Chairman Carper, Ranking Member Capito, and Members of the Committee, good morning. My name is Omar Hammad, and I am an analyst in Environmental Policy for the Congressional Research Services (CRS). On behalf of CRS, I want to thank you for inviting me to testify today. I have been asked by the committee to discuss the state of air quality monitoring sensor technologies, as well as the opportunities and challenges for communities to obtain accurate and reliable information and data about their local air quality.

In serving the U.S. Congress on a nonpartisan and objective basis, CRS does not take positions on legislation and makes no recommendations to policymakers. My testimony draws on my areas of expertise at CRS—the Clean Air Act and air quality monitoring. I work with a team of analysts and attorneys to address related issues for Congress. My CRS colleagues and I remain available to assist the committee in its consideration of air quality monitoring sensor technology issues.

The U.S. Environmental Protection Agency (EPA) defines low-cost air sensors as a class of nonregulatory technology that is lower in cost, portable, and generally easier to operate than the air monitors used for regulatory purposes. Some stakeholders have asserted that EPA, state, and local air agencies should consider the use of low-cost air sensors in their regulatory regimes due to competitive costs, increasingly better technologies, and expanded coverage. Observers noted certain concerns arise regarding such implementation. My testimony aims to introduce and address the elements of this debate. My testimony will discuss ambient air monitors, low-cost air sensors, their uses, and the benefits and challenges of both technologies.

Introduction

Air quality is a term used to describe how much pollution is in the air. Congress recognized the need to address air pollution, establishing the Clean Air Act (CAA) (42 U.S.C. §§7401 et seq.) with the purposes¹

• "to protect and enhance the quality of the Nation's air resources so as to promote the public health and welfare and the productive capacity of its population";
• "to initiate and accelerate a national research and development program to achieve the prevention and control of air pollution";
• "to provide technical and financial assistance to State and local governments in connection with the development and execution of their air pollution prevention and control programs"; and
• "to encourage and assist the development and operation of regional air pollution prevention and control programs."

Air quality management refers to all the activities a regulatory authority may undertake to address the prevention and control of air pollution within its jurisdiction. The process of managing air quality can be illustrated as a dynamic cycle of interrelated elements. The regulatory authority establishes air quality goals, determines the level of emissions reductions needed, develops control strategies, implements programs, and monitors air quality to reevaluate the cycle.²

Criteria Air Pollutants and the National Ambient Air Quality Standards

Under Sections 108 and 109 of the CAA,³ EPA is to issue national ambient (outdoor) air quality standards (NAAQS) for certain listed pollutants (1) whose emissions "may reasonably be anticipated to endanger public health or welfare" and (2) whose presence in ambient air "results from numerous or diverse mobile or stationary sources."⁴ EPA has identified and promulgated NAAQS for six principal pollutants, commonly referred to as criteria pollutants:

• 1. particulate matter (PM),
• 2. ozone (O₃),
• 3. nitrogen dioxide (NO₂),⁵
• 4. sulfur dioxide (SO₂),
• 5. carbon monoxide (CO), and
• 6. lead (Pb).

The CAA directs EPA to establish two types of NAAQS:

• 1. primary standards, "the attainment and maintenance of which in the judgment of the [EPA] Administrator ... are requisite to protect the public health" with "an adequate margin of safety";⁶ and
• 2. secondary standards,⁷ which are necessary to protect public welfare,⁸ a broad term that includes visibility impairment as well as damage to crops and vegetation, and effects on soil and nutrient cycling, water, wildlife, property, and building materials, among other things.

Establishment of NAAQS does not directly limit emissions or compel specific emissions controls; rather, it represents EPA's formal judgment regarding the level of ambient air pollution that protects public health with an adequate margin of safety. In setting the NAAQS, EPA may not consider the costs of implementing the standards.⁹ Promulgation of NAAQS sets in motion a process under which the states and tribes first identify geographic nonattainment areas (i.e., those areas failing to meet the NAAQS) based on monitoring and analysis of relevant air quality data. EPA then establishes nonattainment areas in these locations based on the data and recommendations from states and tribes.¹⁰ States with nonattainment areas then submit State Implementation Plans (SIPs) to EPA. The SIPs identify specific state and federal regulations and emissions control requirements that are to bring areas into compliance, as well as actions for maintaining compliance.¹¹

