Introduction to Air Pollution

Criteria Air Pollutants

In this module, we'll talk about criteria air pollutants, but I want to highlight every breath you breathe in, you're breathing in air pollution. Sometimes more of it, sometimes less. Not all air pollutants though are created equal. Some are much more harmful to our health than others. So in this module that we'll actually be talking about some of the most harmful pollutants and what we've done over time to try to reduce exposures and subsequent health impacts from these pollutants. We've talked in other modules about the responsibility that the Environmental Protection Agency, EPA has for setting standards related to the environment, and so they are also responsible for setting air-quality standards. So we have federal standards for air quality, but they're actually implemented and enforced by each of our 50 states. Now all of this was basically enabled by landmark legislation called the Clean Air Act which was passed in 1970 and then amended in 1977 and again in 1990. The Clean Air Act allowed us to regulate emissions from both stationary and mobile sources of air pollution. This considers basically the relative contributions of each of those types of sources to specific air pollution problems, but also the relative capacity of those different sources to reduce their emissions. So one result of the Clean Air Act has been the passage of what are called the National Ambient Air Quality Standards or NAAQS. So the next specifies six primary or criteria pollutants, but also provides a list of air toxics or hazardous air pollutants HAPs. These were added in 1990 in an amendment to the standards and involve the addition of 189 specific chemicals. So let's look in a little bit more detail at these six criteria pollutants plus these hazardous air pollutants or HAPs. The first one we'll talk about is carbon monoxide. So this is a colorless, odorless gas, that's a very common byproduct of combustion. The second criteria pollutant is nitrogen dioxide or NO2 which is actually a highly reactive reddish brown gas, unlike carbon monoxide, this has a very sharp and biting odor. Ozone, specifically ground-level ozone or O3 is the third criteria pollutant. This is another highly reactive with colorless gas that has an odor similar to chlorine. The fourth criteria pollutant is lead. So you may already know, this is a very toxic heavy metal that's released either through combustion or through industrial smelting typically. The fifth criteria pollutant is sulfur dioxide. So this is another colorless gas, but has an odor like burnt matches. The sixth and final criteria pollutant is particulate matter. So as we've discussed in other modules, this is small particles and aerosol droplets suspended in the air. So in addition to these six, we have the 189 hazardous air pollutants, and these are a range of chemicals with varying properties. So those are the criteria pollutants and the HAPs. Let's talk more specifically about their health impacts. So starting again with carbon monoxide, the key issue here is reduced oxygen in your tissues, and it turns out the impact of carbon monoxide is much worse on those with preexisting heart disease. Nitrogen dioxide aggravates respiratory diseases and especially asthma and can also cause other diffuse respiratory symptoms. Ozone is associated with reduced lung function and respiratory symptoms and again, aggravates asthma. For the fourth criteria pollutant lead, we're very concerned about neurological learning and behavioral impacts particularly on children and lead can also have cardiovascular impacts. Sulfur dioxide causes respiratory symptoms, difficulty breathing, and again aggravates asthma. Finally for particulate matter the sixth criteria pollutant, this is very harmful to your cardiovascular system. It's been linked to heart attacks and strokes, and again, is associated with asthma. So you can see that all of these things in some way are going to impact either your respiratory or your cardiovascular system. Then for the hazardous air pollutants because this is so many different kinds of chemicals, it's difficult to summarize their impacts, but many of them do cause eye, nose, and throat irritation, or difficulty breathing, and some of them are linked to cancer and immune effects. So where do all these criteria pollutants come from. The image you're looking at here is basically an inventory of our national emissions here in the United States by source category, and so the size of the bars indicates the fraction of emissions coming from the different sources listed across the bottom. So let's look at a couple of these in detail. If you look over at the right-hand side towards sulfur dioxide, you can see the majority of that bar is blue. So most sulfur dioxide emissions are coming from stationary fuel combustion. On the other hand, if you look at the left side at NH3 which is one of the HAPs, this is ammonia, the primary source here is industrial and other types of processes. So we have a couple of pollutants that mainly come from one source. On the other hand, if you look at carbon monoxide on the far left, you can see that this comes from multiple sources. The leading source here though only by a little bit is highway vehicles. So again, the pollutants