Frequently Asked Questions for Reports on Air Sensor Performance Testing Protocols, Metrics and Target Values
EPA developed a series of reports outlining recommended testing protocols, metrics, and target values for evaluating the performance of air sensors. In February 2021, EPA released two reports, one for ozone (O3) air sensors and the other for fine particulate matter (PM2.5) air sensors. In February 2024, two additional supplemental reports were released; one for particulate matter with aerodynamic diameters of 10 microns or less (PM10) sensors to accompany the PM2.5 report, and the other for nitrogen dioxide (NO2), carbon monoxide (CO), and sulfur dioxide (SO2) sensors to accompany the O3 report. The objective of these reports is to provide a consistent approach for evaluating air sensors specifically for non-regulatory supplemental and informational monitoring (NSIM) applications for use in ambient, outdoor, fixed-site environments.
The reports are:
Frequently Asked Questions
Why did EPA develop these reports?
Approaches to evaluate air sensors vary along with the data quality metrics used to describe their performance. Prior to these reports, there were no consistent testing protocols to uniformly evaluate air sensors. Additionally, there were no recommended and testable performance metrics to guide technology improvement. As a result, this can lead to confusion for both sensor manufacturers, sensor developers and consumers.
EPA developed these reports to 1) provide guidance to testers on how to appropriately evaluate air sensor technologies, and 2) help consumers understand the performance of different air sensors and which sensors best suit an application of interest. These reports will provide a consistent set of testing protocols along with metrics and target values to evaluate the performance of O3 and PM2.5 air sensors. Additionally, these reports provide a consistent approach for reporting testing results. Testing results do not constitute certification or endorsement by EPA.
Who is the intended audience for the reports?
The intended audience for the reports includes sensor manufacturers, sensor developers, and testing organizations. Consumers may also wish to conduct testing, but they would need access to either testing equipment/facilities, funding to build such infrastructure or access to technical expertise.
What are the intended applications for the testing protocols?
The testing protocols are intended for non-regulatory supplemental and informational monitoring applications only. Examples of these applications include daily air quality trends, hot-spot detection, emergency response, supplemental monitoring, citizen science activities, long-term air quality trends, epidemiological studies, and model verification.
Does EPA verify and approve air sensor technology for use?
EPA researchers are involved in the evaluation of some air sensors for use by the public and provide the information in reports, but the Agency cannot legally endorse or recommend any sensor. For compliance with the National Ambient Air Quality Standards (NAAQS), monitoring instruments, including sensors, must meet the applicable requirements in the Code of Federal Regulations (CFR) -- Part of Title 40, Protection of Environment or other state environmental regulations (see, e.g., 40 CFR Parts 50, 53, and 58). Technical requirements include detailed sampling, siting, and quality assurance requirements. Data from new air sensor instruments should not be used in a regulatory context at this time unless those instruments meet all applicable regulatory requirements.
Is there a third-party independent organization that reviews air sensors for performance?
No, there are currently no third-party independent organizations that evaluate or review air sensor performance.
Are the testing protocols recommended in the reports required?
No, the testing protocols are entirely voluntary. The protocols are being provided to offer consistency in evaluating the performance of air sensors and reporting testing results.
Will testers receive certification for completing the testing protocols?
No, testers do not receive certification for conducting the testing protocols. The protocols are voluntary and do not constitute certification or endorsement by EPA.
Is EPA providing training on the testing protocols?
No, EPA does not provide training on the testing protocols. If testers have specific questions on the testing protocols, they can use the “Contact Us” link at the bottom of the page to contact EPA.
Are the testing protocols meant for specific air sensors?
The testing protocols can be used for any sensor that measures ozone (O3) or fine particulate matter (PM2.5). These are not specific to any particular sensor technology or sensor device.
Is EPA providing funding to conduct the testing protocols?
No, EPA does not provide funding to conduct the testing protocols. Testers are expected to fund the testing if they choose to follow these testing protocols.
What are the anticipated costs to conduct these testing protocols?
EPA is not able to provide anticipated costs for conducting the testing protocols. Testers may choose to share this information when reporting their testing results. The testing protocols recommend a field evaluation and laboratory evaluation. Please keep in mind that costs will likely vary for the different evaluations.
How will the testing results be shared?
EPA recommends that testers share their results on their respective websites for consumers to view.
Will EPA put together a list of who has conducted the testing protocols?
At this time, EPA does not keep track of who has conducted the testing protocols. We recommend that consumers ask sensor manufacturers/developers if their technologies have gone through any performance evaluations.
How would a consumer verify if a tester claims to meet the recommended target values provided in the reports?
EPA does not keep track of claims or evaluate any claims to meet target values. We are encouraging testers to provide their data and graphs of their results. By doing so, we hope that this will give consumers the ability to trace data from the tests to the results to have some confidence in the results. Additionally, if testing is conducted by many organizations and at different sites, this may also help give confidence in the results.
