California Air Resources Board

Toxic air pollutants, also known as air toxics, are chemicals that can cause cancer or other serious health effects when people are exposed to them in the air. As scientific knowledge and chemical use evolve, some pollutants are identified as potentially posing greater risks than previously understood. These pollutants are referred to as emerging air toxics. 

Emerging Air Toxics include air toxics that have:  

• newly identified toxicities not previously understood, including quantified health values 

• recently detected in the environment through air monitoring studies and usage 

• recently found to be accumulating in the bodies of humans identified through biomonitoring studies    

This website will provide information and resources for emerging air toxics of concern. Recent emerging air toxics of concern include:  

Acrolein  

On May 14, 2026, the Office of Health Hazard Assessment (OEHHA) released a new draft cancer risk value for acrolein. 

Resources: 

Frequently Asked Questions (FAQ)  

Factsheet (coming soon)

OEHHA Report Webpage 

Ethylene Oxide 

On May 14, 2026, the Office of Health Hazard Assessment (OEHHA) released a draft cancer risk value for ethylene oxide (EtO). 

Resources:

Frequently Asked Questions (FAQ) 

Factsheet (coming soon)

OEHHA Report Webpage 

Methyl Bromide 

Methyl bromide is a toxic air contaminant that can affect the respiratory and nervous systems. In 2023 air monitoring around fumigation operations near residential neighborhoods identified releases of methyl bromide (MeBr). For more information regarding the ambient air monitoring for MeBr please refer here: https://ww2.arb.ca.gov/capp/cst/ch2/wcwlb/methyl-bromide  

Other Toxic Air Contaminants 

A Toxic Air Contaminant (TAC) is an air pollutant which may cause or contribute to an increase in mortality or an increase in serious illness, or which may pose a present or potential hazard to human health. Under the state’s Toxic Air Contaminant Identification and Control Act, TACs are identified through a formal health evaluation process led by the Office of Environmental Health Hazard Assessment (OEHHA) and the California Air Resources Board (CARB). Once a substance is identified as a TAC, it may move into a risk management phase where strategies are developed to reduce or eliminate public exposure. Please note not all emerging toxics of concern have been formally identified as a TAC.  

Program Overview

The Off-Road Diesel Equipment Surveillance Program is an in-use emissions testing program designed to better characterize real-world emissions and operational performance of off-road diesel equipment. The data collected through this program support various CARB internal efforts, including emissions inventory development and policy design. The program is conducted at CARB’s Depot Park facility (8311 Galena Ave., Sacramento, CA 95828) using portable emissions measurement systems (PEMS).

Background and Objectives: Why This Program is Important

Off-road diesel equipment plays a critical role in California’s economy, supporting construction, emergency response, power generation, transportation, and public services. Many types of ORE operate in diverse real-world environments, including residential and business areas, as well as major transportation hubs, such as airports, rail stations, and seaports.

Portable diesel equipment, including power generators, is often operated in disadvantaged communities identified under California’s environmental justice frameworks (AB 617 and SB 535). These communities frequently experience disproportionate and cumulative air pollution burdens from multiple sources. Collecting real-world emissions data from these equipment types is essential to support CARB’s efforts to reduce emissions and address cumulative impacts.

Accurate estimation of the environmental impact of mobile emission sources is essential for effective air quality management and plays a key role in California’s clean air programs and policy development. Therefore, this program investigates real-world operational patterns and emissions characteristics of off-road equipment.

This program prioritizes diesel equipment because off-road diesel engines account for a significant share of statewide emissions, including diesel particulate matter (29%) and nitrogen oxides (11%). These pollutants contribute to the formation of PM2.5 and ozone, which are key air quality and public health concerns in California. The data obtained through this program helps improve CARB’s emissions inventory and to inform evaluations of emerging and zero-emission technologies.

