The Air Quality Index (AQI) helps provide a general indication of air quality safety and is a measure of common air pollutants like carbon monoxide, sulfur dioxide, nitrogen dioxide, ozone, and small particulate matter (PM2.5 and PM10). However, there are caveats to the AQI score that should be considered. Especially during events like wildfires. So how is AQI measured? And how is AQI calculated?
How is AQI measured?
There are various kinds of air pollutants that are used when determining AQI, and they require different kinds of technologies to measure them accurately.
How is particulate matter (PM2.5 and PM10) measured?
The U.S. Environmental Protection Agency (EPA) primarily uses gravimetric analysis and beta attenuation methods for its official air quality monitors to measure particulate matter (PM2.5 and PM10). These methods are considered Federal Reference Methods (FRMs) or Federal Equivalent Methods (FEMs) for regulatory purposes due to their high accuracy, reliability, and compliance with stringent standards.

Gravimetric analysis is considered the “gold standard” for PM measurement. Here, air is drawn through a size-selective inlet to collect particles on a pre-weighed filter. After sampling, the filter is conditioned and weighed again to determine the particle mass.
Gravimetric analysis is highly accurate and serves as the benchmark for other methods, providing direct measurement of mass concentration (µg/m³). However, it typically provides 24-hour averages rather than real-time data, and is labor-intensive and time-consuming.
Beta attenuation and Tapered Element Oscillating Microbalance (TEOM) are used as equivalent methods due to their automation and ability to provide continuous data. The beta attenuation technique measures the attenuation of beta particles as they pass through particulate matter collected on a filter, whereby the reduction in beta radiation correlates with particle mass. This technique can be automated and capable of near-real-time monitoring (e.g., hourly or daily data). It is a reliable and widely used monitoring method, but must consider environmental factors like humidity to ensure accurate measurements.
TEOM also can continuously measures particle mass by monitoring the change in frequency of a vibrating element as particles accumulate for real-time data (hourly or less) and tracking rapid changes in PM levels. This method, however, also needs to consider environmental factors and requires temperature control to prevent errors due to moisture.
These methods meet the stringent requirements for air quality data used in regulatory decisions, such as compliance with the National Ambient Air Quality Standards (NAAQS). However, newer technologies like optical particle counters (OPCs) and low-cost sensors using optical light scattering are being explored for supplemental monitoring.
For example, in optical light scattering sensors, airborne particles are illuminated by a light source (usually a laser or LED), and scattered light is measured. The intensity and angle of scattered light are analyzed to estimate particle size and concentration.
How are gaseous pollutants measured?
Particulate matter (PM2.5 and PM10), described above, measure small particles that can enter your respiratory system, and potentially enter your bloodstream, posing harm to cardiorespiratory health.
But small particles are not the only airborne pollutants that can be harmful to health. Specific gases, often emitted from human sources, are also harmful and are a part of AQI measurements.
Most of these gases are measured using various light-based spectroscopy methods that are capable of specifying whichgas is measured and quantifying how much(concentration) is in the air.