How do you ensure that a tester has not provided inaccurate information during the testing?
In the testing protocols, EPA strongly encourages testers to be transparent about how they collect and process the data. We encourage testers to provide information about quality control criteria used, reasons for missing or invalidated data, and any maintenance conducted on the equipment. We also encourage testers to report in detail any problems encountered during testing for example inclement weather events, power outages, equipment failures, etc. This information will help consumers better understand the accuracy of the testing results.
How frequently will the information in these reports be updated?
EPA anticipates updating these reports as technology advances and more information is gained from sensor evaluations. At this time, we do not have an exact timeframe of when updates to these reports will be made. However, any updates in the future will be posted to the Air Sensor Toolbox website.
The testing protocols are only for O3 and PM2.5 and air sensors. Why did EPA only choose these pollutants?
The testing protocols focus on O3 and PM2.5 and as there is more widespread use and more technical information and understanding available for sensors measuring these pollutants.
Is EPA planning on developing testing protocols and targets in the future for other air pollutants?
Yes, EPA is developing similar performance testing protocols and target values for air sensors that measure nitrogen dioxide (NO2), carbon monoxide (CO), sulfur dioxide (SO2), and particles with aerodynamic diameters less than 10 microns (PM10). The applications will focus on non-regulatory supplemental and informational monitoring applications. These reports are expected to be released in the future.
Is EPA planning on developing testing protocols in the future for different applications such as mobile monitoring or wearable monitors?
EPA has developed testing protocols for ambient, outdoor, fixed site environments. At this time, we are not developing testing protocols for different applications. We anticipate that the protocols we developed for non-regulatory supplemental and informational monitoring applications can serve as an example from which additional protocols for other use cases can be developed.
Will following these testing protocols help qualify a sensor to be put on the AirNow Fire and Smoke Map?
EPA recognizes that testing results will provide useful information on air sensor performance. However, if and how these results might be used to inform the addition of sensors to the AirNow Fire and Smoke map is being determined.
Specific Testing Procedure Questions
What types of testing are recommended for non-regulatory air sensors?
EPA recommends base testing (field testing) and enhanced testing (laboratory testing).
How many sensors are tested to understand sensor performance?
For both the base and enhanced testing protocols, EPA recommends testing at least three identical sensors at the same time.
How long should the tests be conducted for base testing and enhanced testing?
For base testing (field), we recommend at least a thirty consecutive day testing period. For enhanced testing (laboratory), we recommend collecting 20-30 data points or three consecutive hours of data for each test therein, which will vary depending on the equipment used during testing.
Do the laboratory tests include evaluation of sensors at more extreme weather conditions (for example very cold temperatures or very high humidity)?
The laboratory testing to understand the effects of temperature on sensor performance recommends testing at a moderate temperate (20°C/68°F) and elevated temperature (40°C/104°F). Testing to understand the effects of relative humidity (RH) recommend testing at a moderate RH (40%) and elevated RH (85%). Lower temperatures (< 20°C) and higher humidities (> 85%) are hard to achieve in laboratory testing chambers and can also cause condensation (water vapor turning into liquid water). Water can damage some sensors or impact how they perform. EPA anticipates that some of the field tests may be conducted in colder temperatures or higher humidities, which will help provide information on how sensors perform in those environments.
What reference instrument is recommended to compare sensor testing results to in the testing protocols?
EPA recommends that EPA-designated Federal Reference Method (FRM) and/ or Federal Equivalent Method (FEM) monitors serve as the reference instrument. More information about FRM/FEM monitors.
For field testing, are equipment siting criteria provided?
Yes, the protocols recommend a number of siting criteria including: 1) Mounting sensors within 20 meters horizontal of the reference monitor, 2) mounting sensors so they are exposed to unrestricted airflow, 3) sampling inlet for sensors should be within a height of +/- 1 meter of sampling inlet of reference monitor, 4) identical sensors should be 1 meter apart from each other, and 5) if needed, install sensors in weather proof enclosures that allow ample air flow around sensor. We have included cautions to discourage setting up sensors in areas that can affect air flow to the sensors, alter temperature or humidity levels near the sensor, or alter pollutant concentrations.
For the PM2.5 laboratory testing, what types of test aerosols are recommended?
For the laboratory tests, EPA recommends potential proxies for aerosols including ammonium nitrate, ammonium sulfate, oleic acid or sucrose, and smoke from smoldering wood chips or other biomass material. We are aware that is challenging to recreate ambient particulate matter in a laboratory setting.
Do the testing protocols provide recommendations for maintenance of the sensors?
EPA recommends that testers follow any maintenance instructions provided by the sensor manufacturer or sensor developer.