Current Focus (2025~2026): Portable Diesel Power Generators

During the first two years of the program, CARB is prioritizing testing of portable diesel power generators (gensets). Common uses of gensets include emergency power supply, power for remote facilities, operation of appliances and tools, recreational activities, and backup power for critical operations.
Power generators support a wide range of businesses and events and can also be integrated with battery energy storage systems (BESS) to create hybrid or microgrid configurations. This category has shown significant market potential and growth over the past five years, partially due to the increasing demand for emergency power at artificial intelligence data centers.

Future Focus (202X): Mobile Off-Road Equipment

The ORE Surveillance Program is a continuous program and will expand its focus to mobile off-road equipment upon the completion of the generator testing phase. Potential test subjects include common, high-volume off-road equipment types, such as excavators, wheel loaders, and skid-steer loaders.

Procurement Requirements and Frequently Asked Questions

• Procurement Requirements and Required Information

• Off‑Road Equipment (ORE) Diesel Surveillance Program - Frequently Asked Questions

Exploratory Work and Emerging Technology Insights

Zero‑Emission Technology Evaluation

As part of CARB’s broader transition toward zero-emission technologies in off-road sectors, the ORE Surveillance Program also supports limited evaluations of zero-emission technologies when resources are available. In 2025, the program conducted a performance test of a hydrogen fuel cell (H2FC) power generator, which enabled a side-by-side comparison with a power-equivalent diesel generator.

Zero‑Emission Technology Knowledge Sharing and Demonstration Event

A knowledge-sharing and demonstration event was held on July 25, 2025, at Cal EPIC, with more than 70 attendees from CARB, ARCHES, non-profit organizations, research institutions, rental companies, and original equipment manufacturers (OEMs). The event included three technical presentations covering hydrogen production, fuel cell fundamentals, and H2FC generator systems. The OEM also demonstrated a live electric vehicle charging session.

Knowledge Sharing at Conference: Diesel Power Generator Emission Characteristics

Emissions characteristics obtained from the surveillance program were presented at the 36th CRC Real World Emissions Workshop on March 10, 2026. The conference provides a strong platform for knowledge exchange among government agencies, industry, and academia on topics such as air quality policy, emissions data, alternative fuels, and zero-emission technologies. The presentation highlighted the program's value and implications for upcoming CARB initiatives.  

Alignment with CARB Zero-Emission Programs

• Enables early evaluation of emerging ZE technologies under in-use conditions.

• Supports CARB zero-emission programs through real-world performance data.

• Enhances cross-division coordination on ZE policy, research, and incentives.

Please ask any questions or request further discussions via junhyeong.park@arb.ca.gov.

CARB comments on Federal Aviation Administration information requests and plans

The Toxics and Risk Managers Committee (TARMAC) Emission Factor Tool is a collaborative online resource developed with several California Air Districts and the California Air Resources Board (CARB). This online resource is designed to provide emission factors and pertinent information from industrial and commercial sources.

Key Features & Search Functionality

The tool is designed for high-level filtering and granular data retrieval. Users can search the database using the following criteria:

• Source Identifiers: SCC Number, SCC Name, and TARMAC Number.
• Equipment & Fuel: Equipment type and specific fuel types.
• Pollutants: Pollutant name and Emission Factor (EF) descriptions.
• Regulatory: Agency source.

Data Organization

Information is structured into six specialized tables to provide varying levels of technical detail:

User Resources & Exporting

• Export Formats: Data can be exported directly from the dashboard in PDF or CSV formats for external analysis.
• Documentation: A comprehensive user manual and field definitions list are available within the tool to assist users.

Important Note:

Regional Variability: Emission data and estimation methods vary by location. Always contact your local air district to confirm specific emission factors, reporting thresholds, and regulatory requirements applicable for a given facility.

Simplified Table Definitions 

• Agency – The agency (district) claiming this emission factor record. 

• TARMAC Number – The unique emission factor profile number by district. 

• EF Description – Written description of the emission factor profile, which may contain more specific information than the Equipment field. 

• Equipment – General description of the equipment.  