Air Quality Index

EPA and other agencies have developed tools to measure air quality conditions and alert the public if air pollutants reach a certain level. For example, EPA manages AirNow, a multiagency website that reports air quality based on monitoring data received on a regular basis from state, local, and federal agencies.¹² AirNow contains data in a consistent format and displays it through interactive maps. AirNow reports air quality information using the Air Quality Index (AQI), a nationally uniform index. EPA calculates the AQI for a criteria pollutant based on the ambient concentration of that pollutant.¹³ AQI values range from 0 to 500. The higher the AQI value, the greater the level of air pollution. EPA describes AQI values of 100 or lower as "satisfactory" or "acceptable." AQI values fluctuate throughout the year, due to a number of factors. For example, ozone levels tend to be higher in the summer months for most states, and particulate pollution is typically affected by winter temperature inversions (in a temperature inversion, cold air at the surface is under a layer of warmer air) and the wildfire season.¹⁴

Figure 1 is an example of the data AirNow provides for an area. EPA relates current hourly readings from ambient air monitors to AQI values for ozone and particle pollution.¹⁵ Also, most state and local air quality agencies issue forecasts for ozone and particle pollution. A few areas also issue forecasts for nitrogen dioxide and carbon monoxide.

Figure 1. AirNow Current Air Quality in the Washington, DC, Metropolitan Washington Reporting Area

Source: CRS, using AirNow, https://www.airnow.gov/?city=Washington&state=DC&country=USA. Notes: AirNow real-time data was obtained on April 3, 2024, at approximately 9:30 PM. Air quality at that time was good or "satisfactory," with an AQI value between 0 to 50 for ozone and particulate air pollution.

Ambient Air Monitoring

Ambient air monitoring is "the systematic, long-term assessment of pollutant levels by measuring the quantity and types of certain pollutants in the surrounding, outdoor air."¹⁶ CAA Section 319 directs EPA to promulgate regulations that establish an ambient air monitoring system throughout the United States which¹⁷

• "utilizes uniform air quality monitoring criteria and methodology and measures such air quality according to a uniform air quality index,"
• "provides for air quality monitoring stations in major urban areas and other appropriate areas throughout the United States to provide monitoring such as will supplement (but not duplicate) air quality monitoring carried out by the States required under any applicable implementation plan,"
• "provides for daily analysis and reporting of air quality based upon such uniform air quality index," and
• "provides for recordkeeping with respect to such monitoring data and for periodic analysis and reporting to the general public by the Administrator with respect to air quality based upon such data."

In addition, CAA Section 319 directs EPA to develop requirements and guidance for various aspects of these networks.¹⁸ In accordance with Section 319, the ambient air monitoring system required for NAAQS implementation under CAA Section 110 would "utilize the standard criteria and methodology, and measure air quality according to the standard index, established under such regulations." Most of the ambient air monitoring networks supporting air quality management are designed and operated by tribal, state, and local governments.¹⁹

Criteria Pollutant Ambient Air Monitoring and Monitoring Networks

The national ambient air monitoring system measures air pollution levels at fixed locations across the country.²⁰ EPA, state, and local agencies cooperatively manage this system's infrastructure. Various methods and instruments are available to measure ambient air pollutants. The selection of the appropriate device is generally based on the application.

Per EPA-established regulations, states, tribes, and local air program managers develop five-year network assessments and annual ambient monitoring network plans. The five-year network assessments are used to determine if the ambient monitoring network is meeting the regulatory objectives. The annual ambient monitoring network plans ensure networks comply with design requirements.²¹ EPA conducts on-site reviews and inspections to assess compliance with the regulations governing the collection, analysis, validation, and reporting of ambient air monitoring data.²²

The number and type of required monitors and pollutants monitored differ at each site. These monitors must meet either a designated reference or an equivalent method for monitoring. The Federal reference method (FRM) is a method of sampling and analyzing the ambient air for an air pollutant that EPA has specified as a reference method in regulation. FRMs are EPA-accepted standards for analyzing an air pollutant. The Federal equivalent method (FEM) is a method for measuring the concentration of an air pollutant in the ambient air that has been designated as an equivalent method in regulation.²³ FEMs are methods that have been approved through regulation to be equivalent to FRMs.²⁴ Figure 2 shows examples of ambient air monitoring sites and monitoring equipment.

