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CHAPTER 1: POISONS AND POISONING |
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After studying this chapter you should:
A poison is any substance
that causes harm if it gets into the body. Harm can be mild (for
example, headache or nausea) or severe (for example, fits or very
high fever), and severely poisoned people Almost any chemical can be a poison if there is enough in the body. Some chemicals are poisonous in very small amounts (for example, a spoonful by mouth or a tiny amount injected by a snakebite); others are only poisonous if a large amount is taken (several cupfuls, for example). When people are in
contact with a poison they are said to be exposed to it. The effect
of exposure depends partly on how long the contact lasts and how
much poison gets into the body, and partly on Exposure may happen only once or many times. Chronic exposure to small amounts of poison may not cause any signs or symptoms of poisoning at first. It may be many days or months before there is enough chemical inside the body to cause poisoning. For example, a person may use pesticide every day. Each day the person is exposed to only a small amount of pesticide, but the amount of pesticide in the body gradually builds up, until eventually, after many days, it adds up to a poisonous dose. Only then does the person begin to feel unwell. The way poison gets into the body is called the route of exposure or the route of absorption. The amount of poison that gets into the blood during a given time depends on the route. Through the mouth by swallowing (ingestion) Most poisoning happens this way. Small children often swallow poison accidentally, and adults who want to poison themselves may swallow poison. If people eat, drink or smoke after they have been handling poisons, without first washing their hands, they may accidentally swallow some of the poison. This is a common cause of pesticide poisoning. When poisons are swallowed they go to the stomach (Fig. 1). Some poisons can pass through the gut walls and into the blood vessels. The longer a poison stays in the gut the more will get into the blood and the worse the poisoning will be. If a person vomits soon after swallowing a poison, it may be expelled from the body before a poisonous dose gets into the blood. So, if the person does not vomit straight away, it is sometimes useful to make the person vomit. There are two other ways to stop poisons passing from the gut into the blood: (1) give activated charcoal because this binds some poisons so that they cannot pass through the gut walls; or (2) give laxatives to make the poison move through the gut and out of the body more quickly. The circumstances when it is useful to make a patient vomit or to give activated charcoal or laxatives, and the circumstances when these procedures may be dangerous, are described in Chapter 9.
Fig.1.
Poisons that are swallowed can pass Poisons that do not pass through the gut walls do not get into the blood and so cannot affect other parts of the body. They move along the gut and leave the body in the faeces. For example, mercury metal cannot pass through the gut walls; if mercury from a thermometer is swallowed, it passes out of the body in the faeces and does not cause poisoning. Through the lungs by breathing into the mouth or nose (inhalation)
Fig.
2. Poisons that are breathed in pass very quickly Poison that gets into the lungs passes into the blood vessels very quickly because the air passages in the lungs have thin walls and a good blood supply. Through the skin by contact with liquids, sprays or mists People working with chemicals such as pesticides may be poisoned if the chemical is sprayed or splashed onto the skin, or if they wear clothes soaked with chemical. The skin is a barrier
that protects the body from poisons. However, some poisons can pass
through the skin (Fig. 3). They pass through warm, wet, sweaty skin
more quickly than through cold, dry skin, and they pass through
skin damaged by scratches or burns more quickly than through undamaged
skin. A poison that damages the skin will pass through more quickly
than one that does not damage the skin. It may be possible to wash
poison off the skin before a poisonous dose
Fig.
3. Some poisons can pass through the skin By injection through the skin Poisons can be injected through the skin from a syringe, or a pressure gun, or during tattooing, or by the bite or sting of a poisonous animal, insect, fish or snake. The injection may go directly into the blood vessels, or under the skin into muscle or fatty tissues. Poison injected into the blood has a very quick effect. Poison injected under the skin or into muscle has to pass through several layers of tissue before reaching the blood vessels, so it acts more slowly. What happens when poison is inside the body How poison is carried round the body Once a poison gets into the blood it is carried to the whole body as the blood is pumped round the body by the heart (Fig. 4).
Fig.
