Air Quality

From Ultrafine Particles to Wildfire Smoke: Research Priorities for Cleaner Air

Explore the evidence gaps shaping California air quality policy — including ultrafine particles, air toxics, wildfire smoke, and oil and gas setbacks — and the research needed to address them.
Explore More Research Priorities

 

Ultrafine Particulate Matter 

Ultrafine particles are produced in large numbers by combustion activities – including vehicle emissions & wildfire –  but their extremely small size means that they are currently unregulated by state and federal mass-based air quality regulations (PM 2.5 and PM 10). Their small size also makes them more mobile in the human body than larger particles, leading to distinct mechanisms of harm and associated health effects.

Research Needs 

Research is needed to support the establishment of an evidence-based, health protective regulatory framework for ultrafine particulates.   

  • Research on innovative, cost-effective methods to measure ultrafine particles in the environment at scale 
  • Research that assesses the health impacts of ultrafine particles, particularly in communities that face high rates of ultrafine and/or mixed/cumulative exposures 
  • Research on effective mitigation strategies to reduce exposure to ultrafine particles and improve related public health outcomes 

Background & Policy Context 

Particulate matter (PM) is a diverse and variable mixture of human-generated and natural components. It includes a range of particle sizes, categorized as coarse (PM10 or ≤10 μm in diameter), fine (PM2.5, ≤ 2.5 μm), and ultrafine (≤ 100 nm). US air quality standards regulate PM10 and P2.5 based on their mass concentration (μg/m³), which refers to the total weight of the particles of a certain size in a square meter of air, regardless of composition. Ultrafines are not currently regulated separately from PM2.5, and while technically included in that category, are undetectable using traditional optical particle counters. 

While practical for monitoring and enforcement, a mass-based approach PM10 and 2.5 does not fully reflect health risk or particulate matter, as smaller particles like ultrafines (or chemically complex or toxic particles; see Air Toxics section) can be harmful even at low mass concentrations. Effective regulation of ultrafine particulate matter will likely require both improved methods for measuring them and a greater understanding of their unique health effects. 

Additional Resources 

 

Air Toxics 

Air Toxics, also known as hazardous air pollutants (HAPs) or Toxic Air Contaminants(TACs) are air pollutants that are known or suspected to cause cancer or other serious health effects. Real time monitoring of air toxics is limited, and communities — particularly in areas with multiple potential sources — are concerned that screening tools like the EPA’s AirToxScreen, which relies largely on modeled dispersion from known sources rather than direct monitoring, may inadequately represent actual exposure and health risk levels. 

Research Questions 

Community research questions center on the ability of existing tools to protect public health, including (a) reliance on modeling rather than emissions monitoring and (b) concerns regarding the adequacy of the health risk assessments used in current models.   

Both technological innovation (i.e. lower cost, enhanced monitoring technology) and empirical evaluation of how well risk screening tools like US EPA’s AirToxScreen correspond to real-world observations (i.e. measured emissions, biomarker levels, or health outcomes) are needed.  

How accurate is the US EPA’s AirToxScreen when compared to measured concentrations of air toxics in various community settings? 

Examples of responsive studies could include: 

Do areas flagged by AirToxScreen as “high risk” show corresponding increases in disease incidence or biomarkers of exposure/effect? 

Examples of responsive studies could include: 

  • Conducting epidemiologic studies linking AirToxScreen risk estimates with health records 
  • Measuring biomarkers to assess exposure in high-risk census tracts 

How can AirToxScreen outputs be better linked to enforceable actions or community-led interventions? 

Examples of responsive studies could include:

  • Evaluating whether and how local agencies use AirToxScreen to trigger permitting, enforcement, or land-use decisions 
  • Partnering with high risk communities to co-design “risk reduction pilots” based on AirToxScreen hotspots, then assess changes in air quality and health metrics 

Background & Policy Context 

Air toxics (e.g., perchloroethylene, methylene chloride, dioxin, asbestos, toluene, and metals such as cadmium, mercury, chromium, and lead compounds) are covered by the Clean Air Act, but are not currently included in the EPA’s National Ambient Air Quality Standards (NAAQS), which only apply to six common air pollutants that the EPA refers to as Criteria Air Pollutants.

Unlike criteria air pollutants – for which allowable concentration levels are set and monitored – federal regulation of air toxics primarily focuses on requiring the use of “best available” pollution control technologies at known industrial sources, rather than monitoring these emissions directly or setting air concentration limits. While direct monitoring does take place at the national level, it is limited and not spatially representative. As a result, EPA’s AirToxScreen screening tool relies largely on dispersion-based modeling using estimated operating data and emission factors and health risk assessments that in many cases are decades old.* 

California regulates air toxics at the state level through the Air Resources Board’s Air Toxics Program. It maintains its own list of toxic air contaminants and reference exposure levels and monitors ambient air levels both via state reference sites and by providing support for community-level monitoring. However, monitoring coverage remains limited by the cost of available technology. Current state monitoring focuses on the 12 air toxics that are estimated to collectively constitute 95% of the combined health risk in California based on their emissions and associated toxicity value – these include Diesel Particulate Matter (DPM), five toxic heavy metals: cadmium, hexavalent chromium, lead, arsenic, and nickel, and six toxic volatile organic compounds (VOCs): benzene, 1,3-butadiene, formaldehyde, acetaldehyde, perchloroethylene, and p-dichlorobenzene. 