are coming from many different sources which makes control of those sources somewhat challenging. Let's talk a little bit more about the National Ambient Air Quality Standards. So here is a table summarizing the NAAQS, both in terms of primary standards but also secondary standards. Primary standards are intended to protect human health, and secondary standards are designed to protect the health of the environment around us. So let's look at a couple of these in detail. Carbon monoxide has a level of nine ppm, nine parts per million with an averaging time of eight hours, but you'll see there's another standard of 35 parts per million a higher level for a one hour averaging time, a shorter exposure period. So immediately, we can start to see that some of these criteria air pollutants are regulated based maybe on more chronic exposures, a longer exposure period and some of them are based on more acute exposures, lesser or smaller averaging time. For lead the next pollutant down, you can see we have a level of 0.15 micrograms per meter cubed, but that's computed over a rolling three-month average, and then for nitrogen dioxide, you can see a 53 parts per billion limit on an annual, or one-year basis. You'll also see at the bottom here particulate matter with a course fractions. So this is called PM10. These are particles that don't make it so far into your respiratory system, and the limit here is based on a 24 hour average. Then if we look at the lower part of the table, we have a different limit for PM2.5. So these are the fine particles that can make it deep into your lungs and may be more harmful to your health. So for PM2.5, we have 15 microgram per meter cubed limit on an annual basis and so on for ozone and sulfur dioxide. So again, six criteria air pollutants with very different limits specified over different averaging times and all that's based on their anticipated health impacts. So now we know what the pollutants are that are of key concern, how do we actually evaluate air pollution exposures? On the left here, you can see a map of all of the monitoring stations that are looking for PM2.5 in United States and you can see all 50 states are represented here. Zooming into Michigan on the right here, you can see we have a number of stations here, particularly in Southeast Michigan where much of the population is concentrated and a few monitoring stations that are relatively close to our campus here in the University of Michigan in Ann Arbor. I want to highlight that pollutant emissions have actually gone down over time. So this is is a success story. So the graphic we're looking at here is basically laying out trends for a variety of different criteria air pollutants including PM2.5 and PM10, and we're looking at the total emissions on the y-axis over time from 1990-2016. So you'll see a couple things here. One is a few of these lines have showed pretty dramatic reductions. So if you look at NOX, the orange line on here, that's nitrogen oxides, or oxides of nitrogen, you can see those have declined pretty rapidly over time, you can also see that sulfur dioxide, the dark blue line, has dropped off pretty precipitously, but you'll also see for a few of these contaminants at the bottom of the chart, the trends have been relatively stable. We haven't made such advances there. We can look at the same criteria air pollutants in a slightly different way. So in this graphic, the dashed line at zero percent represents our most recent National Standard for each of those criteria air pollutants, and here we can look at trends over time in terms of whether or not we're achieving these standards. So if you look at the dark blue line sulfur dioxide, you can see way back in 1990, we were about 75 percent above the current limit, but if you follow that line all the way forward to 2016, now you can see we're about 75 percent below the national limit. That's great news. For some of these contaminants, the group along the bottom, you can see that even back in 1990, we were actually below the national standard and our levels have continued to go down though at a much slower rate than things like sulfur dioxide. So all of these pollutant concentrations have declined over time. That's great news, but the specific trends differ by the type of pollutant. We can also think about changes not just in the six criteria air pollutants, but in concentrations of these other 189 air toxics over time. So I'm just showing you an example here of the monitoring network for one specific type of hazardous air pollutant. This is benzene. In this case, this is an air pollutant that actually is associated with cancer. So on this graphic, you can see that for each monitoring station, we can see whether trends have been decreasing over time in blue, or flat over time in green, or increasing over time in red. So the good news here, you can see there are many stations where there has been a decrease over time in benzene concentrations, and for the most part, the rest of the stations are either no trend or indeterminate. So taking this information collectively, we can see that we have a number of harmful air pollutants. We have limits for those and we're actually able to monitor for those so we can determine what the exposures and likely health impacts are on the public.