Carbon Monoxide (CO) is usually measured using Non-Dispersive Infrared (NDIR) Spectroscopy. Here, CO absorbs infrared (IR) light at specific wavelengths. An NDIR sensor measures the reduction in IR light intensity as it passes through the air sample. This can be highly specific to CO and provides real-time, continuous measurements.
Sulfur Dioxide (SO₂) is typically measured using Ultraviolet Fluorescence (UVF). SO₂ absorbs ultraviolet (UV) light and re-emits it as fluorescence. The intensity of the fluorescence is proportional to the SO₂ concentration. Again, this technique has high sensitivity and specificity, differentiating SO₂ from other gases. It also can provide real-time measurements.
Nitrogen Dioxide (NO₂) is often measured via chemiluminescence. Nitric oxide (NO) reacts with ozone (O₃) to produce excited nitrogen dioxide (NO₂*), which emits light (chemiluminescence). The intensity of the emitted light is measured to determine NO concentration. To measure NO₂, it is first converted to NO using a heated catalyst. This technique is highly accurate and widely used in regulatory monitoring.
Ozone (O₃) is measured using UV Absorption Spectroscopy. Ozone absorbs UV light at specific wavelengths. A UV absorption monitor measures the reduction in UV intensity to determine the ozone concentration, rendering this technique very specific and accurate for real-time monitoring of ozone.
What don’tair quality monitors measure?
Air quality monitors broadly deployed are not able to specifically identify the rare, but very harmful, air pollutants released from certain industrial processes or, in the cases of urban or wildfires, burning buildings and other man-made structures.
For example, lead, arsenic, asbestos, and dioxins have been detected where wildfires have burned through neighborhoods. As man-made structures like homes, and the plastics, paints, and other materials inside of the burn, these harmful chemicals get released. The AQI values do notreflect these pollutants.
Common methods of detecting PM2.5 values cannot differentiate between particle chemistry or particle shape. Therefore, asbestos fibers or lead bound to dust or ash will not be uniquely identified and does not get reflected in the AQI value.
Similarly, particles like ash can be too large (over 10 microns) in size and may not be detected by existing monitors, yet still are breathable.
Although permissible limits exist for these pollutants, there is no safe level of exposure, and a low AQI number may not accurately reflect the current levels of these kinds of pollutants and the current harm they may pose to health.
Pollutants like lead, arsenic, asbestos, and dioxins are not typically detected in most places away from industrial processes. In normal conditions, EPA reported values can provide a relatively good assessment of air quality based on typical harmful pollutants that may be present.
But in unique situations like wildfires, the most important places to find accurate information are from local incident response teams performing additional testing of air and water systems with technologies that can detect and quantify these abnormal pollutants.
How accurate is AQI in my neighborhood?
EPA sensors are also not located in all neighborhoods. In fact, around 2,000 out of the roughly 3,000 counties in the United States don’t have EPA air quality monitors.

AQI numbers may be more accurate in places with exposure to similar atmospheric conditions. The air pollutants measured far away from your location may still be relatively accurate if there is not a large variation in atmospheric conditions, like wind, between you and the sensor.
But in cases with point sources of pollution (like wildfires, or specifically, burning buildings) or complex wind patterns like those near mountains and canyons, air quality monitors deployed miles away may not accurately represent the air quality at your home or precise location.
How is AQI calculated?
AQI does not provide an aggregate, average value of the listed pollutants. Each measured pollutant is scaled according to its concentration and negative health impacts as a “sub-index.” The AQI is the highest sub-index of the listed pollutants that are regularly measured.
For example, if PM2.5 has an AQI sub-index of 150 and ozone has an AQI sub-index of 100, the overall AQI is reported as 150.
This is important to understand in cases like wildfires that release pollutants not normally in the air at significant concentrations. If no single pollutant is particularly high, but all other measured pollutants are still relatively high, you may still have a seemingly low AQI value but there might still be a combination of large concentrations of a variety of harmful air pollutants.
For example, wildfires may release large quantities of volatile organic compounds (VOCs), which can mix with nitrogen dioxide in cities derived largely from car exhaust that can form ozone in the presence of sunlight. Thus, only the highest pollutant will generate the reported AQI value, even though relatively high levels of PM2.5, ozone, and nitrogen dioxide are all present in this case.
And as mentioned earlier, a variety of airborne pollutants are not measured by most air quality monitors and thus will never impact AQI readings.
Staying Safe
Despite its shortcomings, paying attention to AQI measurements is still very valuable. The readings provide insight into the most common, airborne pollutants and should dictate your exposure, especially if you have asthma or cardiorespiratory concerns.
During acute wildfire events, leaving the area, if possible, is the best way to avoid harmful air quality conditions. These will gradually dissipate over time.
For those who cannot leave the area, air purifiers with HEPA filters and activated carbon can help capture harmful pollutants like PM2.5, and more specifically, asbestos and much of the dust and ash-bound lead or arsenic. N95 masks can also help filter some of these pollutants, as well.
It is more difficult for those who cannot wear masks, like infants and pets. In addition to air purifiers, wet-cleaning dust to avoid disturbing it into the air can help gradually clean potential indoor pollutants. However, if the dust or ash is thick, professional cleaning services who have the personal protective equipment (PPE) and experience cleaning hazardous materials may be warranted.
Outdoor activities should be limited during acute events like wildfires, but also generally high AQI days, as running with poor air quality can exacerbate cardiorespiratory health.
You can explore becoming a citizen scientist, measuring your own local air quality conditions to contribute to broader environmental science. Learn more about Canary Upcycled!