 So let's talk a little bit about environmental contaminants, what a contaminant is, and how we sort of separate the study of contaminants. Basically, we have three groups here that we're going to talk about, transport, fate, and exposure, and define what these terms are. First of all, let's talk about being a living being. Living beings interact with our environment, and indeed most of our primary bodily functions are about interacting with the environment. As a human, you're regularly breathing, you're regularly eating, so we have our respiration from breathing, we have ingestion, we're eating things, we have perspiration, where we're exchanging temperature and water with the environment through our skin. We have excretion and secretion, where we release different chemicals into the environment and or get rid of our wastes. And we also have the ability to add things through topical absorption, in other words, we can absorb certain things in through our skin. And so we're regularly exchanging things with our environment. So most of our functions involve sort of exchanging different substances with the environment. So substances that we use for survival are often called nutrients, that were nutrition, anything that we sort of use that helps us and improves our survival. And a substance that doesn't improve our survival but that actually damages some of our bodily functions is what we call toxic, something that damages bodily functions. Now I will point out that toxicity is a matter of amount. Anything, if you can think about anything that's, well, anything that's harmless, let's say we drink water regularly, we breathe in air regularly. Anything that we actually ingest into our bodies, if it's in the wrong amount, in the wrong place, there's a possibility for too much of it in the wrong place that it can be damaging. For example, we know that you breathe in a little bit of water vapor regularly in the air, but if you fill your lungs with liquid water, you drown, that's damaging. So liquid water can be toxic. In fact, even if you drink too much water, that can be toxic as it can create an imbalance in your body. Things like eating a banana. A banana is actually good and full of nutrients. But if you eat too many bananas, your body will actually reject it because there will be an imbalance in the amount of different ions in your system and it will be bad for your body. So too much of anything can be toxic at a certain level. On the same token, a small enough amount of anything can be harmless. Your body can usually repair it even if it creates damage. Your body can repair that damage in a very short period of time. So we are regularly breathing in things that will be toxic in large enough amounts, and sometimes they're even beneficial if they're small enough, but again, too much of it's bad. Arsenic or mercury, or think of anything that you consider to be a poison, there's probably some small, tiny amount of that in your body at any one time, and your body can easily deal with that. So the key idea here is that this idea of toxic or toxicity, how toxic something is, obviously depends on how much of the thing there is, how much of this substance there is. And a substance that is toxic or actually a substance that when mixed with other substances increases the toxicity of the overall mixture is called a contaminant. So it's going to increase if you have something harmful or even beneficial, for example, if you're breathing in air or drinking your water, and it's mixed in with something that is toxic, that thing is called a contaminant. And there's typically three ways that we interact as humans. We breathe air, air is breathable, we drink potable water, and we ingest edible food. Well, any of those things can be contaminated or made more toxic than when we intend by the addition of something that damages our body. Contaminants all have a source. They all have something, some place where they get into the environment. That source could be a natural source or the source could be something man-made where we create something that has a different purpose but ultimately ends up being toxic if mixed in to our food or our air or our water. Usually we separate these into two types of things. We can call them a point source. We can identify a point source. A point source is something like a volcano or a leaking gas tank, one single location in space that is generating or has generated some amount of contaminant. If instead we're gathering the contaminant from lots of different locations, this is what we call a non-point source or NPS, and that would be the example of maybe there's a little bit of something that's bad throughout the soil, but there's not a lot of it, but then the water flows through the soil and as that water does it gathers up little bits of pollution from different points. That's true with things like pesticides where we spray them over large areas and then they can be accumulated into a water source or in smoke that's in the air that there's lots of different locations that maybe it has come from. If you have a large forest fire burning, for example, that would probably be more of a non-point source. So once we have contaminants in the environment and they've originated, they've gotten into the environment, there are three sort of methods or three pieces that we care about to study. And these methods line up sort of with our standard sciences. The first method is transport. The contaminants are going to move from their source and eventually end up in contact with a living being. In our case we care most frequently, we care about humans. So transport is the movement of the contaminant, and usually that movement is governed by physical processes. For example, fluid mechanics applied to water or air or the primary processes. Some motion of solids actually occurs too. You can have pollutants that are in solid form that move along with water or fall or drift in the air. But transport processes are one of the first major things that we can sort of study when we're talking about environmental contaminants. How do the contaminants move from point A to point B? The second piece here is the fate. Fate is basically chemical processes. It's really what happens in how the chemical or the contaminant changes over time as a result of interacting with other substances or other contaminants or maybe non-contaminants, just other substances. Again, it evolves chemistry and usually what happens is either the chemical becomes more or less toxic by combining it away. For example, salt and ACL is sodium and chloride. Well, salt itself is harmless and actually beneficial. But chlorine, if the salt is broken apart to the chlorine ion, the chlorine ion can be very damaging. Well, the fate of a chemical or a contaminant in the environment will change whether or not we have to be concerned about it. It might become more toxic. It may become less toxic. It might also become less inclined to move. It might stick onto something and stay there and be trapped more or less permanently and stop moving. So we are concerned about the fate of chemicals because we know whether they're being removed or added to our system that we're concerned about. And then the final piece that we're interested in is exposure. An exposure is basically a description of the toxic interactions between the contaminant and a person or some other biological entity. And these, again, are pretty much biological processes. And they depend upon a number of different things. They'll depend on usually something about how long you're exposed and also depending upon the toxicity. So basically exposure is an interaction of the contaminant with the biology and we measure it by talking about how long you've been exposed to the chemical and how much chemical you've been exposed to and the mechanisms whereby you are exposed. For example, if you touch something, you're going to have a very different kind of response to that as opposed to if you breathe something or if you ingest something. So those three processes, the main processes you should remember from here, transport, basically the physics of the environment, fate, basically chemistry of the environment and the contaminant, and finally exposure, basically the biology of the environmental contaminant.