Figure 2. Examples of Ambient Air Monitoring Sites and Monitoring Equipment

Source: U.S. Government Accountability Office, Air Pollution: Opportunities to Better Sustain and Modernize the National Air Quality Monitoring System, GAO-21-38, December 7, 2020, p. 21, https://www.gao.gov/products/gao-21-38.

A collection of monitoring sites makes up an air program's monitoring network. Figure 3 illustrates the District of Columbia's air program's existing five-station ambient air monitoring network. These sites collectively make up national ambient air monitoring networks. These networks include²⁵

• Air Toxics
• Lead Monitoring
• National Core Network (NCore)
• Near-Road Monitoring
• Ozone: Photochemical Assessment Monitoring Stations (PAMS)
• Particulate Matter (PM) Networks
• Fine Particulate Matter (PM_2.5)
• Chemical Speciation Network (CSN)
• Interagency Monitoring of Protected Visual Environments (IMPROVE)
• Susceptible and Vulnerable Populations—NO₂ Monitoring

Figure 3. The District of Columbia's Ambient Air Monitoring Network

Source: District of Columbia, Department of Energy and Environment (DOEE), "District of Columbia's Calendar Year 2024 Draft Annual Ambient Air Monitoring Network Plan," DOEE, https://doee.dc.gov/release/public-comment-period-2024-annual-ambient-air-monitoring-network-plan. Notes: AQS = Air Quality System, NO2 = Nitrogen Dioxide, CO = Carbon Monoxide, SO2 = Sulfur Dioxide NO = Nitrogen Oxide, NOy = Nitrogen Oxides.

Air Toxics Ambient Air Monitoring and Monitoring Networks

In addition to monitoring the ambient air for criteria air pollutants, EPA monitors for the 188 hazardous air pollutants (HAPs) informally referred to as "air toxics." Section 112 of the CAA directs EPA to promulgate emission standards for the sources of HAPs that are listed in Section 112(b).²⁶ Ambient air toxics monitoring provides air toxics data, which have a critical role in characterizing HAP concentrations across the country. The monitoring data help support trend analyses across cities, regions, and specific areas of interest over time; provide exposure assessments to help examine the relationships between ambient HAP concentrations, human activities, and the related personal exposures that are used as inputs for HAP modeling; and help HAP model evaluations.

EPA established the National Air Toxics Trends Station (NATTS) Network in 2003. The current network configuration (as illustrated in Figure 4) includes 26 sites (21 urban, 5 rural) across the United States. Typically, each NATTS monitors over 100 pollutants; 19 of those are formally required. Target HAPs include volatile organic compounds (VOCs), carbonyls, PM₁₀ metals (PM₁₀ is particulate matter with a diameter smaller than 10 micrometers and greater than 2.5 micrometers), and polycyclic aromatic hydrocarbons (PAHs).²⁷

Figure 4. The National Air Toxic Trends Monitoring Sites

Source: U.S. Environmental Protection Agency (EPA), "Air Toxics Ambient Monitoring," https://www.epa.gov/amtic/air-toxics-ambient-monitoring. Note: No National Air Toxic Trends Stations (NATTS) are located in Alaska or Hawaii.

Ambient Air Monitoring Regulatory Context

For NAAQS compliance, air monitors must meet regulations promulgated by EPA and any applicable state, tribal, or local regulations. Technical requirements include detailed sampling, siting, and quality assurance requirements. Air monitors used in policymaking and regulatory decisions provide the data needed to calculate design values, a statistic that describes the air quality status of a given location relative to the level of the NAAQS.²⁸ For example, NAAQS designations are based on the most recently available design values computed using air quality data reported by state, tribal, and local air monitoring agencies to EPA's Air Quality System (AQS).²⁹ An area's attainment and implementation of the NAAQS rely on ambient air monitors.³⁰ An air agency's permitting decision for new sources of air pollution and the levels of source-specific controls or offsets required are determined by the design values established through regulatory ambient air monitoring.³¹ A nonattainment area's level of emission reductions and control requirements, needed to meet nonattainment progress goals or achieve attainment, are partially determined by the design values established through regulatory ambient air monitoring.³²

