Understanding Air Quality - Criteria Air Pollutants and More
Air quality is a hot-button topic in today's world, as many people are understandably concerned about what they're breathing in. This concern has only grown as our understanding of our air and the effects of poor air quality has developed. But recent advancements in the field of air quality sensing now allow individuals, more than ever before, to take informed decisions to safeguard their health.
Defining Air Quality
Air quality is, broadly, a term that describes the makeup of things in the air. The air can be clean, when there is a low level of harmful things in the air, or dirty, when there is a high level of harmful things in the air. Air quality is in essence a spectrum that describes the amount of pollutants present in a given sample of air. You will often hear the air described as poor when there is a high level of pollutants, or clean when there is a low level of pollutants.
Many different air pollutants can contribute to poor air quality, but they are not universal. One type of air pollutant may be an issue in one area but not necessarily in another. To better understand what pollutants may be affecting people, the EPA identified six criteria air pollutants that most commonly contribute to poor air quality.
- Ozone is a “highly reactive gas composed of three oxygen atoms.” Ozone is “good” when found in the upper atmosphere because it “reduces the amount of harmful UV radiation reaching the Earth's surface.” But “bad” when found in the lower atmosphere because “it can trigger a variety of health problems.”
- Particulate Matter (PM) is the “term for a mixture of solid particles and liquid droplets found in the air.” Essentially, particulate matter, or PM, is solid particles in different size ranges that float around in the air.
- Carbon Monoxide (CO) is a “colorless, odorless gas that can be harmful when inhaled in large amounts” and is “released when something is burned.”
- Lead is a naturally occurring metal that you’ve probably heard of before. Disregarding the typical concern about Lead, it is also a significant air pollutant.
- Sulfur Dioxide (SO2) is an air pollutant that belongs to the sulfur oxide family of pollutants. Sulfur Dioxide is used as an “indicator for the larger group of gaseous sulfur oxides.” This means that if sulfur dioxide levels are rising, it is likely that other sulfur oxides are as well.
- Nitrogen Dioxide (NO2) is an air pollutant that belongs to the nitrogen oxides family of pollutants. The same as sulfur dioxide, Nitrogen Dioxide is used as an “indicator for the larger group of nitrogen oxides.” This means that if nitrogen dioxide levels are rising, it is likely that other nitrogen oxides are rising as well.
The EPA has identified these six air pollutants as the most common. But that does not mean that other pollutants aren't critical to consider from region to region.
Sources of Air Pollution
The air pollutants above originate from various sources. While quite diverse, they can generally be sorted into two categories: Indoor Sources and Outdoor Sources. Going a bit further, this article is also going to sort the outdoor sources into those found in Urban Environments and those found in Rural Environments. This organization will give you a good idea of what to look out for based on where you live or frequent.
It is also important to note that while this article will do its best to provide you with an extensive list of different sources of air pollution, it will not be exhaustive. Additionally, the information presented here is sourced from the various EPA pages for each criteria pollutant unless otherwise specified.
Outdoor Urban Sources of Air Pollution
Outdoor urban sources of air pollution include those that originate from or are primarily found in urban environments such as cities, industrial sectors, or other busy population centers. The table below outlines some common urban sources of outdoor air pollution and what pollutants those sources produce.
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| Vehicles | Power Plants | Industry | Fuel Burning | |
|---|---|---|---|---|
| Ozone | ■ | ■ | ■ | |
| Particulate Matter | ■ | ■ | ■ | |
| Sulfur Dioxide | ■ | ■ | ■ | ▲ |
| Nitrogen Dioxide | ■ | ■ | ■ | |
| Lead | ▲ | ■ | ▲ |
There are quite a few common sources of pollutants in urban areas. These are caused by an unknowable number of factors, but proximity to industry and population density are two that contribute a fair bit. As per the table above, vehicles, power plants, and other various industrial facilities collectively output a number of the criteria pollutants. However, there are some conditions required for the emission of sulfur dioxide and lead.
- Sulfur Dioxide can sometimes result from the burning of fuel. This will only occur if the fuel being burned is of a high sulfur content.
- Lead is marked as being emitted by both vehicles and by the burning of fuel. For both of these sources, the fuel has to be leaded. Nowadays, leaded fuel is mostly used in uncommon vehicles, such as piston-engine aircraft.
Outdoor Rural Sources
Outdoor rural sources of air pollution are those that can be found in, no surprise here, rural areas. But what do those sources actually look like? Some pollutants drift in from urban centers, while others are created during certain activities such as agriculture. The table below goes over some common sources of outdoor air pollution in rural environments.
| Wildfires | Agriculture | Dusty Roadways | |
|---|---|---|---|
| Particulate Matter (PM) | ■ | ■ | ■ |
| Lead | ■ | ▲ | |
| Ozone | ■ | ||
| Nitrogen Dioxide (NO2) | ■ |
While also outdoors, rural sources of air pollution vary quite a bit from their urban counterparts. This is because the composition of land, people, business, and just about everything is different. Now, the two can share pollution sources, especially when urban pollution drifts into rural areas. However, this doesn’t mean that rural areas are devoid of their own sources. As per the table above, things such as wildfires, agricultural activities, and particularly dusty roads can contribute to rural air quality.
Due to the previously mentioned piston-engine aircraft, airborne lead can be a problem in rural communities. Crop dusters, also known as agricultural aircraft, are commonly piston-engine aircraft. Thus, when leaded fuel is used to power them, lead is released into the air.
Indoor Sources
Indoor sources of air pollution include those that originate from or are commonly found indoors. Unlike the previous two sections, this blog does not separate indoor sources between urban and rural. It is important to note that external infiltration is a large contributor to indoor air pollution. That is, outdoor pollutants that make their way indoors. Given that those pollutants originate outdoors, they will not be included in the following table.
| Combustion | Cooking | Pets | |
|---|---|---|---|
| Particulate Matter (PM) | ■ | ■ | ■ |
| Carbon Monoxide (CO) | ■ | ▲ |
Indoor sources are more uniform across areas than their outdoor counterparts. This means that there are a number of things you should look out for inside your home or workplace. Anything that utilizes combustion (e.g., candles, fireplaces, or space heaters) is a source of pollutants. Additionally, cooking is a very common source of indoor air pollution. It produces particulate matter and can be a source of carbon monoxide if you’re using a gas stove.
Air pollution is an issue that you can't defeat all on your own. However, that doesn't mean that you should despair. There are many things you can do and actions you can take to reduce your exposure to poor air quality. For more information, refer to the EPA's guide here.
Interested in monitoring your air quality? Check out PurpleAir sensors here