Air Quality: Other Pollutants Beyond Particulate Matter (PM)

Understanding the dangers of air pollution can be a conundrum. Not only do we have to wrap our heads around the notion that the air we breathe may  be filled with hazardous particles, we must also assess how we can effectively protect ourselves and our loved ones from these pollutants. Unfortunately, the current industry standard method of measuring the performance of an air purifier only takes into account the filtration of particulate matter (PM). While this is certainly a crucial component of how effectively an air purifier can help improve the air quality in your living or work space, it is not the only way to gauge its overall performance. To help you better understand this complex topic, we discuss how there is more to indoor air pollution than just particulate matter.

Indoor air: A brief snapshot

When it comes to the air you breathe inside your home, there is a spectrum of pollutants that can impact air quality. Take, for example, particulate matter (or PM, for short), the tiny, free-floating dust particles that become visible when sunbeams shine through a window in your home. Only the largest of PM is visible in sunlight, however, and most cannot be seen with the naked eye. Inhaling this invisible PM is associated with several different serious health problems, which is why it is a very well-researched and often-discussed pollutant.

Bear in mind, however, that it is not the only pollutant in the air, and importantly, it is not the only pollutant that can affect your health. There are also infectious microorganisms, airborne toxins released from everyday household furnishings and building materials, mold spores that float through the air, and many other contaminants that we need to consider if we want our air to be clean.

The current approach to air quality testing

The oldest and most well-developed method of testing air quality is to measure the amount of PM floating in the air. This type of technology comes in many different forms but the simplest and often the most accurate are devices that count the size and number of individual particles. This is useful for measuring the approximate levels of particulate air pollution emitted from a source like a freeway, power plant or cooking stove. Smaller particles are thought to be more unhealthy since larger particles are more easily caught and expelled by the lungs, but regardless of their size, these particles are usually emitted from the same source. As a result, this approach is useful for getting a rough gauge of particulate air pollution.

When testing the efficacy of air purifiers, the current industry standard is to measure the filtration efficiency of three different particle size ranges roughly corresponding to the average smoke, dust and pollen particles.

What is missing in the current testing approach

The current testing approach is useful, but it has major drawbacks. For one, it does not take into account the toxicity or physical nature of the measured particles, which can have varying effects on your body. A single particle of smoke, for example, is not healthy to inhale but nowhere near as harmful as inhaling a viable virus of the same size, which can cause a long-lasting infection. In this situation, particle counters would be unable to tell the difference. For those who suffer from allergies, inhaling small particles of sand would probably not cause an allergic reaction, but inhaling similar sized particles of pollen would be more likely to cause an allergic reaction. In addition, different particles can be easier or more difficult to filter. Small droplets of liquid could easily wrap around the fibers of a filter, but a similar sized mold spore that is hard and spiky might bounce off the fibers and not be trapped in the same way.

Most importantly, individual molecules of gaseous substances cannot be detected by a particle counter, they are just too small to trigger the detector. So while the particle-counting approach certainly tells us something about the quality of the air, it is definitely not the whole story.

The composition of indoor air: an emerging science

The study of the composition of air is an emerging science, and is rapidly developing as measuring instrumentation becomes simpler for scientists to use, and cheaper for consumers to buy.

Both the normal and pollution components of the atmosphere represent a very complex mixture of matter. Traditionally the easiest way of divvying up the different parts of the atmosphere to understand them better has been to consider gases and PM separately and to develop techniques for measuring each one.

Typically, particles are defined by their approximate size, measured in microns, or one ten-thousandth of a centimeter. Gases, on the other hand, are typically given names that correspond to their chemical properties such as ozone, nitrogen dioxide, and formaldehyde, to name a few. As the research progresses, the chemical nature of particles and the physical nature of gases is becoming more important.

To complicate the matter, until recently it was assumed that indoor and outdoor air quality were roughly equal. However, the EPA has recently been saying that indoor air quality is worse than outdoor air quality. There are sources of PM and gaseous pollutants in the home that can build up over time if ventilation is inadequate.