• Fuel or Process – Either the fuel type or specific process for the equipment. 

• Pollutant Name – Name of the pollutant for the emission factor record. 

• CAS Number – The Chemical Abstracts Service (CAS) registry number of the pollutant for the emission factor record. 

• SCC Number – The Source Classification Code (SCC) number used by EPA https://sor-scc-api.epa.gov/sccwebservices/sccsearch/. 

• SCC Name – The Source Classification Code (SCC) name used by EPA https://sor-scc-api.epa.gov/sccwebservices/sccsearch/. 

• Emission Factor (EF) – This emission factor record is the numeric value that acts as a multiplier, converting activity/usage data into total emissions. 

• EF Units – The units that the emission factor record is presented in. 

• EF Date – The year that the reference document was published with the emission factor. 

References Table Definitions 

• Agency – The agency (district) claiming this emission factor record. 

• Document Link – Link to the (first) reference document for the emission factor record. 

• References – The reference document for the emission factor record. 

• TARMAC Number – The unique emission factor profile number by district. 

• Section – Refers to the specific section in the reference document that the emission factor record is based on. 

• Author – The creator of the emission factor/reference document that the emission factor is based on. 

Emission Factor Description Table Definitions 

• TARMAC Number – The unique emission factor profile number by district. 

• Agency – The agency (district) claiming this emission factor record. 

• Agencies Share – This is all the known districts with the same emission factor record.  Additional districts may use the same emission factor record but have not yet participated in the TARMAC EF Clearinghouse project.   

• EF Description – Written description of the emission factor profile, which may contain more specific information than the Equipment field.  

• EF Date – The year that the reference document was published with the emission factor. 

• Equipment – General description of the equipment.  

• Fuel or process – Either the fuel type or specific process for the equipment. 

• SCC Name – The Source Classification Code (SCC) name used by EPA https://sor-scc-api.epa.gov/sccwebservices/sccsearch/. 

• References – The reference document for the emission factor record. 

• Notes – Additional information that should be reviewed prior to using the emission factor. 

Emission Factor Table Definitions 

• TARMAC Number – The unique emission factor profile number by district. 

• EF Description – Written description of the emission factor profile, which may contain more specific information than the Equipment field. 

• CAS Number – The Chemical Abstracts Service (CAS) registry number of the pollutant for the emission factor record. 

• Pollutant Name – Name of the pollutant for the emission factor record. 

• Emission Factor (EF) – The numeric value that acts as a multiplier, converting activity/usage data into total emissions. 

• EF Units – The units that the emission factor record is presented in. 

• References – The reference document for the emission factor record. 

Prescribed fire is planned, intentional ignition applied by land managers to reduce vegetation fuels that feed potential future wildfire, reduce wildfire risk on communities and infrastructures, restore ecosystem health, and support landscapes that depend on periodic burning. It is a planned, carefully managed tool used across California and other parts of the United States and the world. Unlike large wildfires, prescribed burns are conducted under specific weather and fuel conditions to keep fire behavior low intensity and predictable. California has set ambitious goals to expand the use of prescribed fire to reduce hazardous fuel buildup, improve forest resilience, and help protect communities from severe wildfire.

This webpage provides estimates of air pollutant emissions produced by the vegetation and other natural fuels intentionally consumed during prescribed fire operations. These data represent the amount of pollutants released at the point of combustion on the day and at the location where burning occurred.

Where smoke travels after ignition depends on many factors, including weather, terrain, and the operational conditions of the burn. As a result, smoke movement can vary considerably from one prescribed fire to another, even when they have similar objectives or fuel types. The emissions presented here describe only the quantity of pollutants released—they do not reflect how smoke disperses or what levels of smoke may be experienced by nearby communities. For more information about how prescribed fire smoke may affect local air quality and tips for reducing smoke exposure, please see the additional links at the bottom of this webpage.

