Recent advances in technology have sparked rapid development of air quality sensor devices for a variety of known air pollutants. The North Carolina Division of Air Quality (DAQ) is supportive of the research and development of air sensors that can provide important insights into local air quality. As more of these devices enter consumer markets, DAQ is committed to engaging with federal regulators, manufacturers, and communities to provide feedback and answer frequently asked questions.
Please note that while many manufacturers are producing sensors meant to assess indoor air quality, DAQ’s regulatory authority is restricted to outdoor air quality, and all information below relates to sensors and monitors meant to assess outdoor air quality only.
Air sensors are devices for assessing air quality that are designed to be more accessible to the public than traditional federally certified regulatory monitors. As compared to the regulatory monitors in DAQ’s network, sensors are typically smaller, lower-cost, less accurate, and usually function based on different technological principles. Often, several different types of sensors will be assembled into a single device so that it can measure more than one pollutant, while regulatory monitors are generally specialized to measure a single type of pollutant. For more information on regulatory monitoring of criteria air pollutants, please visit DAQ’s Air Quality Monitoring page.
The United States Environmental Protection Agency (EPA) has a rigorous testing and certification process for regulatory monitors, with two designations: Federal Reference Method (FRM) and Federal Equivalent Method (FEM). FRM/FEM air quality monitors are highly regulated from site establishment through data collection.
The EPA lays out stringent guidelines for:
- siting regulatory monitors;
- calibration and maintenance according to specific criteria; and
- required equipment and data quality audits.
All of North Carolina’s regulatory monitors are installed, maintained and operated in accordance with EPA requirements, ensuring that policy decisions in our state are informed by high-quality monitoring data.
For air sensors, there are no federal standards on accuracy, calibration, siting, maintenance or data quality assurance/quality control (QA/QC). There is currently no certification program for sensors and the quality of both the hardware and software is highly variable between manufacturers and models. There is no governing body to verify that manufacturers are accurately characterizing the performance of their sensors. Often, software modifies the sensor data to account for different environmental conditions, but those correction algorithms are typically not available for review by customers or researchers. This makes it difficult to assess the true performance of the sensors and the quality of the resulting data.
However, the EPA has developed two reports detailing step-by-step testing protocols, performance metrics, and performance targets for sensors that measure ozone (O3) and particulate matter <2.5 microns (PM2.5). While sensor manufacturers are not required to follow these protocols, sensors that are able to meet these performance targets are more likely to provide users with useful data.
EPA’s Performance Targets and Testing Protocol Reports:
DAQ groups the possible applications for air sensors into three main categories: Recommended, Recommended with Reservations, and Not Recommended Until Technology Improves.
Research: As the sensor field continues to evolve and costs decrease, research into sensor capabilities will become increasingly accessible to community science groups and research scientists. DAQ encourages continued research into the capabilities and limitations of sensors.
Education: DAQ supports and recommends the use of sensors to educate students at all levels about air quality and the process of collecting data. Teachers and mentors should take care to match the type of sensor to their teaching goals and emphasize the importance of quality assurance/quality control (QA/QC) in data collection.
Awareness: Nearly all air sensors can be used to raise awareness about air quality, even if the data results are not robust enough for research or education purposes. A well-placed ambient or wearable sensor can spark discussion with friends and neighbors about air pollutants and how concerned residents can minimize their negative impacts on air quality.
If you are interested in additional ways to raise air quality awareness or engage with students about air quality issues, please visit NC’s Air Awareness page, or reach out to the Air Awareness team at email@example.com.
Recommended with Reservations
Personal Exposure Monitoring: Community members may be interested in using sensors to better characterize air quality around them during their daily activities. For this application, DAQ recommends extensive research into the pollutant of concern and selectivity when purchasing a sensor for personal exposure monitoring. Even if a sensor is not accurate enough to consistently provide numeric exposure values, it may help community members determine if short-term air pollution levels are increasing or decreasing or provide feedback on relative exposure between locations.
Not Recommended until Technology Improves
Source Identification and Characterization: Due to the current limitations of sensor technologies, DAQ cannot recommend use of sensors to identify sources of air pollutants or to characterize pollution from existing sources. Facilities that have air emissions at levels requiring a permit are routinely inspected and evaluated to ensure they are complying with the emissions limits in their permits. For a searchable database of facilities with air permits, please visit DAQ’s Public Records Search. Mecklenburg, Forsyth and Buncombe counties issue their air permits separately from NC DAQ, and citizens interested in permitted facilities in those counties should contact the local air quality programs for more information.
Supplement Existing Monitoring Information: For air quality professionals, one of the most interesting aspects of low-cost air sensor technologies is the potential to supplement data from regulatory monitors. However, due to the current limitations of sensor technologies and the varying levels of data quality from sensors, their data cannot be verified, validated or integrated into DAQ’s regulatory data. This is an active area of research and DAQ will continue to engage with federal regulators, researchers, and manufacturers to explore ways to use sensor data without sacrificing our high standards of data quality.
For any application, DAQ encourages air sensor users to be mindful of the limitations of the sensor technology before drawing conclusions from any sensor data.
With increased access to affordable sensor technology, the number of air sensors on the market keeps growing each year. However, the ability of sensors to provide accurate readings varies wildly by the manufacturer and type of pollutant. Cost is not a reliable indicator of quality, and sensors that measure a variety of pollutants may not measure each pollutant equally well. Some sensors are sensitive to temperature, humidity, or are prone to interference from other chemicals. This makes buying a sensor often frustrating and confusing for people trying to better understand their local air quality.
