Introduction to Air Pollution

Controlling Air Pollution

In this module, we'll talk about how to control air pollution. I want to note that in environmental health, anytime we identify a hazard that's potentially harmful to people, we have a moral and ethical and professional responsibility to address that problem. Despite the fact that air pollution remains a very common exposure around the world, we've actually made huge strides in reducing people's exposures, particularly in certain regions. So in this session, I want to talk about how we've gone about making those gains. I'd like you to hark back to the hierarchy of controls. So this still serves as an organizing framework for us to think about how do we approach and reduce air pollution. Previously, we talked about it in an occupational contexts, but this absolutely applies to any environmental health situation. So when we think about the hierarchy of controls, we always want to start with elimination, then moving to substitution, then engineering controls, then administrative controls, and last and absolutely least comes personal protective equipment for individuals. So let's talk about some particular examples of applications of the hierarchy of controls to air pollution. I'd like to start with elimination here. In terms of elimination, there's something in the United States called the National Emission Standards for Hazardous Air Pollutants. This is essentially a permit program that puts limits on how much pollution an individual point source can actually emit. So in other words, we're basically placing a cap on the amount of air pollution that a specific facility can put out. That cap is actually enforced by both monitoring and also by regulatory inspections. There's different ways we can eliminate air pollution. So you may have heard of zero emission vehicles. The idea here is we don't have any tailpipe on an electric car. So there's no direct emission of air pollution. But an important thing to consider here is where does the electricity come from that's used to charge that zero emission vehicle. If you're in an area that's using solely renewable energy, that's great. But if you're charging your car with electricity that was created by a coal-fired power plant, that's not so great. We can think about substitution, the next level down on the hierarchy of controls. You may heard of ultra-low sulfur diesel fuel. So this is fuel that's been mixed basically to reduce very purposely the amount of sulfur in the fuel, to reduce the amount of sulfur that's emitted when that fuel is burned. Actually, by making changes and reducing sulfur content, we've enabled other changes to engines that can make them even cleaner as a secondary benefit. If we move down one level further on the hierarchy of controls, now we're to engineering controls. So what could we do from an engineering controls perspective on a point source like a factory or a power plant? Well, if we're emitting gases in particulate matter, we could build a smokestack. This basically elevates the pollutants, so they're not being released right at ground level, and helps disperse them better in the atmosphere. If we have dust or particulate matter being emitted, we could install something called an electrostatic precipitator, which actually uses an electric charge to collect and clean particulate matter out of exhaust air. Finally, we could use something called a scrubber. So a scrubber is actually a piece of equipment designed to run exhaust gas through a liquid, and that helps strip out some of the dangerous hazards in the gas. So if we look at a more detailed example of a scrubber here, the orange arrow that you see in the image represents exhaust gas. So let's say we have an industrial process that's creating air pollution. We would basically pipe that air pollution into this scrubber. You can see at the top of the scrubber is a spray tower. So that tower is spraying liquid which is going to basically capture some of the gas, and you'll see there's actually an oxidizing gas being introduced into the liquid pool at the bottom of the scrubber. In this case, we can also inject limestone, and through a chemical reaction in that liquid slurry, we can create gypsum or wallboard. So what this scrubber is doing is taking in dirty exhaust air, exhausting clean air from the top, and from the bottom, we have gypsum which can then be used to create a new product. So this is a great way to control air pollution. We can also think of engineering controls on motor vehicles. So when you're sitting in a car or bus, the tailpipe at the back of the vehicle is designed to direct that exhaust away from the passenger compartment. We could also think about the catalytic converter in your car which uses a chemical reaction to capture hydrocarbons and carbon monoxide and NOx and convert those to less toxic materials. We also have this concept of exhaust gas recirculation that basically says instead of sending all of the exhaust out the tailpipe, let's recirculates some of it into the engine which actually reduces the production of NOx specifically. So we can look at an image here of the exhaust system. So at the upper right-hand corner, we've got basically where the exhaust enters the exhaust system from the motor, and so we're passing it through the catalytic converter, which is again going to strip out some of the contaminants. It also gets passed through a muffler, which is designed specifically to reduce noise pollution. Then finally, we have much cleaner exhaust gas coming out of the tailpipe. So we've already