Particulate matter: an important aspect of air quality

PM is an unhealthy component of air pollution and has been linked to a wide array of diseases that include not just cancer, but also neurological diseases like dementia and multiple sclerosis, and diseases of the internal organs like diabetes.

Early research was most easily done on PM composed of particles 10 microns or less in size, also called PM10. The emission of PM10 from most sources is correlated with the emission of other particle sizes, so it was a handy proxy for measuring the presence of particulate pollution in the air. However it has since been  discovered that PM representing particles with smaller widths such as PM2.5 (2.5 microns or less in size) and PM1.0 (1 micron or less in size) are more dangerous because they more easily bypass the lungs’ defenses and can make it into the blood and therefore into internal organs.

Source: Encyclopedia of the Environment

This is in addition to the fact that larger particles settle to the ground more quickly and smaller particles are more likely to remain suspended where they can be inhaled. So while they are usually emitted from the same source, their concentrations in the environment may be different.

Source: EPA

As you can see in this graphic, PM2.5 is much smaller than a human hair, which is about 50-70 microns in diameter. Most naturally occurring particles, such as dust, pollen and mold spores, are larger and closer to 10 microns. Like most forms of air pollution, dangerous concentrations of PM2.5 in the air is a relatively new phenomenon and has really only been a problem since the industrial revolution about 150 years ago. It primarily comes from the combustion of fossil fuels like petrochemicals and coal outdoors, but also can be formed from the machining of wood and metal or from welding fumes.

In the home, the most common source of PM is cooking, but there are many other sources, such as burning candles or just kicking up dust while walking around. PM can also enter the home from outdoors when you live closer to traffic such as busy roadways and highways along with severe air quality events such as  wildfire.

What is missing from a traditional picture of air quality: Inorganic gases, VOCs and bioaerosols

While unhealthy, PM is certainly not the only air pollutant that results from modern industry, nor is air pollution even the only thing floating in the air that could make you sick. Moreover, air quality measurements have traditionally been focused on outdoor air pollution from industrial and vehicular sources, but indoor air might have even higher concentrations of pollutants. While outdoor air pollutants are subject to being blown away by the wind, destroyed by the rays of the sun or otherwise mitigated by the natural environment, indoor pollutants can simply build up with no place to disperse.

As we mentioned above, indoor air pollutants are becoming more complex and less defined. To show you how complicated things can be, we separated out some of the various components of indoor air pollutants in a modern society with this graph below. You will see how often various organic pollutants can intersect with particulate matter and vice versa.

VOCs: Found in the modern home

A group of air pollutants that become gases, but should be considered separate from inorganic gases, are volatile organic compounds, or VOCs. These substances are typically the byproducts of industrial manufacturing or components of cleaning chemicals and are often found in the home. They are sometimes used as solvents for products like paints, adhesives, and lacquers and sometimes used as the actual building blocks for plastic components. There are numerous VOCs found in a typical home, and it can become exhaustive to cover, but common substances include formaldehyde, benzene, and methylene chloride.

Different VOCs have different degrees of toxicity for different people. Acetone, a common solvent for nail polish, is not known to be particularly toxic. Formaldehyde, however, which is a common off-gas from building materials, furniture, carpets, and mattresses, is considered to be toxic and carcinogenic. Benzene is considered to be cancer-causing by the Department of Health and Human Services and is emitted from cigarettes, vehicle exhaust, and many other sources.

Not all VOCs are inherently dangerous. Almost all smells emitted by animals, plants, and other organic life are technically considered VOCs floating in the air. While these VOCs may cause allergic reactions in some people, they are generally non-toxic to everyone else. There are exceptions, for example, one component of the smell emitted by bacteria digesting rotting organic matter is a chemical called putrescine that is poisonous in large doses.