Summary of Prescribed Fire Activity and Emissions

The graphs below summarize statewide prescribed fire activity and associated PM_2.5 emissions across California. In Figure 1, daily emissions patterns show that most PM_2.5 from prescribed burning occurs during concentrated seasonal burn windows, particularly in the fall, with both flaming and smoldering combustion contributing to emissions. Emissions are generally low and sporadic through most of the year. The largest emission peaks occur in October and early November, where both flaming and smoldering emissions increase sharply, with smoldering generally higher during the largest events. This pattern indicates that most daily PM_2.5 emissions from prescribed burning occur during a concentrated fall burn period.  Figure 1 does not include additional prescribed burns for which the burner did not record a burn date and there was no satellite fire detect to fill in the burn date.

Figure 2 presents the geographic distribution of prescribed burn PM2.5 emissions and burn acreage across California air basins. Majority of prescribed burns in California are in the northern part of the state. In other parts of the state, prescribed burn is conducted mostly in mountainous regions and areas covered with natural vegetations.

Prescribed Fire Emissions Data

California Air Resources Board (CARB) staff revamped the methodology and the data sources of the Prescribed Fire Emission Inventory in the 2025-2026 inventory cycle. Published in 2026, the 2023 emission inventory differs from previous inventory vintages in the following ways: it integrates activity datasets collected and compiled by local/state/federal air and land management agencies, the most extensive data reconciliation effort among similar datasets to date; uses satellite fire detect data to fill in burn date that was not recorded by the burner; refines fuel consumption assumptions specific to fuel compartments; and employs emission estimation pathways that are specific to combustion phase (e.g., flaming vs. smoldering) and burn type (broadcast burn, pile burn, or air curtain incinerator or carbonizer). It is spatially and temporally explicit inventory with day-by-day emissions and location information. The following inventory data are available:      

• A detailed table of 2023 prescribed fire emissions, categorized by burn type (broadcast, pile burn, and air curtain incinerator or carbonizer) and further separated into flaming and smoldering emissions. 
• Technical documentation of inventory methods
  • Appendix 1:  Documentation of Fuel Characteristic Classification System Fuelbeds lookups for Air District and Interagency Treatment and Tracking System prescribed fire records.

Archive of previous inventory vintages:

• Documentation of the methodology prior to the 2023 inventory:
  • Forest Management, Range Improvement. 
• Look up historical inventory data using the CEPAM query tool 
  • Instructions: Go to the CEPAM Query Tool, select the inventory year and the geographic area that you want to look up, choose Source Type “Area,” and click “Run Report.” Navigate to Rx Fire Emissions by subsequently clicking the following: “MISCELLANEOUS PROCESSES,” “MANAGED BURNING AND DISPOSAL,” and scroll down to “664-RANGE IMPROVEMENT” AND “666-FOREST MANAGEMENT.”   

For a summary of the inventory methodology, see the Oct 16, 2025, public webinar presentation in the Webinar Presentation section below.

Webinar Presentation

Additional Information & Resources

Prescribed Fire Data Reporting & Smoke Management

Prescribed Burning (General Information Webpage)

Smoke Management Program for Prescribed & Agricultural Burning

Prescribed Fire Information Reporting System (PFIRS)

Fire & Exceptional-Event Assessment, Screening, & Toolkit (FEAST)

California Smoke Spotter Application

California Wildfire & Forest Resilience Task Force, Interagency Treatment Tracking System (ITS)

Emission Inventory

Criteria Pollutant Emission Inventory (all emission source categories)

Wildfire Emission Inventory

Other Related Programs

Natural & Working Lands Inventory

SB 901 Report

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This webpage provides summaries of public webinars, FAQs, and reports produced by CARB’s Fire Emission Inventory Team. The webinar summaries include the announcement, presentation slides, and recording. The information is listed chronologically with the most recent first. For more information about fire emission inventory, see the additional links at the bottom of this webpage.

Fire Emission Inventory Data & Information:

• Wildfire Emissions Estimates 

• Wildfire Emissions Summary Graphs 

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