*Note that this largely reflects a regulatory implementation gap rather than a lack of scientific evidence. While academic research continues to advance regarding the health impacts of air toxics exposure, official regulatory benchmarks frequently lag years or even decades behind. This disconnect means that tools like AirToxScreen may underestimate risk in cases where official toxicity values have not been updated to reflect current knowledge. 

Additional Resources 

 

 Air Quality Monitoring and Notifications 

In addition to extensive regulatory monitoring, California has some of the most advanced community air monitoring initiatives in the world. Academic partnerships can strengthen these systems by providing access to new monitoring technologies and helping to ensure that the data they are collecting is communicated to maximize public health impact. 

Research Needs 

Research that supports the establishment of public air quality notifications that are timely, community–responsive, and effective in improving public health outcomes, such as: 

  • Research on effective and accessible health risk communications in highly impacted communities, including farmworkers and other outdoor workers 
  • Research on the utilization and efficacy of existing air quality alerts to protect outdoor workers and other sensitive populations 
  • Research on safe, effective strategies for outdoor workers and other at risk populations to respond to public notices about exposure to poor air quality in high heat settings 

(i.e. when an air quality intervention like a mask could increase the risk of heat related illness) 

  • Research on the development of low-cost and mobile air quality monitors usable in a community setting 
  • Research on methods of integrating community-based and regulatory air quality monitoring data 
  • Research on effective, community-based analysis and communication of monitoring data in advocacy and public health contexts 

Background & Policy Context 

Air quality monitoring is a central part of air quality policy management. Historically, public air quality monitoring systems have tended to cover large geographic regions and have been in formats not easily accessed by the public. More recently, various public and private air quality notification systems have been developed to try to make air quality information available to the public in real time, allowing health protective action. These include the Air Quality Flag Program, Spare the Air alerts, and mobile apps and websites like AirNow, IQAir, and 

Breezometer, as well as public agency systems such as the San Joaquin Valley Air Pollution Control District’s Real-Time Air Advisory Network (RAAN) and the California Air Resources Board’s AB 617 Air Mapping Tool. 

In addition to public agency and private air monitoring, a new generation of community-based air quality monitoring is rising. These include the national PurpleAir network, California’s IVAN Air Monitoring Network and the SJVAir Collaborative,. 

Both the IVAN Air Monitoring Network and SJVAir Collaborative allow users to view maps and individual monitors online also and sign up to receive alerts associated with monitors anywhere in the network. These networks can also be used to generate reports to guide enforcement actions by public agencies. However, little research has been done on how effective these tools are at improving public health outcomes and how they could be improved. 

Some of the challenges facing agency-based air quality monitoring and notification include designing systems that are effective and accessible in the diverse, multilingual communities that need them most. Conversely, community-based monitoring and notification systems may struggle with issues of quality assurance and quality control (QA-QC), data interpretation, integration with public systems, and long-term sustainable funding.    

Additional Resources

 

Oil and Gas Setbacks 

California is a significant producer of oil and gas, and unlike more rural gas producing states, many active operations are located in close proximity to residential areas. 

Community groups and academic researchers have raised concerns for many years about the health risks to those living near oil wells, and after years of community pressure the state enacted 3200 ft setbacks between oil and gas facilities and sensitive receptors (residential areas, schools, hospitals, etc.) in 2024. These setbacks are currently facing legal challenges based in part on the scientific evidence base used to justify them. 

Research Needs 

California-specific research is needed to demonstrate actual exposure levels and health outcomes associated with living or spending significant amounts of time within 3200 feet of oil and gas production. 

  • Research on actual emissions from oil and gas production facilities located within 3200 feet of sensitive receptors. 
  • Research on health outcomes associated with proximity to oil and gas production facilities. 

Background & Policy Context 

Community advocates have raised concerns for many years about the health risks associated with residential proximity to oil wells, including attempts to establish health protective setbacks through Assembly Bills AB 345 in 2019 and AB 467 in 2021, both of which ultimately died in committee. 

In 2021, the Department of Conservation’s Geologic Energy Management Division (CalGEM) convened a national Public Health Science Advisory Panel to review the scientific evidence on the public health implications of oil and gas production. Following their report, which found consistent evidence of harm within 1 kilometer of drilling operations, CalGEM released draft public health regulations, including a 3200 ft setback from sensitive locations. The setback regulations developed by CalGEM were expected to take effect in 2023 . However, in August 2022 California Governor Gavin Newsom asked the legislature to consider adopting a law establishing the setbacks, rather than continuing through the lengthy regulatory process. 

