Air pollution has always been a big deal, but the pandemic was a catalyst for many of us to start thinking about the air–and what’s in it–much more seriously.
More than four years after Covid-19 first struck, air quality remains a topic of major consideration, particularly once fire season starts. Is your air dirty? Should you try to clean it? If so, how?
Air purification is a confusing topic that merits a deep dive. Besides walking you through the basics, we’ll also tick off the six most common–and most dangerous–airborne pollutants, from smog and soot to carbon monoxide to lead.
The question of which pollutants are the most worrisome depends on who you ask, how healthy you are, and what you’re trying to protect.
When you’re done, check out our top air purifiers buying guide. An air purifier won’t filter out all the worrisome pollutants in the air, but it can help scrub the more troublesome bits of particulate matter and volatile organic compounds (or VOGs) that can build up in your home.
What are the types of pollutants you need to worry about?
Jeez, a lot.
The EPA breaks down outdoor pollutants into two main types: criteria (or common) air pollutants and hazardous air pollutants (aka toxics).
This feature is part of TechHive’s in-depth coverage of the best smart air purifiers.
First, we’ll focus on criteria air pollutants–the more common variety. Later, we’ll take a look at the scarier pollutants in the “hazardous” category.
The six types of common–or “criteria”–air pollutants
Criteria air pollutants are broken down into six types of compounds, and each has an EPA-set National Ambient Air Quality Standard (NAAQS) associated with it.
“Together with state, Tribal, and local air agencies, we work to attain the standards throughout the country,” says EPA spokesperson Remmington Belford. “Criteria air pollutants are ubiquitous and can harm health, the environment, and properties.”
The six criteria pollutants are:
1. Ozone
In the Earth’s stratosphere, the ozone layer protects us from the sun’s deadly ultraviolet radiation. That’s the good kind of ozone.
Then there’s ground-level ozone, better known as “smog.” A product of industrial and vehicular pollution, ground-level ozone is considered one of the most dangerous types of airborne pollutants.
2. Particulate matter
Also known as “PM,” particulate matter is a mixture of airborne particles of various shapes, sizes, and composition. Examples of particulate matter include dust, dirt, pollen, mold, and soot.
We’ll delve more deeply into the specific types and sizes of particulate matter in a moment.
3. Carbon monoxide
An invisible and odorless gas, carbon monoxide is a byproduct of such common burning fuels as coal, charcoal, kerosene, natural gas, and propane.
Tricky to detect, carbon monoxide can build up in poorly ventilated indoor spaces, and it’s blamed for roughly 200 deaths a year in the US.
4. Lead
An insidious and deadly pollutant that’s particularly harmful to infants and children, airborne lead can come from a variety of sources, including incinerators, smelters, battery manufacturers, and the combustion of coal and leaded gas.
5. Sulfur dioxide
A product of the fuels burned in power plants and other industrial venues, sulfur dioxide can mix with other material in the air to make particulate matter (the aforementioned “PM”), causing a variety of immediate and longer-term health effects.
6. Nitrogen dioxide
Yet another component of smog, nitrogen dioxide is a dangerous gas that gives urban haze that distinctive reddish-orange hue, and it’s also a key ingredient in acid rain.
Needless to say, breathing in too much nitrogen dioxide can lead to a host of respiratory problems.
How much of these pollutants is hazardous to your health?
That’s a complicated question, as the Clean Air Act specifies two types of standards–primary and secondary–for most of them.
Primary standards are designed to protect public health directly, while secondary standards protect the environment at large (such as animals, vegetation, and even buildings).
Primary and secondary standards differ for most of the pollutants. For example, the primary standard for nitrogen dioxide is 100 parts per billion averaged over an hour, while the secondary standard is 53 parts per billion averaged over a year. You can really go down the rabbit hole while delving into the standards. The EPA website has all the details you could ask for.
Five of the criteria pollutants–all but lead–are monitored by the EPA’s widely cited U.S. Air Quality Index (AQI). Importantly, there are separate AQI ratings for each of those pollutants rather than an overarching measurement of air conditions, and each is calculated differently.
The most widely used AQI measurement is a particulate matter measurement of PM2.5 particles (which we’ll elaborate on in a bit), followed by ozone.
AQI is a color-coded index designed to quickly let you know whether air is healthy or not. The EPA revises the AQI every five years to reflect the state of scientific knowledge and health concerns. In fact, on May 6, 2024, the amount of allowable particulate matter for “green” AQI was reduced from 12.0 micrograms per cubic meter to 9.0 micrograms per cubic meter.
The AQI chart looks like this:
Christopher Null/Foundry
If you’re not already familiar with the AirNow website, bookmark it now. It’s an excellent resource to get real-time, hyper-local AQI data.