4. Once poisons gets into the bloodstream, it is How poison is broken down by the body How poison leaves the body Unchanged poisons or their metabolites usually leave the body in the urine, faeces or sweat, or in the air that a person breathes out. The movement of poison from the blood into urine takes place in the kidneys, and the movement of poison from blood into breathed-out air takes place in the lungs. Poison in the faeces may have passed down the gut without being absorbed into the blood or it may have been absorbed into the blood and then passed out into the gut again. Some poisons, like DDT, pass into body tissues and organs where they may stay for a long time. The effects of a chemical substance on the body may be described as either local or systemic. A local effect is limited to the part of the body in contact with the chemical: the skin, the eyes, the air passages or the gut. Examples of local effects are skin rashes, skin burns, watery eyes, and irritation of the throat causing coughing. Many poisons cause local effects, but there are also many poisons that do not. Further details about local effects are given below. A systemic effect is a more general effect that occurs when a poison is absorbed into the body. Some poisons cause both local effects and systemic effects. If someone has local effects from exposure to a chemical it is important to check whether they also have signs or symptoms of systemic poisoning. Local effects On the skin Chemicals that damage the skin cause reddening or a rash, pain, swelling, blisters or serious burns. The burns are like the burns caused by fire. An irritant chemical causes itching, a burning feeling, or pain when it first touches the skin, but does not cause burns if washed off straight away. However, if it is in contact with the skin for a long time, for example when people wear contaminated clothes for several hours, it might cause burns. Some irritant chemicals have no effect the first few times they touch the skin, but with continued contact they cause reddening or a rash. This might happen with repeated use of a household cleaner. Sometimes people become sensitive to a chemical after they have used it many times. They may have no effects at first but after a few weeks or months they get a rash every time they use it. A corrosive or caustic chemical very quickly causes painful burns and destroys the skin. There may be blisters and the skin may turn grey-white or brown. On the eyes Irritant or corrosive chemicals can cause severe pain if they get into the eyes. They may very quickly burn the surface of the eye and cause scars or even blindness. The eyes will look red and watery. The person may not want to open the eyes and bright light will hurt. Inside the gut Irritant or corrosive chemicals may damage the mouth and throat or the inside of the gut. The person will have belly pain, vomiting and diarrhoea, and the vomit and faeces may contain blood. If the throat is burnt it may swell very quickly, so that the person cannot breathe. Inside the air passages and lungs Some gases and vapours can irritate the nose, throat and upper air passages and cause coughing and choking. Some gases and vapours damage the lungs in a way that causes them to fill with water. This may happen very soon after a person breathes in the substance, or it may happen up to 48 hours afterwards. A person with water in the lungs cannot breathe properly and may drown. He or she must be taken to a hospital as quickly as possible. Water in the lungs is called lung oedema. Some of the gases that cause lung oedema also irritate the nose, throat and upper air passages, and make people cough and choke. When people start to cough and choke, they will quickly move away from the gas into fresh air, if possible. This often means that they are not exposed to the gas for long enough to get poisoned. Some poisonous gases, such as carbon monoxide, have no effect on the nose and throat. Poisonous gases that do not cause coughing and choking are very dangerous because people may not know they are breathing poison. Petroleum distillate liquids, such as kerosene, may cause lung oedema when swallowed. When people swallow any liquid or solid, the air passage closes and keeps most of it out of the lung but small amounts of liquid may still get in. With most liquids this does not harm the lungs because the amount is too small, but even very small amounts of petroleum distillates can cause lung oedema. More importantly, when people are unconscious the air passage does not close, so there is nothing to stop food, drink or vomit getting into the lungs and blocking the air passages or causing lung oedema. That is why it is very dangerous to try to give any food, drink or medicine to unconscious people. At injection sites Irritant poisons that are injected into the skin, such as poisons from insect stings and snake bites, may cause pain and swelling where they are injected. People who accidentally inject themselves with veterinary products, when giving injections to animals or birds, may get local effects. Systemic effects There are many ways in which poisons can cause harm:
Effects on unborn babies Some poisons can harm a baby inside the womb. This is most likely during the first three months of pregnancy when the nervous system and all the major organs begin to form. The parts of the baby usually affected are the bones, eyes, ears, mouth and brain. If the damage is very bad the baby will stop growing and die. Some poisonous chemicals may harm a baby in the womb without harming the mother. This is serious because there is nothing to warn the mother that her baby is in danger. If a mother drinks alcohol or smokes during pregnancy it may harm her baby. Medicines may also harm a baby in the womb. Pregnant women should never take medicines unless they are prescribed by a doctor. Systemic effects only happen when the amount of poison in the body is greater than the amount the body can get rid of, and the poison builds up and reaches the threshold level. Usually, when contact with a poison lasts only a short time (acute exposure), the effects happen soon after exposure and do not last very long. But, in some cases, the effects of a poison are not seen for several hours or even days after an acute exposure. When people have been exposed to a poison for a long time (chronic exposure), the effects may last for a long time. Sometimes a person may be exposed to a poison yet not seem to have any ill effects. This may be because the person has not been exposed for long enough to absorb a toxic dose. Or it may be that the person has absorbed a toxic dose but appears well because it is too soon to see any effects of poisoning. Sometimes it can be hard to know whether a person who has been exposed to a poison is going to develop any ill effects. Before you send a person home always check:
When you suspect someone has been poisoned it is often a good idea to watch the person for 12-24 hours to see if any ill effects develop. In some cases you may need to watch them even longer than that. Exposure to a chemical does not affect everyone in exactly the same way. Some people may be more sensitive than others. For example, young children and old people are more likely to be severely poisoned than young adults; and people who are sick because they are not eating well, or because they drink a lot of alcohol or have a disease, are more likely to be severely poisoned than healthy people. |
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