsenic occurs in many forms, but is most toxic as an ion because it reacts with sulfur-containing groups on certain enzymes. Exposure to non-lethal levels of arsenic over a long period of time may result in chronic poisoning and carcinogenic (cancer-causing) effects. For this reason, arsenic remains a work-safety issue in industries where it is still used, such as in the manufacture of weed killers and insecticides, the preservation of wood, and in the extraction of lead and copper ores. The symptoms of acute arsenic poisoning occur in two forms. In the paralytic form, a severe paralysis develops within 1-2 hours, often accompanied by signs of delirium. In the gastrointestinal form, symptoms such as nausea, headache, intense pain, vomiting and diarrhoea are dominant. The strange case of arsenic poisoning Despite the apparent demise of Agatha Christie-like poisonings, arsenic remains a hazard in today's society. According to a 1996 edition of the Medical Journal of Australia, a middle-aged man, his wife, his father and his dog developed severe abdominal pain, vomiting and diarrhoea after eating a meal grilled on a wood-fired barbecue. Later, symptoms such as tingling of the fingers, facial numbness, painful cramps in the torso and flaking skin on both hands appeared. Tests on one of the men revealed abnormally high levels of arsenic in his urine. This wasn't a murder attempt. In preparing the fire for the barbecue, the family had obtained wood off-cuts from a nearby building site. Although proof was not obtained, it is believed that some of these off-cuts had been impregnated with copper-chrome-arsenate, a substance commonly used to protect non-durable timbers from insect and fungal attack. When burnt, the arsenic in the wood vaporised, contaminating the meat. Fortunately, all victims survived. Related sites
DDT was widely used after it was first manufactured before World War II. Perhaps its greatest triumph has been in the control of the Anopheles mosquito, which transmits malaria. But, after years of widespread use, it was realised that DDT is also a hazard and it is no longer used in Australia. The problem with DDT is that it is chemically stable and doesn’t break down readily in the environment. Because of this persistence in the environment, DDT has been suspected of causing deleterious effects to organisms higher up the food chain. This is because of a phenomenon known as biological concentration or amplification. Biological concentration When a potentially toxic and persistent substance is released into the environment, its concentration may be so low that is causes no obvious damage. It may move into and remain in the plants at the same low concentration in which it exists in water or soil. But a herbivore (a plant-eating animal) must eat about 10 grams of living matter to make 1 gram of itself. So herbivores will, on average, take in as much of the potentially toxic substance as was found in 10 individual plants. A carnivore (a meat-eating animal) will accumulate the toxin to a concentration about 10 times that found in a herbivore. Thus, the animals at the top of the food chain may contain the compound at a high enough concentration to be damaging, even though the concentration in the environment or in other species may be too low to cause harm. Biological concentration only works with non-biodegradable substances (ie, substances that are not easily degraded by the chemical processes of living things.) An example of biological concentration Imagine a field sprayed from the air with a pesticide. A cow grazing in the field will eat the pesticide that is on the plants. The pesticide will remain in the cow's tissues. Assuming that the pesticide is not degraded, the more the cow eats, the more pesticide it will accumulate. If the cow is slaughtered and its meat eaten, the human consumer ingests the pesticide at a much higher concentration than the cow did. If the person regularly eats meat from a herd of cows that grazed in that paddock, as time passes that person will also steadily accumulate the pesticide. It is known that humans have accumulated compounds such as DDT in their tissues, but we cannot be certain that this has caused harmful effects. However, studies on carnivorous birds (eg, the peregrine falcon) have shown that accumulated DDT in their tissues caused a reduction in the amount of calcium deposited in their egg shells. This made the eggs very fragile, breaking even when the mother sat on them to keep them warm, and reduced the number of offspring. Related site
Australian Science September 2006, pages 29-30 Green cleaners may revive sick soils (by Leigh Ackland) Discusses the reclamation of soils contaminated by heavy metals using a mycorrhizal fungus.
October 2005, pages 19-20 How rusting iron can clean up toxic spills (by Andrew Feitz) Describes a way of cleaning up spills of toxic chemicals using iron nanoparticles.
March 2005, pages 14-16 Solutions for a toxic world (by Julian Cribb) Describes new technologies to treat toxic waste.
November-December 2004, pages 18-19 Oyster plan for toxic waste (by Geoff MacFarlane) Describes how pearl oysters can be used to remove heavy metals and microbes from the aquatic environment.
Ecos No. 127, 2005, page 6 A vegetable alternative to toxic transformer oil Describes research into a vegetable oil alternative to a mineral oil used in electricity transformers.
No. 127, 2005, page 7 Natives tackle cotton pesticide residues Looks at using plants to prevent the buildup of pesticides in water recycled from cotton farms.
No. 126, 2005, page 6 Life could be cooler with sugar coated roads Describes an alternative to asphalt bitumen which is less toxic.