To help with the process of buying a sensor, the EPA has released a flyer: Six Questions Before Buying an Air Sensor. Additionally, the Air Quality Sensor Performance Evaluation Center (AQ-SPEC) operates as part of the South Coast Air Quality Management District in California. AQ-SPEC researchers place air sensors side-by-side with regulatory monitors under both laboratory and field conditions to evaluate how closely the sensor readings match the monitor readings. If you are interested in a particular sensor, you can go to their website and see if they have an evaluation for it prior to purchasing. Please keep in mind that the sensors are field-tested in southern California, which has distinctly different climatic conditions and pollutant profiles from North Carolina, and that testing results may not be representative of how your sensor will perform.
There are three primary reasons your air sensor may not show the same readings as DAQ’s regulatory monitor:
- Instrument Limitations:
Air sensors often operate using different technological principles from regulatory monitors and have different detection limits, or perform differently under various environmental conditions. The technologies in regulatory monitors have been designed and tested to reduce bias and increase precision in their measurements, with cost as a secondary consideration. Air sensors often prioritize lower cost and user accessibility over producing the most accurate data. Because of these substantial differences, your sensor may not be able to replicate the measurements of regulatory monitors, even when exposed to the same concentrations of a given pollutant.
- Broad-scale vs Local Scale:
Regulatory monitors serve the specific function of looking at the broad air quality of an area and determining if the levels of criteria pollutants are greater than what the EPA recommends to protect human health and welfare. There are specific rules that govern how many air monitoring sites are needed based on the population size of a given area to ensure that the air quality is being measured in such a way that estimates the exposure of the average resident in that area. However, air quality can have hyperlocal variability. Because there are no restrictions on where a resident places an air sensor, it could be located directly next to a pollutant source (like a road or chimney), or in a location protected from ambient pollutants (such as a walled patio or deck). While it may be interesting to see those types of readings, they cannot be directly compared to a properly sited regulatory monitor that may be several miles away.
- Averaging Times:
Criteria pollutants are regulated by the EPA because they are known to increase the risk of adverse health effects. The National Ambient Air Quality Standards (NAAQS) are levels set by the EPA based on current scientific consensus on the pollutant level at which health effects are seen in a representative population, with an appropriate margin of safety. Because research is limited on the effects of extremely short-term exposures, the NAAQS are based on longer averaging times, such as 1 hour (sulfur dioxide), 8 hours (ozone), or 24 hours (PM2.5).
Air sensors often report data averaged over a 1- to 5-minute duration, which can show a great deal of variability compared to a 1-hour or 24-hour average. If you are seeing spikes and troughs in the pollutant level for your sensor, try averaging the data over a 1-hour, 8-hour or 24-hour period to get a better idea of how your sensor is performing.
As new companies and groups utilize air sensors, more information is being shared on various online platforms related to air sensor readings. Some sensor manufacturers compile data from customer sensors and produce real-time maps of those data which are available online. Due to the lack of quality assurance checks of the data prior to being uploaded to the maps, DAQ has a number of concerns about the accuracy of the information these maps provide. Some of these maps are reporting sensor values as “Air Quality Index (AQI)” numbers, even though they are not using the same calculation as the official EPA AQI. (For more information about the AQI, please see the EPA’s guide, “AQI: A Guide to Air Quality and Your Health.”) Some of the sensors do not have GPS modules to broadcast real-time location, and therefore may not be at the location indicated on the map, or may be installed in an indoor location rather than outdoor. For some manufacturers, there is no automated mechanism for removing sensors from the maps once they start showing signs of substantial malfunction. Until sensor manufacturers are better able to provide quality control on their public interfaces, it is DAQ’s position that concerned residents should view these maps with a high level of skepticism.
However, the AirNow program has integrated into its Fire and Smoke Map the use of PurpleAir sensor data for measuring PM2.5. Before being displayed on this map, the sensor data have gone through basic quality control checks and a mathematical equation has been applied to correct for known limitations of the PurpleAir sensor units. For more details on this correction equation and how sensor data is displayed on this map, please visit EPA’s webpage, Technical Approaches for the Sensor Data on the AirNow Fire and Smoke Map. DAQ encourages anyone interested in viewing location-based sensor data to use the Fire and Smoke Map rather than other third-party maps.
For a map with the more accurate measurements provided by DAQ’s regulatory monitors--as well as information on meteorology, air quality forecasts, and past conditions--please visit our new Ambient Information Reporter.
DAQ is continuing to work with local and federal partners in evaluating and testing air sensors. We are excited to announce that we have received a grant from EPA to install an air sensor collocation shelter at our monitoring site in Raleigh, NC. This shelter will provide researchers, individuals, and community groups with a secure location to collocate their sensors with DAQ’s regulatory monitors in order to better evaluate sensor performance. The shelter is tentatively expected to be available for use in Fall 2022.
If you are interested in more information on the collocation shelter, or would like periodic updates on DAQ’s efforts regarding sensors, please submit your contact information below so you can be added to our mailing list.
- US EPA’s Air Sensor Toolbox – A comprehensive collection of EPA’s current sensor information and projects.
- AQ-SPEC – Sensor testing organization operated out of South Coast Air Quality Management District in California
- Sensor Educational Toolkit - produced by South Coast Air Quality Management District in California, this toolkit includes a number of excellent resources, including a guidebook on community air sensor monitoring and training videos
- DAQ Air Sensor Team: AirSensors@ncdenr.gov