The resulting bill, SB 1137, was signed into law in 2022. It prohibited the permitting of new oil and gas wells within 3,200 feet of sensitive receptors and imposed enhanced environmental and operational requirements for existing wells located within this buffer zone, including mandatory leak detection, repair protocols, and additional health-protective measures. 

Since its passage, SB 1137 has been subject to sustained opposition from industry stakeholders, including challenges through both the legislative process and currently ongoing litigation. 

The law is currently being challenged in court, and one of the lawsuit’s central claims is that the research used to support it  does not prove drilling is harmful to people living within 3,200 feet of a sensitive site. 

In addition to state efforts, Los Angeles adopted an Oil and Gas Drilling Ordinance in 2022 that would phase out oil and gas drilling in the city over the next two decades. This ordinance was struck down on September 12, 2024 by a county judge on the grounds that the state, rather than the city, has jurisdiction over drilling operations. However this ruling was invalidated two weeks later when the governor signed AB 3233, which gave California cities and counties the right to ban drilling and phase out existing wells. 

Los Angeles is currently going through the process of passing new oil phaseout ordinances, which the LA County Board of Supervisors is expected to introduce in early 2026. 

Additional Resources 

 

AB 617 Community Air Protection Program Implementation 

California’s AB 617 Community Air Protection Program was passed in 2018 with the purpose of improving air quality in heavily impacted communities in California. It involves community-scale air quality monitoring and the development of air pollution emission reduction plans in selected communities. 

Research Needs 

Research that determines the extent to which the implementation of the AB 617 Community Air Protection Program is improving public health outcomes in participating communities, such as: 

  • Measurement of changes to air quality associated with AB 617 implementation and/or specific interventions being utilized in participating communities. 
  • Analysis of changes to public health outcomes associated with AB 617 implementation 
  • Examination of the role of specific interventions that have been implemented under AB617 to improve air quality and associated health outcomes. For example, land use mechanisms (e.g., restricting certain land uses in already heavily impacted communities), urban greening, trade-ins for domestic equipment such as lawnmowers and fireplaces. 
  • Research on the integration of community-based and public agency air quality monitoring systems.

Background & Policy Context 

The California Air Resources Board (CARB) manages the Community Air Protection Program for the state, and local Air Districts are responsible for implementation in consultation with Community Steering Committees made up of local residents, organizations, governments, and businesses. 

CARB’s Community Air Protection Program Blueprint 2.0 includes both a strategic plan for the program and practical guidance for partners engaged in improving air quality at the local level. The Program Blueprint was originally established in 2018 when the law was enacted and is required to be updated every 5 years. CARB’s Governing Board approved the current Blueprint 2.0 in October, 2023, which was informed by The People’s Blueprint. 

Many CAC members are closely involved with AB 617 implementation and are organizing to ensure quality community engagement and advocating for improvements to the regulation.

Additional Resources 

 

Wildfire Health Impacts 

Wildfire is a major and growing environmental health concern in California, where in recent years record heat and drought have increased their size, speed, and destructiveness. 

Research Needs 

Research on the impact of wildfire on the health of communities such as: 

  • Research on the environmental health impacts of wildfire smoke exposure on outdoor workers 
  • Research on effective exposure mitigation strategies in high risk occupational settings, like agricultural field work and construction 
  • Research on the environmental health impacts of wildfire smoke exposure in communities also experiencing high levels of air pollution from other sources 
  • Research on effective wildfire smoke mitigation strategies in sensitive locations like schools, day care centers, prisons, nursing homes, and other institutional and medical settings 
  • Research on the after-effects of wildfire on drinking water and exposure to toxic substances in soil. 

Background & Policy Context 

In response to increasing wildfire risk, billions of dollars of new investments are being made in preventing and responding to wildfires at both the state and federal level, including California SB 109, which established a new Office of Wildfire Technology Research and Development in 2022. 

These large, rapidly spreading fires are now also crossing into more populated areas. As more industrial and household materials burn, the composition of wildfire smoke is changing and may include toxic metals and other contaminants. 

Residents of California's Central Valley are particularly vulnerable to wildfire smoke exposure. Smoke from fires in the mountains can collect and settle in the Valley for days or weeks at a time, leaving residents exposed for longer than those in the actual fire zone in some cases. Central Valley communities already face some of the worst air pollution in the country and many residents are low income and have limited access to mitigations like air purifiers, HVAC systems, masks, or the option of staying or working indoors. 

The region is also home to the largest agricultural sector in the country, including half of its agricultural workers, who work outside during fire season with minimal protections, while also facing high rates of occupational illness and injury and limited access to health care. 

Additional Resources