talked about the upper levels of the hierarchy of controls. Now let's focus on the lower levels, specifically administrative controls. So many of you have probably already heard of the air quality index. The air quality index is basically a way for the government to communicate to the public how good the quality of the air is in their community. So as you can see at the top here, in green, we have the lowest levels of the air quality index. These are levels at which we're simply not concerned about human health. We would consider this to be healthy. Below that, we have the yellow or moderate bands. So there's more air pollution in this band, but perhaps not enough for us to get particularly concerned. However, as we consider the lower and lower bands here, we start to get much more concerned. Then as we get further and further down here, we're getting higher and higher pollution levels until we get to the bottom hazardous band where we basically say this is so unhealthy that probably everybody is going to be impacted by this pollution event. Finally, we hit the lowest level of the hierarchy of controls, personal protective equipment. You may have seen images in the popular press of people in cities, often in south Asia or east Asia, that are experiencing very acute air pollution incidents, and people wear dust masks basically to try to protect themselves from the pollution. The bad news here is that dust masks are at best ineffective at reducing exposures to very fine particulate matter, again the type of particulate matter that penetrates furthest into our lungs, and dusk masks are absolutely useless against gases and vapors. So these masks may provide some mental health benefits. But from an actual objective health protection perspective, they really don't do much unfortunately. So it's reasonable to ask, have these control efforts work? Earlier, I said that we've made tremendous strides in reducing air pollution, and that's absolutely true. So here you can see a very strong growth in our gross domestic product also in terms of vehicle miles traveled. The blue line shows our growth in population, and the darker blue line shows our energy consumption. So all of these things have very much increased since 1980. But despite all those increases, we've actually had reductions in so-called criteria pollutants, the most important pollutants that we regulate. The magenta line in the middle, it says 12 percent for 2018, is carbon dioxide emissions. You can see these haven't decreased, but they also haven't increased at the same rate as some of the other growth in economy and population. But aggregate emissions, the bottom most line on the graph, have fallen dramatically even despite all the increases in the other areas of our country. Unfortunately, despite those improvements, over-exposures to air pollution do continue in the US. So this chart is showing the number of people living in counties where the air quality concentrations exceed the NAAQS, the National Ambient Air Quality Standards, in 2018. So the top bar you can see here is counties where at least one or maybe more of these standards are exceeded, and that accounts for 137 million people, more than one in three Americans. That's not so good. A hundred and twenty three million Americans live in a county where the ozone standard is violated. But the good news is, as we move on to PM 2.5 and PM 10, we have steadily reduced number of people overexposed. The great news is, when we get to the bottom of this graph, carbon monoxide and nitrogen dioxide, we have zero people living in counties where there are overexposure. So we've made progress in some areas, but there's still a lot of room for improvement in others. I'd like to finish the lecture by talking about global efforts to reduce air pollution. We've talked in a previous module about stratospheric ozone depletion. So the layer of stratospheric ozone is very important to protect us from ultraviolet radiation, but pollutants that we've emitted in the past were very damaging to that ozone layer. Fortunately, the Montreal Protocol was signed in 1989, and partnering with the United Nations Environment Program to basically stop the reductions in the ozone layer by eliminating the pollutants that we're causing it. We've seen tremendous improvements in the ozone layer as a result. Many people have been concerned about atmospheric deposition of mercury. So fortunately in 2013, there was a global mercury partnership sign. This is a binding treaty among a bunch of countries who have basically vowed to reduce their mercury emissions to as low as possible. We've seen many agencies, including the World Health Organization, working to reduce urban smog, which consists primarily of particulate matter in ozone. So the WHO has set guidelines and establish standards, and they've also worked with countries to set interim targets. So people and countries have a place that they're working towards in terms of reducing their exposures. Finally, we have greenhouse gas emissions. So these are critical emissions that are driving climate change. You may have heard of the Kyoto Protocol or the Intergovernmental Panel on Climate Change or the Paris Agreement. These are all different international agreements to try to curb and reduce our emissions for greenhouse gases. So we have air pollution issues at a very local or micro scale in a particular neighborhood, for example, but we also have to consider air pollution at a global level. The good news is there are things we can do at all of those levels to improve public health.