The topic of this lecture is toxicology. Toxicology is the science of poison and poisoning. Poisoning can happen for many reasons. It can be accidental, when somebody eat something that they shouldn't eat. It can be environmental, when people are poisoned by chemicals present in the environment such as the many people around the world who have to drink ground water contaminated with arsenic. It can be occupational. When workers who have to work with a particular toxic substance start to feel the effects because they are not properly protected. It can also be deliberate. Poisoning for many many centuries now has been used for both suicide and murder. The use of poisoning to commit murder goes back a very very long way in human history. And even up to say, a 100 years ago, it was still relatively common. Nowadays it's much less common. We see fewer cases of poisoning. And the reason is very simple. In the old days anyone could go into a drug store or a pharmacy and could buy over-the-counter the most horribly poisonous chemicals. Nowadays these chemicals are very tightly controlled. So it's much more difficult to obtain these awful substances nowadays than in the past. Well, before we move onto the classic poisons, there are other substances which will cause damage to the human body in other ways, in particular strong acids and strong alkalis are corrosive. And from the GHS symbol you can see very graphically the effect that they have on the human body. The best known and most often abused of these chemicals is concentrated sulfuric acid also known under its old name of Vitriol. And even today this stuff is quite readily available because, of course, it's the acid that's used in car batteries. There have been many cases from around the world of people using sulfuric acid to attack. There have been cases for instance, in Cambodia, in Bangladesh, in Pakistan, recently there were cases of it being thrown rather indiscriminately at markets in Hong Kong. Possibly the most celebrated case is the case of the Iranian lady Ameneh Bahrami. And you can see from the picture here the drastic effects that sulfuric acid can have on the human body. And this is what she looks like after multiple operations and extensive medical treatment. What happened in her case is that a young man proposed to her, she rejected him and he got his revenge by attacking her with sulfuric acid. Other chemicals can also be irritants. For instance, many salts of the elements lead or mercury or arsenic when they get into the gut, irritate the gut and this causes nausea or diarrhea. And this is an automatic reaction of the body to get rid of these substances. So in the gut these things can act as irritants. Of course these chemicals after they're absorbed from the gut into the body can then have other effects. And we'll talk about some of these later in the lecture. Now let's look at poisons themselves, systemic poisons. These things that we normally consider poisons, they will typically attack particular biochemical systems within the body. That is they will interfere with the molecular machinery that keeps us alive. For instance, one of the most famous of all poisons is cyanide. And what cyanide will do is that when it gets into the cells, it will interfere with the action of the mitochondria. And the mitochondria are those parts of the cell which are involved with processing energy to keep us going. So it's essentially like taking a hammer to the cell's fuse box. As we'll see on the next slide, carbon monoxide, interferes with the work of hemoglobin in our blood. And a poison such a strychnine which used to be quite common, like many other poisons, it shuts down the nervous system. Morphine is a poison we will talk about that more in our lecture on drugs. Atropine another poison like morphine and strychnine, it's derived from a plant and atropine also affects the nervous system. And we'll talk about that later in this lecture. Strychnine, morphine and atropine are purely natural compounds. But synthetic pesticides that are used by farmers, some of these are very very toxic indeed. And later in the lecture we'll also look at some of the synthetic nerve agents that have been used. In the last part of the lecture we'll look at some inorganic poisons such as the elements arsenic, antimony, mercury, and thallium. Now, at the beginning of this slide I mentioned the very famous poison cyanide. And normally cyanide is very difficult to obtain. But cyanide poisoning does still occur. And one of the reasons is this plant here. This plant is Cassava. The root of the cassava plant is used as a staple food by many many millions of people around the world, particularly in Africa. And there's a problem because when the root of the cassava plant is cut open, the plant will start to release cyanide. Now, normally what people do when they're preparing cassava to eat, there's a whole procedure they have to go to to process the root. And after that procedure then it's quite safe, the cyanide has been eliminated. But every year there are a few cases of cyanide poisoning due to cassava and simply because they haven't prepared the root properly. Now let's look at one of the poisons that I mentioned, and that is carbon monoxide. Carbon monoxide poisoning remains quite common. So every year from around the world there are cases of carbon monoxide poisoning. Almost all of them are accidental. A few of them are deliberate. Now carbon monoxide works as a poison by blocking hemoglobin. Hemoglobin is the protein in our blood that is responsible for transporting oxygen from the lungs, where it comes into our body, to the cells, where it's consumed as part of our metabolism. Now hemoglobin is a complex protein, it contains four sub-units and in each sub-unit there's an iron atom and it's the iron atom that does the work. So when the hemoglobin arrives in the blood vessels near the lungs, it picks up an oxygen molecule which has come in with the air we breathe and that oxygen molecule will coordinates to the ion atom, sticks to the iron atom and then is transported through the bloodstream to where it's needed. Well if iron can pick up a small molecule