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Clean air, year round.

Air quality has never been more important. A recent Harvard study estimates that 8 million people died globally in 2018 as a result of fossil fuel pollution – almost 1 in 5 deaths. The EPA, the WHO, and many other organizations and individuals have been working for years to find ways to decrease the impact of air pollution. One way that scientists are trying to help is to alert the public when air quality might be bad outdoors.

Conveying poor air quality conditions is not easy, because air quality is very complex. Reporting the concentrations of specific chemicals in the air does not give the public a clear picture of the air quality. Very few people know what levels of chemical concentrations are harmful. To solve this issue the EPA came up with the Air Quality Index (AQI). This composite score is used to indicate the concentration of pollution and its impact it has on human health.

The Air Quality Index was designed to let you know at a glance if the levels of outdoor pollution levels are high enough for the EPA to recommend reducing physical activity or perhaps remaining indoors. The AQI includes lower levels for more vulnerable people like children, the elderly, or people with compromised health. It also has more extreme levels that recommend everyone stay inside. Let’s take a look at how it works.

The EPA Criteria Pollutants

In America, the Environmental Protection Agency is the most prominent organization that provides information about air quality to the public. The Clean Air Act requires the EPA to set standards known as the National Ambient Air Quality Standards (NAAQS). These standards have two primary goals. The first is to protect the health of vulnerable people who may be acutely affected by air pollution. The other is to protect plant life, animals, buildings, and vistas. In order to reach these goals, the EPA monitors six airborne contaminants, known as the “criteria air pollutants.”

Particulate Matter (PM)

Particles small and light enough to remain suspended in the air for extended periods of time are called particulate matter (PM). Particulate matter is classified into coarse particles smaller than 10 microns (PM10) and fine particles smaller than 2.5 microns (PM2.5). When inhaled, PM2.5 is small enough to penetrate deep into the lungs and possibly enter the blood where it could be distributed throughout the body.

PM can be made up of metals, sulfates, nitrates, organic compounds, carbon, minerals, biological material and a few other substances. It is formed directly from fossil and other organic fuel combustion and indirectly by reactions among other pollutants in the atmosphere.

PM damages living cells and has a huge impact on respiratory health. Due to its ability to diffuse into the blood, it is also linked to many other diseases such as cancer, dementia, multiple sclerosis, diabetesblindness, and even depression. Different particles have different toxicities depending on their source, with the most toxic coming from automobile exhaust and the least toxic from road dust.

Despite any variance due to source, PM is often thought of as a convenient proxy indicator of air pollution since it is often produced along with other air pollutants from fossil fuel use. Its links to disease have been very well-studied, and it affects more people than any other pollutant. As a result many air standards around the world are based solely or primarily on PM.


This substance is a high-energy form of oxygen that is strongly oxidizing, reactive, and relatively stable. It is part of natural processes that occur high up in the stratosphere, but can also be formed at ground level by interacting with artificial sources of air pollution like organic compounds and nitrogen oxides.

Ozone reacts with other substances in the air to produce particles known as secondary organic aerosols. Ozone and PM are chemically coupled, meaning that one produces the other. However, which one is being produced varies depending on the amount of sunlight, the weather, and the other constituents of the air. Ozone itself can cause respiratory problems, and any secondary particles it produces may cause the issues mentioned above.

Nitrogen dioxide (NO2)

Nitrogen dioxide is formed from the combustion of fossil fuels that contain nitrogen. It can cause irritation of the airways if inhaled, and plays a role in the series of chemical reactions that create smog, ozone, PM, and other more harmful classes of pollution. As such it is a good indicator of overall air quality, and is included in the AQI.

Carbon monoxide (CO)

Carbon monoxide is a strong oxidizer, which, like ozone, is stable in the atmosphere for weeks or months. It can prevent the body from absorbing oxygen which can be fatal in high indoor concentrations. Outdoors, people who already have trouble transporting oxygen due to heart problems may be more affected by CO.

Though still considered as part of the AQI, levels of CO are generally eclipsed by the presence of PM since CO emissions have decreased considerably but PM remains a concern.