Also note: It’s important to understand that while many air purifiers include color-coded displays that provide a visual cue of ambient air quality, rarely do they align with the EPA’s formal AQI ratings or even use the same color scheme.
What are the different types of particulate matter pollutants?
Particulate matter is a complex category because it can come from a nearly infinite array of sources and exists in a large number of incarnations. However, it all boils down to one thing: Particulate matter is made of particles (solids and liquids) suspended in the air rather than existing as a gas.
Particulate matter is commonly categorized by the maximum size of the particle in the category; each category is smaller than the last: PM10, PM2.5, PM1, and PM0.1. We’ll look at each in turn. (Note that this is not a canonical list. You can create particulate matter categories like PM40, PM5, or PM0.03 if they are useful, but the above four are the most commonly used.)
PM10
PM10 is the largest generally used category of particulate matter, representing particles with a diameter of 10 micrometers or smaller. These include just about everything: mold, bacteria, dust, smoke, and more.
Visible, windblown soil–dust that is “kicked up by the wind”–is a good way to think about PM10. Airborne asbestos particles often fall in the PM10 category, as well.
Note that PM10 includes all particles of this size or smaller, so particles in the PM2.5 and PM0.1 category are also categorized as PM10 particles.
Larger PM10 particles in the outdoor environment aren’t the biggest concern for air quality researchers as they are often just a nuisance, though they can irritate the eyes, nose, and throat. However, they are large enough to become lodged in the lungs and can exacerbate chronic issues like asthma. As such, the EPA and other jurisdictions do set standards for PM10 safety.
PM2.5
The gold standard for particulate matter measurement is PM2.5, or particles that are smaller than 2.5 micrometers in diameter. Because they are so small, PM2.5 particles can lodge deeply in the lungs and can even enter the bloodstream, impacting a wide range of health conditions.
PM2.5 pollution is a significant problem for asthmatics and anyone with heart or lung disease, but long-term exposure to high levels of PM2.5 pollution are known to be create health risks for anyone.
Most commercial air purifiers–and HEPA filters–are designed to reduce PM2.5 pollution specifically and many have PM2.5 monitors built in, providing a real-time look at PM2.5 conditions in the home. Note that these types of purifiers will also trap larger and smaller particulate matter pollutants, which we’ll discuss later.
Christopher Null/Foundry
PM1
PM1 is a lesser-used particulate matter designation, representing particles under 1 micrometer in diameter. The risks of PM1 pollution are largely the same as those for PM2.5, and in fact most PM2.5 pollution is actually in the PM1 category. One source suggests that up to 86 percent of PM2.5 pollution in China is classifiable as PM1.
Of special note is PM0.3, or the 0.3 micrometer level, which is the “most penetrating particle size” for HEPA filters.
HEPA filters, by specification, can remove 99.97 percent of particles with a size of 0.3 micrometers, the size at which they are the least effective: HEPA filters are more effective at capturing particles that are both larger and smaller than 0.3 micrometers.
This may sound counterintuitive, but while larger particles are likely to be physically intercepted, smaller particles are more easily trapped by HEPA filters through a process called diffusion, which causes ultrafine particles to bounce around as they pass through the filter, eventually forcing them into the filter’s fibers. This article offers a deep dive on the topic of HEPA filter operations.
PM0.1
The smallest commonly used category of particulate matter is PM0.1–particles that are 0.1 micrometers in diameter or smaller. At the PM0.1 level, the impact of a particle on the human body becomes more variable and is often toxic, as these particles can easily pass through the lungs and into the bloodstream, much like a gas. Most viruses also fall into this category.
PM0.1 particles are so small they are difficult to filter through (non-HEPA) physical means, as anyone who lived through the cloth-masks-are-fine era of the Covid-19 pandemic will remember. As such, they’re the subject of a significant amount of scientific research.
What are VOCs?
VOCs are volatile organic compounds, which is a scary-sounding term for gaseous chemical compounds with a high vapor pressure and low water solubility.
A lot of VOCs are chemicals that are very bad for you–by-products and vapors emitted from gasoline, paint thinner, refrigerants, pesticides, and more. VOCs are also given off by new carpet and upholstery, glues, photocopy machines and printers, and more.
Formaldehyde, methane, chloroform, benzene, and acetone are some of the big names in this category, and they can have health effects ranging from headaches to liver damage to cancer. Many VOCs have a strong smell associated with them, but plenty do not, and your nose is not a good indicator of whether you have a VOC problem.
However, note that not all VOCs are awful. Scents and other compounds given off by flowers are VOCs, for example, and many of these are fairly harmless. These are, however, the exception, and unlike most pollutants, VOCs exist in far greater concentrations inside the home versus outside, where they more readily dissipate. Some VOCs can be up to 5 times more prevalent indoors, depending on where you live.