Low-Cost Air Sensors

EPA defines low-cost air sensors as a class of nonregulatory technology that is lower in cost, portable, and generally easier to operate than the air monitors used for regulatory purposes.³³ A low-cost air sensor is a relatively low-priced device that uses one or more sensors and other components to detect, monitor, and report on specific air pollutants like particulate matter (PM) or carbon monoxide (CO) and specific environmental conditions, such as temperature and humidity.³⁴ Depending on the sensor or combination of sensors used, it can detect one or more, or a combination of, pollutants and/or environmental factors.³⁵

Low-cost air sensors typically have a price point below $2,500, compared to regulatory ambient air monitors that reach price points of up to $50,000.³⁶ These low-cost air sensors typically provide relatively quick or instant air pollutant concentration measurements, and they allow for the measurement of air quality in more locations.

Many low-cost air sensors fall into one of four types, depending on how they measure air pollution:

• 1. light scattering (used for PM),
• 2. electrochemical (gaseous pollutants including O₃, SO₂, NO₂, total volatile organic compounds (VOCs), and CO),
• 3. metal oxide semiconductor (gaseous pollutants including O₃, CO, NO₂, and total VOCs), and
• 4. photoionization (total VOCs).³⁷

Advancements in technology, microprocessing capabilities, and miniaturization have led to an expansion in the availability of low-cost air sensors to measure a variety of air pollutants. As these sensors have become more readily available, they have been increasingly used for measuring air quality conditions and thus provide additional low-cost air sensor data sets.³⁸ According to the U.S. Government Accountability Office (GAO), the use of low-cost air sensors is "increasing, driven in part by policy and public interest in air quality stemming from wildfire smoke, neighborhoods near pollution sources, and other concerns."³⁹

Figure 5 illustrates the use of low-cost air sensors to measure PM_2.5 in the District of Columbia by various users of PurpleAir sensors. Community members using the sensors are able to link their sensors and provide data to the mapping tool. The number of regulatory monitors noted in Figure 3 were five total monitors, four of which monitored PM_2.5 in the District, compared to the over three dozen nonregulatory low-cost PM_2.5 PurpleAir sensors.⁴⁰

Figure 5. PurpleAir Low-Cost Air Sensor Map for the District of Columbia Daily PM2.5 AQI Data

Source: CRS, using PurpleAir, https://map.purpleair.com/1/mAQI/a1440/p604800/cC0#10.58/38.9186/-77.0803. Notes: Map settings were set to "US EPA PM2.5 AQI," with an averaging period = "1-day", and the remaining settings were left at default values.

Low-Cost Air Sensor Regulatory Context

EPA is involved in the advancement of low-cost air sensor technology, including performance evaluations of sensor devices and best practices for effective use. EPA evaluated low-cost sensors for how well they measured air pollutants and how easy they were to use. Placing the sensors near a regulatory monitor, EPA collected data on air quality with both technologies. By assessing the data collected under the same air quality and weather conditions, EPA compared how accurate and reliable low-cost technologies were compared to regulatory methods.⁴¹ Based on its assessment of the accuracy of low-cost sensors, in a June 2020 EPA memorandum, EPA stated that "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." These requirements would include meeting EPA monitoring-related regulations.⁴² In the memorandum, EPA recognized that these low-cost air sensors may not meet the requirements for use as regulatory monitors; however, it stated, these sensors "could still be very useful in non-regulatory applications."⁴³

Although EPA found that these low-cost air sensors are generally less accurate than their more expensive regulatory counterparts, GAO noted that they can be deployed in large numbers to supplement information provided by the national ambient air monitoring networks.⁴⁴ Among the possible uses are identifying pollution "hot spots," providing local community-scale air monitoring, assisting in the site selection for new or relocated regulatory monitors, and conducting scientific research.⁴⁵

EPA announced 132 community air monitoring projects to be conducted by a range of entities, including nonprofits, state and local agencies, and tribes, that would receive $53.4 million from the American Rescue Plan Act of 2021 (P.L. 117-2) and P.L. 117-169, known as the Inflation Reduction Act of 2022. The majority of the announced projects plan on using low-cost air sensors.⁴⁶ For the announced projects, quality assurance requirements apply to the collection of environmental information.⁴⁷ Environmental information collections are any measurements or information that describe environmental processes, locations, or conditions; ecological or health effects and consequences; or the performance of environmental technology.⁴⁸