No. 123, 2005, pages 26-28, On the trail of sexual chemistry (by Wendy Pyper) Looks at efforts in Australia to measure amounts of endocrine-disrupting chemicals in waterways and their effects on wildlife reproductive systems.
Emagazine.com January 2006 New car smell: It’s not so sweet (by Jim Motavalli) Describes the toxic chemicals that create the characteristic ‘new car smell’.
Issues March 2005 Contains a collection of articles on toxic waste.
New Scientist 1 September 2007, pages 44-47 Toxic cocktail (by Bijal Trivedi) Looks into the effects of mixtures of artificial chemicals on health.
23 September 2006, pages 26-27 Uncovering the hazards in our electronic gadgets (by Duncan Graham-Rowe) Reports on the analysis of laptops for harmful chemicals.
21 September 2006, page 8 Rich nations put something rotten in Africa (by Debora MacKenzie) Reports that toxic sludge dumped in Africa has resulted in fatalities.
22 July 2006, pages 39-41 Where dirty ships go to die (by Duncan Graham-Rowe) Looks at the exposure of Bangladeshi people to toxic compounds from decommissioned oil tankers.
6 December 2005 Toxic slick hits another major Chinese city (by Shaoni Bhattacharya) Describes a toxic chemical slick in a river in China.
14 July 2005 Arctic seabirds create pollution hotspots (by Anna Gosline) Suggests that bird droppings may be a source of toxins in the Arctic.
24 June 2005 Evacuation not best during a chemical incident (by Gaia Vince) Reports that residents may be safer at home, rather than being evacuated, during an accident involving chemicals.
13 December 2004 Dioxin may cause Yushchenko long-term problems (by Will Knight) Describes the long term effect of toxic poisoning of politician Viktor Yushchenko.
18 September 2004, page 4 Warning over toxic stockpile Reports that corroded containers are releasing toxic compounds in poorer nations.
6 December 2003, page 22 For want of a dollar a year (by Hugh Warwick) Discusses the problem of smoke from cooking fires affecting women and children in developing countries.
6 November 2003 Toxic US ghost ships should 'go home' (by Shaoni Bhattacharya) Reports on two ageing US navy ships in UK waters.
29 October 2003 Europe launches chemical safety crackdown (by Rob Edwards) Discusses the need to test thousands of chemicals due to new regulations in Europe.
1 February 2003, page 9 Arctic faces toxic time bomb (by Fred Pearce) Covers the accumulation of PCBs in the Arctic.
Our Planet A collection of articles on chemicals and the environment is available.
Scientific American March 2006, pages 62-69 Little green molecules (by Terrence Collins and Chip Walter) Discusses a new type of catalyst used to destroy pollutants in waste water.
6 March 2006 Decade-long examination of US waterways finds pesticides in most streams (by David Biello) Reports on a ten year study of pesticides found in rivers in the US.
A fact sheet outlining what hazardous substances are and how they should be labelled and handled.
Understanding toxic substances (California Department of Health Services, USA)
Provides a non-technical explanation of the health effects of hazardous workplace chemicals.
Lessons from life cycle analysis (Australian Government Department of the Environment and Water Resources, Australia)
Concern about the environmental impacts of the products and materials we use has led to considerable publicity about the importance of performing Life Cycle Analyses on consumer products. Life Cycle Analysis (LCA) has been promoted as the best way of determining the real impacts of products.
Toxic chemicals running off roads (Australian Broadcasting Corporation)
Looks at the toxic chemicals that wash from roads in stormwater.
food chain. A sequence of organisms including plants, herbivores (plant-eating animals) and carnivores (meat-eating animals), through which energy and materials move within an ecosystem. ion, anion, cation, and divalent ion. An ion is an electrically charged atom or group of atoms. The charge is the result of the loss (positive ion) or gain (negative ion) of one or more electrons. The gain of one or more electrons produces an ion with a negative charge (anion). The loss of one or more electrons produces an ion with a positive charge (cation). Ions that have gained or lost two electrons are called divalent ions. LD50 . The amount of a substance that is lethal to 50 per cent of the experimental animals exposed to it. LD50 is usually expressed as the weight of the substance per unit of body weight of the animal in order to account for weight difference among animals. More information about LD50 and other measures of exposure to toxic substances can be found at Dose-response relationships in toxicology (Extension Toxicology Network, USA) protein. A large molecule composed of a linear sequence of amino acids. This linear sequence is a protein's primary structure. Short sequences within the protein molecule can interact to form regular folds (eg, alpha helix and beta pleated sheet) called the secondary structure. Further folding from interaction between sites in the secondary structure forms the tertiary structure of the protein. Proteins are essential to the structure and function of cells. They account for more than 50 per cent of the dry weight of most cells, and are involved in most cell processes. Examples of proteins include enzymes, collagen in tendons and ligaments and some hormones. More information can be found at Protein structure and diversity (Molecular Biology Notebook, Rothamsted Research, UK). smelter. An industrial plant that uses a high-temperature process to separate out a pure metal, usually in a molten form, from an ore.
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