Unlike particulate matter, VOCs can’t be captured by a standard HEPA filter. Instead, an activated carbon filter is the most effective means of capturing VOCs in the home. The tiny pores on the surface of a carbon filter can capture VOC gases through the process of adsorption, binding the VOC to the carbon chemically, effectively removing it from the air permanently.
Hazardous air pollutants
While criteria air pollutants sound bad, hazardous air pollutants represent the really nasty stuff.
“Hazardous air pollutants come from a variety of sources and have a wide range of health effects and environmental impacts,” says Belford. And none of those are good.
There are 188 hazardous air pollutants that the EPA regulates, including benzene, mercury, dioxin, asbestos, chromium, and hexachlorocyclopentadiene.
These are the chemicals that turn into class action lawsuits that Hollywood makes movies about–and there’s no AQI rating for any of it. Basically, you don’t want to be around them in any amount.
Which of these pollutants is most dangerous? Where do pollutants come from?
The question of which pollutants are the most worrisome depends on who you ask, how healthy you are, and what you’re trying to protect.
Children under 6 are especially vulnerable to lead. Sulphur dioxide is (as previously mentioned) the primary component of acid rain, which can destroy the environment. All pollutants carry health risks, but to different degrees.
Particulate matter is the most ubiquitous of pollutants, as these compounds are produced by all types of natural and industrial processes, including fires, combustion engines, smokestacks, and more. Pollen generated by plants, volcano ash, and dust kicked up by driving on an unpaved road are all particulate matter generators.
Inside the home, things get more complex. Pollutants come from appliances and cooking activities, heating and cooling systems, tobacco use, and building materials such as new carpet or insulation (particularly old insulation with asbestos in it). Pollutants from outdoors can migrate inside via open doors, windows, and poor insulation.
Ironically, cleaning products can even generate indoor pollutants. Pet dander and dust brought in from outdoors can contribute to particulate matter, but there’s one cause of particulate pollutants that is much more difficult to abate: One source suggests that up to 90 percent of indoor particulate matter is composed of human skin cells.
How should considerations around indoor and outdoor air quality differ?
“Generally, individuals can exert more direct control over their indoor air quality than over outdoor air,” says Belford. “Outdoor air, on the other hand, is subject to more regulations and quality monitoring. For outdoor air, keeping informed about air quality can help minimize exposures to high levels of pollutants, such as by reducing time spent in outdoor activities.”
Indoors, you can opt to close windows when outdoor air quality is poor, increase ventilation, or run an air purifier to clean pollutants out of the air. In the long term, the most effective way to impact indoor air quality is to find the source of any pollutants and remove or abate it.
Should you get an air purifier? What kind?
If you have determined (through a measurement device or simply out of an abundance of caution) that your indoor air quality is poor, then an air purifier probably makes sense.
If for no other reason, an air purifier can be a huge help if someone has a mishap in the kitchen and burns something; a good air purifier can remove foul smells in a matter of minutes.
But as you’ll find in our guide to the best air purifiers, not all air purification devices are the same, and it’s important to understand the differences among them, as well as the technologies they employ.
HEPA and carbon filters
At a bare minimum, any quality household air purifier should have both a HEPA filter and a carbon filter. Note that many purifiers have a combined filtration unit that sandwiches a HEPA filter and a carbon filter together. When the filter reaches end of life, both units are replaced at once.
This combination of HEPA and carbon filter is the most effective way to clean your air of all common hazards.
Ionizers
Ionizers are another purification technology, and they’re quite controversial. Ionizers electrically charge particles in the room so that those particles stick to walls, the floor, and everything else, effectively removing them from the air (but, technically, not the room). Ionizers only work with particulate matter, though, and are less effective the larger a particle gets. As such, many independent tests claim that ionizers aren’t useful for most environments.
Another problem with many ionizers is that they create ozone, a known pollutant that users are actively generating and introducing into the home. The EPA and other sources note that some ionizers generate so much ozone that it’s thought to be harmful to human health.
Ultraviolet light
One final purification system you’ll occasionally find uses ultraviolet light. UV-C light is often an add-on feature to the above systems and is used not to clean the air but rather to destroy bacteria and viruses as they pass through the filter. Studies have found that UV-C is an effective way to kill germs such as Covid-19 and staph, though those tests have all been in controlled lab settings using large-scale industrial equipment.
A tiny ultraviolet bulb in an air purifier lacks the power of a six-figure hospital sanitizer, severely limiting its impact (studies have said as much), but unlike an ionizer, having a UV feature on your purifier certainly can’t hurt.
Note: Please bear in mind that none of the information in this story should be construed as advice of a medical nature. If you have questions about air quality and its impact on your health, please consult a physician.