Bioaerosols: Mold, bacteria and more

A second category of pollutants that could be considered particles, but need special consideration, are known as bioaerosols. Bioaerosols are particles released from living things but are not necessarily alive, though some are. Bioaerosols are airborne viruses, bacteria, mold spores, pollen and other fragments of organisms.

  • Viruses are very common in the air, but only a tiny percentage infect humans and make them sick with diseases such as the flu and measles. While airborne transmission of viral diseases can happen, most of the time viruses are suspended in relatively large droplets of liquid that settle to the ground within a few minutes.
  • Bacteria are also fairly common in the air and are also mostly benign. They are hardier than viruses but still are easily killed by lack of water or by direct sunlight. Bacteria such as strep and staph can potentially cause serious infections. In addition, many bacteria have toxins inherent in their structures which can cause allergic reactions and other health issues.
  • Mold spores are reproductive bodies produced by many different fungal species and are well-suited to float through the air. They contain dangerous toxins associated with a host of health effects that are mostly restricted to allergy-like reactions. In some cases, continued exposure to mold spores can sensitize some individuals and create stronger asthmatic reactions over time.
  • Allergens are substances that react with the immune system to initiate an allergic reaction. The most common allergens occur in pet dander (skin flakes), dust mite remnants, and plant pollen. Allergic reactions can also be initiated by a wider array of substances in the air and sometimes simple smells.

Inorganic gases

Toxic gases in the air are primarily a result of industrial activity but also occur naturally. The EPA has been conducting research just in the past few years on how these toxic substances are common components of air pollution, and how they can even build up in indoor air and contribute to health problems. The EPA considers the most problematic inorganic gases to be ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), carbon monoxide (CO), and radon. Each one is a little different:

  • Ozone is made of oxygen atoms like the oxygen we breathe but is toxic when it comes into contact with living things. It is naturally produced by sunlight in the ozone layer high in the atmosphere. The ozone in the ozone layer is not considered air pollution. Down on the ground, however, ozone is most commonly formed from the reaction of VOCs and nitrogen dioxide with sunlight and is a harmful pollutant.
  • Nitrogen dioxide is emitted into the air when burning fuels in vehicles and power plants. It is also a product of wildfire smoke, though this represents a less common source.
  • Sulfur dioxide is also emitted from the burning of fuels. Less common sources include volcanoes and the extraction of metals from ores.
  • Radon is a completely natural but radioactive gas that is emitted from certain types of rock. It can build up in basement spaces where its radioactivity can cause drastic health effects.
  • Carbon monoxide is formed from the burning of anything, including fuel in cars but also any type of wood or gas for cooking.

The tiny individual molecules of these gases are toxic to inhale and require chemically-based solutions to remove them from the air. Carbon filters can remove all of these substances except for radon, but may re-release them at a later time if heat or humidity conditions change.

Know what is in your air and how to deal with it

Indoor air contains a spectrum of pollutants that can impact the air you breathe. Regardless of the complexity of indoor air, the current approach to testing air quality only revolves around one kind of pollutant: PM. And whether the particles come from the outdoors or are generated inside through everyday activities such as cooking or burning a candle, the levels of PM in a modern home is certainly a significant concern.

However, this is also the major drawback of the current testing approach: Though particle counting tells us something about air quality, it does not give the full story. What is missing from the traditional picture of air quality are the other harmful pollutants: VOCS, bioaerosols like mold, inorganic gases and allergens. Therefore a better approach is to test for these missing pollutants for a more accurate gauge of what is truly clean air.

It is most important to understand air quality so we know how to keep ourselves safe from what is in the air and to practice good methods of preventing the emission of further pollutants. Researching your local environment can inform exactly what pollutants are present, and paying attention to your health, and asking your doctor if any problems you have may be the result of inhaling airborne pollutants, is a great way to stay healthy. For future generations it is also necessary to keep up-to-date on the latest methods of reducing the air pollution you are responsible for.

If you would like to learn more about indoor air quality, check out our blogs on air pollution and sleep apnea, how to keep kids safe from air pollution and the top ten most polluted cities in the US,

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