Air pollution affects everyone. Increasingly, people are using low-cost air quality sensors to measure this microscopic phenomenon. Can you trust the readings? The answer holds real-life implications, especially when it comes to your health.
Consumer-grade sensors are not yet as reliable as laboratory-grade sensors, and more research is required for them to become more reliable and accurate. But they can still offer value. This article explores the benefits and challenges of sensors and what you can learn from them today.
What are consumer air quality sensors?
Consumer-grade air quality sensors are relatively inexpensive. Some can fit in the palm of your hand and many provide real-time data. Very inexpensive air quality monitors like a smoke or carbon monoxide detector simply alert you when there is a problem. Compared to sophisticated laboratory equipment and federal air monitoring stations, consumer sensors and the information they produce can be more accessible to the general public, and they can also cover more locations. While consumer sensor technology standards are evolving, more effort is needed to interpret the results.
What kinds of pollutants are measured by air quality sensors?
Regardless of whether you are measuring inside or outside, there are two major categories of pollution that consumer sensors can monitor: 1) particulate matter, and/or 2) gases. While there is a much wider range and components of airborne pollutants (mold, bacteria and more), each with different health implications, these are the two categories that most consumer sensors currently can monitor.
Particulate matter (PM)
Particulate matter (PM)is a mixture of solid particles and liquid droplets. Some you can see with the naked eye (e.g., visible dust) while others are so small you would need a microscope to see them. Two primary PM size fractions of concern are PM10 (PM with a size up to 10 micrometers) and PM2.5 (size up to 2.5 micrometers).
For some context, the average human hair is 70 micrometers in diameter—about 30 times the largest diameter of PM2.5. Size is important. PM2.5, for example, is small enough to be inhaled deeply into your lungs where it can even enter the bloodstream.
Many gaseous pollutants can be found both outside and inside. When fuel is burned, whether in the town factory, in your gas stove or furnace or in your car, gases like carbon monoxide (CO) and nitrogen dioxide (NO2) will form. You may run into these types of gases in your home, too.
In addition to CO and NO2, some portable sensors attempt to measure volatile organic compounds (VOCs), which are airborne chemicals. You may encounter VOCs from the off-gassing of household products or when using cleaning sprays, for example.
What standards exist for air pollution?
Consumer sensors are not intended to replace expensive, large-scale air quality stations mandated by the federal government. These federal stations monitor and regulate a list of outdoor pollutants like ozone, particulate matter (PM) or nitrogen dioxide. Many consumer sensors are tested and developed in relation to these regulatory-grade monitors.
The mix of indoor air pollution can be very different from outdoors. Levels of individual common indoor air pollutants like mold and pollen cannot be separately reported from the mix of other pollutants by a typical consumer sensor. Today, not as many standards exist for testing indoor air quality with sensors in residential buildings, with the notable exceptions of carbon monoxide and smoke.
How do consumer-grade air quality sensors work?
Consumer sensors give you an estimate of particulate matter (PM) or gaseous pollutants in the air, often based on a measurement of a different thing that is related to the actual pollutant but is much easier to measure. For example, the sensor may measure the amount of light scattered by particulate matter, which typically increases as pollution levels rise. Sometimes, these results are put on a scale of low, medium, and high, or in the case of sine very basic air quality monitors like smoke detectors, on a binary scale (that signals whether smoke is present or not). Fun fact: some of the first low-cost PM sensors were built from smoke detectors!
A laboratory-grade air quality sampler, on the other hand, can give you a direct measurement of pollutants in the air, like how much dust is in 1 cubic meter of air (micrograms/m3).
Challenges of low-cost PM sensors
Many particulate matter (PM) sensors are photometers, which means they estimate the amount of PM in the air based on how much light from a laser or LED is blocked or scattered when that light is sent through the air. There are many factors that can interfere with these processes, such as:
- Different kinds of particulate matter can scatter light differently based on their size, shape and color. For example, darker-colored particles will scatter less light than lighter-colored particles. Calibration against these factors is needed for highly accurate PM assessments.
- Some very (very) small PM can be missed by the light passed through the air.
- Some PM is so large that it might not make it into the sensor chamber.
- Dust particles themselves can build up on the device over time and block the sensor. Cleaning sensors with compressed air is one way to help with this.
- The LED light source or the laser can dim over time, incorrectly reducing the sensor’s estimate of blockage or scattering.
- A sensor needs a reasonable amount of air passing in and out of its detection chamber in order to produce reliable estimates of pollution levels as they change over time. If it is in a location where the air flow is blocked or substantially decreased, then it can underestimate the concentration of particles.
- Humidity and temperature can greatly affect the readings. For example, if you bring the sensor near a shower, it will “sense” a lot of particulate matter in the air, but it is actually detecting water vapor!
Challenges of portable gas sensors
Monitors that show the concentrations of gases like CO and NO2 tend to be expensive, ranging from a few hundred to thousands of dollars.
Portable VOC sensors are less expensive but have limitations because of the sophistication required to measure gas concentration. As they are today, consumer-grade VOC sensors are generally not very sensitive. Some limitations include:
- Usually, a portable VOC sensor will not measure one particular VOC. Instead, it will likely measure a total mixture of several VOCs (referred to as TVOCs). If this is the case, the sensor may vary its response to different types of volatile organic compounds. For example, it could magnify the estimate of one compound over another, possibly by a significant factor. The final reading could then be inflated. This becomes very important because some VOCs are more hazardous than others. If you see a high TVOC reading of less-hazardous VOCs, it could be less dangerous than a low TVOC reading of more-hazardous VOCs.
- Contamination from other gases can give you an inaccurate reading. For example, if you are measuring VOCs, the sensor may detect other gases like CO and NO2 without telling you.
- Gas sensors must be calibrated regularly to maintain their reliability. Otherwise, their measurements could “drift” and become less accurate over time.
- Temperature and humidity must be controlled or calibrated for when you use a gas sensor or it may interfere with readings.
What are the benefits of air quality sensors?
Though consumer air quality sensors have their limitations, they also have benefits. They can show a general trend in air pollution and alert you to the kinds of activities that create pollution indoors. They may also show a trend in outdoor air quality in your neighborhood. This is localized data that you may not get from the federal air quality monitors in your general region.
Researchers at Lawrence Berkeley National Laboratory compared consumer sensors with laboratory-grade sensors (Singer & Delp, 2018). They measured particulate matter (PM) from normal activities like burning toast, lighting a candle or cooking green beans. Among seven consumer particle sensors, four devices were off by a factor of about two. The other three consumer sensors were off by well over a factor of two.
Are consumer-grade air quality sensors good enough?
As of now, most consumer air quality sensors have technical limitations that can affect their accuracy and reliability. The readings may not yet be accurate enough to compare against regulatory standards or sensitive enough to reliably portray ultra-low concentrations of ambient pollutants, but they can help guide or inform your personal health decisions. If you are merely looking for hotspots or trends, consumer sensors can sometimes help tell you whether air quality interventions are working over time or which behaviors cause changes in concentrations (like cooking, vacuuming, or opening a window). And while many consumer sensors may not report air pollution concentrations with great accuracy, emerging technologies and calibration techniques show great promise.
Air quality sensors offer the most value when they highlight problem sources, illustrate pollution patterns and inspire solutions. If the devices can help change air quality in your home or community, this is a step in the right direction.
Special thanks to Drew Hill, PhD MPH for his input. Drew is a colleague at SHAIR, a Ramboll-powered startup working to help cities, communities, and businesses do more with their air quality measurements.