Introduction to Air Pollution

Key Concepts in Air Pollution

In this module, we'll be talking about some key concepts with regards to air pollution. Now, it's very common that we refer to air pollution as though it were a single exposure or hazard. But of course, air pollutants actually number in the hundreds or thousands in many areas. In this lecture we're going to focus in on some parameters of air pollution that are particularly important from a public health perspective. I want to return us to the Exposure-disease model. So for air pollution and all other types of pollutants in the environment we have a source. We are very interested in how the air pollution might move through the environment and ultimately come into contact with us. Of course, in terms of dose for an air pollutant, we're typically thinking about something that's inhaled the dose is occurring in the lungs. And there are unfortunately many different health effects associated with excessive exposure to air pollution. Let's talk for a moment about different pathways and routes of exposure that we might have between for example a source of air pollution and us. So in this image here we're looking at lots of different pathways and these are highlighted with the red arrows for chemicals as they move through the environment. If we focus on the topmost red arrow, which is labeled atmospheric release, let's imagine we have a source perhaps the factory in the background that's releasing some type of hazard into the air. Well, that hazard may float along through the air until finally, it comes into contact with a person, and we have an inhalation route of exposure. That's maybe the most direct pathway for air pollution. But there are other ways that are pollutants can end up in our body. So if we imagine for example a chemical like mercury, it can actually deposit out of the air and it may deposit onto the ground or on to plants where it's then eaten by animals, and we then consume those animals. So that's an air pollutant that's end up coming into our body actually through ingestion of food. We could also think about a chemical again, mercury is a good example depositing out into water and us then drinking that water and becoming exposed that way. So although we're talking about pollutants that start in the air these pollutants can get into our bodies through multiple pathways and at least two routes of exposure inhalation or ingestion. Let's think about different types of air pollutants for a moment. So you may have heard about particulate matter previously. This is basically a complex mixture of extremely small particles as well as liquid droplets that are suspended in the air. This could include things like acids or organic chemicals that contain carbon or metals as well as soil and dust. We also have to worry about gases and vapors, so things like hydrocarbons and carbon monoxide can be present in air pollution. And the third type would be what we consider to be smog. Now we talked in a previous module about the Great London Smog event in 1952. Smog is actually a portmanteau of smoke and fog. In more modern times we consider smog to be a mixture of pollutants but were primarily concerned actually about ground-level ozone, which I'll talk about more in a moment. So if we focus even further in just on particulate matter it turns out there's two types of particulate matter. There are primary particles, so this is particulate matter that's basically emitted directly from an air pollution source. The smoke you see coming out of a diesel stack on a truck is a great example. We can also think about secondary particles, so these actually form in the atmosphere from a combination of gaseous and particulate air pollutants. So for example, sulfur dioxide it's emitted from power plants can combine with other pollutants in the atmosphere to form sulfates. The image you're looking at here is designed just to convey the scale of air pollution to you. So you can see in the lower left corner we're looking at a fine grain of beach sand about 90 micrometers in diameter. So a micrometer is a millionth of a meter a very, very small distance. You can also see here Illustrated a human hair which is typically somewhere between say 50 and 70 micrometers. Now the blue and pink circles on this image represent different sizes of particulate matter. PM10, the blue circle, is particulate matter with a diameter of 10 micrometers or less and so you can see that's much smaller than a human hair or a grain of sand. And PM2.5 is four times smaller than that, so two and a half microns in width or less. So we're talking about very ,very small microscopic particles. Now, why are we worried about the different sizes of particulate matter? So this is another way to look at the size distribution of different particles. So across the top of this image, you can see different biological substances from down to the level of a molecule all the way up to a human hair and you can see that the range of sizes is quite broad. We're interested in the distribution of sizes here because not all particles can actually enter our body. So something, for example, the size of a human hair is very difficult to inhale it's simply too big to get into our airways. On the other hand something like pollen that might have a lower or smallest diameter of say 10 micrometers that's something that's considered a thoracic particle. In other words, it's of a size where it can get into our thoracic cavity in our body. If we think about something like a bacteria down around 1 micrometer, that would be what we consider a fine particle that can actually penetrate to the deepest regions of our lung. Viruses are even smaller we might consider those to be ultrafine particles. So let's talk a little bit in depth about gases and vapors. I mentioned ground-level ozone a moment ago, I want to highlight that this is different from stratospheric ozone. So the ozone layer up in the stratosphere actually is quite good for us it absorbs harmful ultraviolet radiation from the sun. Ground-level ozone unfortunately is down here where we're breathing. It's created by chemical reactions and it causes a number of health outcomes. We're also concerned about nitrogen oxides or NOx. These are highly reactive chemicals that actually lead to the formation of ozone among other things. We're interested in volatile organic compounds. So these are organic or in other words carbon-containing compounds that actually evaporate at normal temperature and atmospheric pressure. Finally, we're also very concerned about greenhouse gases. So these are gases that can contribute to climate change which we'll talk about this more later in the course.