like oxygen, it can also pick up a small molecule like carbon monoxide. So if you breathe in some carbon monoxide, some of your hemoglobin molecules will pick up CO instead of O2. That means that that hemoglobin molecule is not available for transporting oxygen. Now at any given time a certain amount of your hemoglobin is in this form with CO bound to it. The CO bind strongly so the oxygen cannot compete. Now the reason we always have a certain amount of hemoglobin carrying carbon monoxide in our blood is because there's always carbon monoxide in the air. Whenever hydrocarbon fuels are burned, a small amount of CO will be produced. So when someone drives a car, a little bit of CO is emitted and therefore we are going to breathe it in. The amount of CO in ordinary air is too small to do us any harm. It's only when the amount of CO in the air gets above a certain level, that we can get poisoning. Now, blood of course, it's red. It's red because of the hemoglobin. But the color of the hemoglobin depends on the small molecule that's bound to the iron. So of course, blood is red because when oxygen is bound to hemoglobin it's a fairly bright red color. When carbon monoxide is bound to hemoglobin, it's a slightly different color. It's a pinkish color. So in fact one of the simpler signs that someone is suffering from carbon monoxide poisoning is that they will look a bit pink. Now let's look specifically at the chemistry of carbon monoxide poisoning. So let's consider the burning a butane gas, and butane is commonly used around the world as a gas for cooking. So butane is the molecule C4H10. If you burn butane in the presence of plenty of air, then for each butane molecule six and a half oxygen molecules are going to be consumed and you will get four carbon dioxide molecules and five water molecules. This is not a problem. But suppose that same gas is burned in a limited amount of air. Suppose we don't have six-and-a-half oxygen. Suppose we have one oxygen left. We only have five-and-a-half oxygens. In that case for each butane molecule you will get two molecules of carbon dioxide, five molecules of water, but two molecules of the toxic carbon monoxide. So if you burn hydrocarbon fuels where there's a restricted amount of air and there's improper ventilation, then the concentration of carbon monoxide in that place can easily build up to toxic levels. And this happens for instance if you have a room with a gas heater and the flow of the gas heater is not properly maintained. Carbon monoxide will easily build up to lethal levels. Another way carbon monoxide can build up is in a car. Suppose you've been driving a car for a long time. Maybe you're a taxi driver. You've been driving a car all day and you're tired. You want to take a nap. So you pull over, you park somewhere. It's hot, so you leave the engine running because you like the air conditioning. So you close your eyes, you go to sleep. Now perhaps this is an old vehicle. The exhaust pipe is a bit leaky. So that means carbon monoxide as well as with the other exhaust gases can leak out of the exhaust system. Maybe the cab of the car is a bit leaky as well which means that carbon monoxide from the exhaust can seep into the cabin of the car. And there have been cases of people taking naps in cars, engine running, and never waking up due to carbon monoxide poisoning. Here's another example of carbon monoxide poisoning. And this is a tragedy that killed five people a few years ago. And this is a group of teenagers who checked into this motel in Miami to have a party. So this is a rather unusual motel in that the rooms are up above and below the room there's a parking garage, and then there's a stairway leading from the garage up to the room. So what these guys did was to park their car in the garage, and they decided to leave the car engine running. And apparently this is because it was a rather old car and they were worried that if they turned the engine off they might not be able to start it anymore. Well they were eventually found sometime later, they were all in the room, and they were all dead. And what had happened was that carbon monoxide produced by the engine had seeped up the stairs into the room and had tragically killed all of them. Carbon monoxide's cousin is carbon dioxide. A molecule that a lot of people have been talking about the last few years. Now carbon dioxide is much less toxic than carbon monoxide. It does not bind to hemoglobin in blood, so it can't do the same trick as carbon monoxide. The principal danger with carbon dioxide is the danger of asphyxiation and that is, if you're in a place where so much carbon dioxide can be released that it simply replaces the air in that environment. And then there's no oxygen around for people to breathe. The most tragic and dramatic incidents of this in recent years was at a place called Lake Nyos. Now Lake Nyos is an old volcanic crater. It's in Cameroon in West Africa and it's an old volcanic crater that has filled up with rainwater and it's become a lake. But the ground under the lake is still geologically active and it's giving off carbon dioxide. So carbon dioxide from underground is slowly seeping into the lake. Now, the lower layer of the lake is cold water. It's building up carbon dioxide and in fact, has become super saturated in carbon dioxide. The carbon dioxide can't escape because on top of the lower layer, there's the top layer of warmer water acting as a kind of lid. And that's warm because, of course, it's warmed by the sun. And this situation is unstable. But, for the carbon dioxide in the lower layer to be released, there has to be some event that disturbs the lake. And it's believed that what happened is that at one side of the lake there was a landslide. Okay, rocks crashed down into the lake, that stirred up the lake and then rather like a Coca-Cola bottle that's been shaken, and then the lid has been taken off, the CO2 from the supersaturated lower layer was released. Because CO2 is denser than air, the carbon dioxide coming out of the lake flowed over the ground, spread down the valley and approximately 1,700 people were asphyxiated by that carbon dioxide.
