In this module, we'll be talking about pathways and routes of exposure. I'll define these terms as we go through the lecture. But first, I want to highlight. We're standing here at the beautiful University of Michigan campus farm, in front of some crops that are being grown. You might ask, "What does this have to do with environmental health sciences?" Well think about ways that harmful substances in the environment can get into your body. Food represents one very obvious way that we can potentially become exposed. So this seem like a great setting, to introduce these concepts. If you recall back to the exposure disease framework, we talked about in another module, you hopefully remember that the first box in that framework is the source. We can classify sources in many different ways. One of them, is whether they're anthropogenic, created by humans or natural. So standing here in front of a farm, this is very much an anthropogenic creation, but notice that it occurs in an otherwise very natural setting. We could also think about sources as being stationary, something that doesn't move around in space. So an example there might be a power plant or mobile, something that does move through space like cars, or planes, or trains. Finally, we could think of sources as either being a point source or an area source. A point source is a fixed point in space that emits some harmful hazard, an example here might be an incinerator in the middle of a community. Or an area source when that's more dispersed over a broad area and farms are another excellent example of an area source. So for example, when rain falls onto a field any chemicals that have accumulated on that field, pesticides, for example, or fertilizers can then run off with that rainwater, and that run-off becomes a very dispersed source of pollution for the surrounding environment. We can also think about ways that chemicals can move throughout the environment to potentially come into contact with people. In the graphic that you see here, we've illustrated multiple sources for a single chemical. So the child, the baby sitting in the middle of this picture, has many many different sources of potential lead exposure. So lead, a very harmful neurotoxic and for children especially, can come from lots of different sources. So you see here, there's the potential for this child to inhale lead that might be emitted from an industrial source out of a smokestack. Or it could come out of the exhaust of automobiles that are using leaded gasoline. You'll see also there's the potential for this child to get lead exposure, for example through the lead solder that's present in cans of food. So when that solder comes into contact with the food, the lead can be transferred into the food that the kid then eats. We could also think about older homes, which often have lead paint. As that paint degrades over time, it falls in flakes to the floor. The kid can eat those flakes, which actually are sweet, or the flakes can turn into dust as they're stepped on or otherwise crushed, and then maybe there's a pet that goes and rolls in that dust and then the child pets the pet. So here's one case, a single chemical where there are many potential sources of exposure. We can also think about a single source that emits lots of different types of hazards. So a classic example here, might be a factory or a power plant, in the middle of a community. The smoke stacks from that factory might be emitting volatile organic compounds, sulfur dioxide, carbon dioxide, ozone, lots of different hazards. So a single source, but many many different hazards emitted from that source. In environmental health, we think about vectors or media that chemicals particularly can travel through to get through the environment from the source to us. When I say us, the proper term there is a receptor. So in environmental health sciences, we're interested in receptors that are people. So we can think about the different media that a chemical might collect in and move through the environment through. Those are air, soil like we find here, water and food, like the crops that you see behind me. So let's dive into some examples here of specific pathways that a chemical might take to move through the environment. So in the image you're looking at, we have a couple of drums of a chemical, a harmful chemical, and we can see arrows that are highlighting different pathways of exposure. So the one at the top, indicates that perhaps this chemical is volatile. So it turns into a vapor in the air, then it moves with the air until it comes in to contact with the person, and the person has a chance then to breathe in that air. So that's a media of air, and you can see the pathway here is it basically blowing down wind from the spill. We could also imagine that chemical going into the ground, and once it's in the ground it may come into contact with water until eventually perhaps it comes into contact with a well pipe. Well water is quite common source of water here in Michigan. Now when someone is pulling water through that well pipe, they're also pulling that chemical and potentially drinking that chemical. We could also think about that chemical getting into the air or getting into the soil, and then either being taken up by the crops growing in that area from the soil, or getting into contact with that chemical through water that's applied to the crops. Or if the chemical is in the air and then deposits on the leaves of this great-looking broccoli behind me, then the food becomes contaminated, and we become exposed when we eat that food. So again, we've got four primary media here; air, soil, water, and food and different pathways that a chemical can take from the source through the environment to come into contact with a person. Once a chemical has moved through the environment and come into contact with a person, we then have to consider what we call the route of exposure. There are four main routes of exposure. So this is the way that a chemical or other type of hazard transitions from being in the environment, to being in your body. So the first one we'll focus on here is ingestion. Imagine drinking a glass of water. You're drinking the water, hydrogen, and oxygen but you're also drinking any other chemicals that are present in that water. Or imagine eating food, you're eating whatever the food is, but also pesticides and potential other materials that might have contaminated that food. All of those things are an ingestion route of exposure. We could also think about inhalation. So every time I breathe in air, I'm breathing in hundreds if not thousands of chemicals, even in a clean environment like this. So each inhalation I take, is a potential inhalation route of exposure for chemicals or other types of hazards. The third route of exposure we'll talk about is skin contact. So if you've ever come into contact with the poison ivy plant, you've had probably an unfortunate experience of a skin contact route of exposure. The oil from the leaves of the plants gets on your skin, it doesn't travel through your skin but it causes a very acute, very localized skin rash that can be incredibly unpleasant. Finally, we can also have skin absorption. So certain types of hazards will pass through the dead layer of skin on the outside of your body and end up, for example, in your bloodstream. An example here would be nanoparticles. You might think, "Well, where do I come into contact with nanoparticles?" Actually, if you wear sunscreen, and you should wear sunscreen to protect against skin damage, the active ingredient in sunscreen is a bunch of nanoparticles. So as you apply that sunscreen to your skin, some of those nanoparticles immediately start traveling through your skin and end up in your bloodstream. Now we know that sunscreen on the outside of your body is very good for protecting your skin. What about these nanoparticles when they're inside your body? We don't know so much about that. So collectively, we need to think about where's the source, in the environment? What is the type of source? What sort of media can that hazard from the source accumulate in and move through the environment? Then once a human comes into contact with that hazard, how might it actually enter their body? What route of exposure does it take?