Play video starting at :6:21 and follow transcript6:21

So let's imagine two pollution sources as we have illustrated here the tailpipe of a car and a power plant. Both of those sources might be emitting NOx as you can see in red as well as volatile organic compounds in blue. And it turns up the interaction of those two things in the presence of heat and light as we get from the sun combine to form ground-level ozone, which again is another type of toxic air pollutant.

Play video starting at :6:47 and follow transcript6:47

Now I want to return here for a moment just to highlight why the size of particulate matter especially is of such concern to us. So in the image here, you see highlighted in blue the upper airways of the human body and in green and red the lower airways. You'll also see to the right of that actually a couple of different graphs showing different colored curves. So the blue curve corresponds to the upper airways, green to the middle airways and red to the lowest airways. So if we focus in on the blue area, your upper airways, it turns out the body has many ways of keeping particulate matter out. You have hairs in your nose that are very effective at filtering large particles. Larger particles will also tend to impact onto your mucus membranes both in your nasal cavity and in your throat. And both of those mechanisms will capture the particulate matter and then ultimately expel it by swallowing or blowing your nose or coughing. You can see in the graph on the right with the blue line this is showing basically the impaction effectiveness of different size particles in those upper airways. And so you can see we have this U-shaped curve here meaning that the upper airways are really good at trapping very small particles and really good at trapping large particles, but they let those medium-sized particles through. Once the medium sized particles are through the upper airways there now into the mid and lower airways. Here we have particles that might impact into our bronchial tubes and embed in the lung tissue or the very smallest particles can actually get down into our alveoli spaces. So the alveoli in your lung are where your body exchanges oxygen with the atmosphere and with your bloodstream. And so if you look at the graphs with the green and red lines here you'll see there's a different size particle that tends to get trapped in the middle airways then in the low or red airways. And I want to highlight specifically that red line, so you can see between about .01 and .1 micrometers or microns is where the lower airways tend to capture those very smallest particles. In other words, that size particle range tends to go very deep into our lungs and potentially can have the worst health impacts. Now switching gears for a moment here I want to talk a little bit more about smog. You may have experienced or heard of smog episode, we certainly talked about the one in London. But why does a smog episode occur? Why isn't it always smoggy? And it turns out here we actually have to turn to weather. So if you look at a normal circumstance for most weather conditions in the left graphic here, we have warm air rising and it can continue rising through cooler and cooler air until eventually, it hits the ambient temperature. In the case of an inversion, we have a flipped situation where cool air is trapped down at the ground by an upper warmer layer of air. So here we imagine the pollutants rise because they tend to be warm, but then they hit this warm layer of air, and they can't rise anymore which effectively results in all those pollutants particularly ozone and particulate matter being trapped right down at the ground level where we are trying to breathe.

Play video starting at :9:52 and follow transcript9:52

I want to wrap up here by noting that it's not just outdoor air pollution that we're concerned about we also have to be concerned about indoor air pollution. So here we have an image of a house you can see all of these red lines coming in are different types of pollutants that we might be exposed to in the home. In the upper right corner, you can see outdoor air pollutants, of course, our house does not exist in a vacuum and whatever's outside can make it inside. But in the home, we might have exposure to example for things like asbestos from building materials. At the bottom you see we may have radon actually coming in from the ground through our basement or crawlspace. If we have just the wrong combination of temperature and humidity we can have mold and bacteria very common in bathrooms. If we have a fire going somewhere in the house either for heating or for cooking or both we're going to have combustion products and smoke from the fire. And unfortunately a lot of the building materials we use as well as paints tend to omit volatile organic compounds. So again, we're very concerned at a global scale about outdoor air pollution, but we also need to be concerned about what I'll say is a microclimate. Basically the indoor environment in homes where people can also get exposed.