Concluding Observations

Ambient air monitoring networks have provided reliable air quality data throughout the country for decades. The data are standardized and accessible through EPA's AQS and other air quality data tools. Regulators, researchers, communities, and others have relied on the network to provide the data needed for studies, source permitting, NAAQS attainment and implementation, air quality alerts, and a host of other applications.⁴⁹ Figure 6 is an example of one of the visualization tools EPA provides. The figure illustrates the daily AQI in the Washington, DC metro area from 1999 to 2023.

Figure 6. Daily Ozone AQI Values, 1999 to 2023 Washington-Arlington-Alexandria, DC-VA-MD-WV

Source: EPA, "Air Data—Multiyear Tile Plot," https://www.epa.gov/outdoor-air-quality-data/air-data-multiyear-tile-plot. Note: Generated April 4, 2024.

Some observers have raised concerns about the ambient air monitoring networks. In particular, observers have noted the increasing costs to establish and maintain ambient air monitoring networks. For example, according to a 2020 report from GAO, "modern monitoring equipment technology is significantly more expensive than its predecessor technology." In the report, GAO found that the level of funding EPA provided for air quality management programs from 2004 to 2019 remained relatively steady. When adjusted for inflation, the amount of federal funding declined by an average of $4 million per year over the same time frame.⁵⁰ According to the 2020 report:⁵¹

The ambient air quality monitoring system is a national asset that provides standardized information for implementing the Clean Air Act and protecting public health. The Environmental Protection Agency (EPA) and state and local agencies cooperatively manage the system, with each playing different roles in design, operation, oversight, and funding. For example, EPA establishes minimum requirements for the system, and state and local agencies operate the monitors and report data to EPA. Officials from EPA and selected state and local agencies identified challenges related to sustaining the monitoring system. For example, they said that infrastructure is aging while annual EPA funding for state and local air quality management grants, which cover monitoring, has decreased by about 20 percent since 2004 after adjusting for inflation.

While GAO highlighted funding issues associated with regulatory monitoring, EPA noted that ambient air monitoring networks might not properly identify hot spots or community-level air pollution issues if no ambient air monitor is within the direct vicinity to properly characterize the possible air pollution issue.⁵²

Considering these concerns, a question facing policymakers is what role low-cost air sensors could play in support of ambient air monitoring networks. Some state and local air agencies contend that low-cost air sensors have been successfully used to supplement regulatory monitors and fill data gaps. The low-cost sensors help decisionmakers address specific needs. For example, some air agencies have used low-cost air sensors to help direct limited enforcement resources.⁵³ This nonregulatory use of low-cost air sensors may help an air agency ensure it is achieving the maximum emission reductions through its regulatory enforcement actions, saving it time and money. The emission reductions achieved may also be pivotal in maintaining a lower NAAQS design value and maintaining or achieving the attainment of NAAQS.

In addition to their support of compliance programs, low-cost sensors have been used in nonregulatory contexts. For example, federal agencies have deployed low-cost air sensors to monitor smoke during wildfires and communicate possible risks to stakeholders.⁵⁴ In addition, some agencies make low-cost air sensors available for deployment to wildland fire locations upon request of firefighting agencies. These low-cost air sensors can help inform firefighting agency decisions on allocating resources.⁵⁵ Low-cost air sensors have been particularly useful for monitoring wildfire smoke in areas without regulatory monitors.

Agencies and stakeholders may need to consider the proper siting, use, and understanding of the data obtained from low-cost air sensors deployed for wildfire smoke initiatives. Furthermore, during a wildfire event, the readiness and availability of low-cost air sensors to be deployed during an emergency is a key feature of the technology, but a sensor's data may be questionable if the device was not previously calibrated with a regulatory monitor.

According to state and local air agencies, one of the challenges with low-cost sensors regards communication with community members to address the limitations and nonregulatory aspects of low-cost air sensors.⁵⁶ This raises questions about how low-cost air sensors and their air quality data are perceived and used by the public.

I thank the committee for its time. I am available to answer any questions you may have about these technologies and their implementation. CRS can assist with any additional research and analysis regarding this issue.