Sulfur dioxide (SO2)

Like NO2, sulfur dioxide can also cause respiratory damage but doesn’t take part in the same degree of nefarious atmospheric chemistry, though it may contribute to particulate matter formation. SO2 has been reduced nationally by 92% since 1980, and spikes have only ever been detected in Hawaii near the volcano, so it is not often used in the AQI, but is still monitored and regulated.


This criteria pollutant is a success story from a time when airborne lead exposure was more of a concern. The EPA’s efforts phased out leaded gasoline in 1996, and along with other efforts decreased lead air pollution 98% between 1980 and 2014. While standards must still be set and followed, the EPA does not include lead in its AQI.

Air Toxics

While not part of the original criteria pollutants, the EPA has started to monitor “air toxics,” which are toxic substances in the air of widely varying chemical constituencies. They can be loosely grouped into heavy metal particulate matter like arsenic, chromium, and nickel, and volatile organic compounds like benzene, naphthalene, or formaldehyde.

Air toxics have never directly influenced the outdoor AQI and are not measured continuously. However, they may be more relevant when measuring indoor air quality.

Not every pollutant pulls its toxic weight

While all six pollutants are monitored, they don’t have equal influence on policy at this time. Since they were first codified by the Clean Air Act almost fifty years ago, our understanding of the impact of different pollutants and efforts to stop pollution have changed the EPA’s approach., the EPA’s AQI website, only offers action plans for particulate matter and ozone pollution, and primarily reports on these two pollutants. In their 2020 report, they show that PM and ozone are declining at the slowest rate, and offer special reports on these two criteria pollutants but not the others.

Measurement of the reactive oxides (carbon monoxide, sulfur dioxide, and nitrogen oxides) still has an impact on AQI but efforts to curb their sources have caused drops in the years since the Clean Air Act. Lead air pollution has fallen far enough that it is not included in the EPA AQI at all.

The AQI is based on recommended exposure levels

Most of us do not have a reaction to seeing figures like “250 micrograms of fine particles per cubic meter of air” or “ozone levels are 150 parts per billion.” We rely on organizations like the EPA to do the science and tell us what concentration of particles or ozone is linked to health problems.

The EPA, the WHO, the Chinese government, and many, many other organizations around the world recommend safe exposure levels for specific pollutants over a certain amount of time. Anything above these levels raises the risk of the health problems associated with the pollutant. The EPA recommends taking action if exposed to a concentration of more than 35 micrograms of PM2.5 per cubic meter over a 24-hour period, for example. The action they recommend isn’t drastic, just that people sensitive to air pollution reduce their physical activity levels. At around 150 micrograms, however, they recommend everyone reduce their physical activity. Since it’s a little difficult to remember these numbers, and there are different values for other pollutants, the EPA developed the Air Quality Index.

The actual equation to convert the concentration into a single number is complex, and the EPA adds a color-coded scale, as well. The colors are meant to inform at a glance what to do about the air quality:

Coloring chart indicating varying degrees of air quality

The AQI shown on the AirNow website and reported by any organization that relies on the EPA’s AQI always reports the worst of all pollutants. If ozone is moderately high but PM2.5 is very high, then the AQI will show the AQI for PM2.5.

Organizations around the world may vary considerably in what numbers they use to convey air quality, but more or less all use yellow for caution, orange for levels that have exceeded guidelines, and red for extreme events. The color indicating good air quality is usually green or blue.

Different organizations may also put emphasis on different pollutants when reporting the AQI. Canada uses PM, O3, and NO2 only for its AQHI, based on a 2008 study.

Beyond the AQI

Since we’re concerned with indoor air here at Molekule, we think about more than just the six criteria air pollutants. The sources of air toxics like VOCs can be indoors, which means they can concentrate and potentially be a greater concern than outside. We are always doing research to find new ways to detect and remove pollutants like VOCs from indoor air, and how to think about what exposure levels could be problematic.

Also, there is more research coming out about how air pollution may still have an impact at levels below the recommended exposure. This could lead to more stringent color coding of the AQI and definitely lets us all know that any air pollution is bad air pollution.

We will be announcing any new research along these lines on this blog, on Facebook, and Instagram, thanks for reading.

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