The Air Toxics Hot Spots Information and Assessment Act — AB 2588 — has been on the books since 1987. For most of its history, it affected a relatively stable set of large industrial facilities. That changed when CARB significantly expanded the program's reach starting with 2024 operations, reported in 2025.
If your facility wasn't previously subject to AB 2588, it may be now. And if you were already reporting, the expanded Criteria and Toxics Reporting (CTR) requirements may have added substances and sources you weren't tracking before.
Here's what the program requires, who it applies to, and why the accuracy of your emissions data matters more than you might expect.
How AB 2588 Works
The program follows a four-step process. Not every facility goes through all four steps — most stop at step one or two. But the process is sequential, and where you land depends on what your emissions data shows.
Step 1: Emissions Inventory
Every subject facility must prepare and submit an inventory of toxic air contaminant (TAC) emissions. This inventory quantifies what you emit, how much, and from which sources. It covers over 700 listed substances, including metals, volatile organics, semi-volatile organics, PAHs, dioxins, and other compounds.
The inventory uses a combination of source test data, emission factors, material balance calculations, and engineering estimates. The more actual test data you have, the more defensible your inventory is.
Step 2: Prioritization
The air district — SCAQMD in Southern California — reviews submitted inventories and assigns a prioritization score. The score weighs the quantity and toxicity of emitted substances, proximity to sensitive receptors (schools, hospitals, residential areas), and the number of people potentially exposed.
Facilities that score below the district's threshold stay in the inventory-and-report cycle. Those that score above the threshold move to Step 3.
Step 3: Health Risk Assessment (HRA)
Prioritized facilities must conduct a Health Risk Assessment. The HRA uses air dispersion modeling to estimate pollutant concentrations at receptor locations, then applies toxicity values to calculate cancer risk and non-cancer health hazard indices for the exposed population.
The HRA is only as good as the emissions data feeding it. Modeled concentrations derive directly from the emission rates in your inventory. Inaccurate emission rates produce inaccurate risk numbers — in either direction.
Step 4: Risk Reduction
If the HRA shows cancer risk exceeding 10 in a million or a chronic hazard index above 1.0 at any receptor, the facility must prepare and implement a risk reduction audit and plan. This can mean installing controls, changing processes, substituting materials, or other measures to bring risk below the thresholds.
What Triggers AB 2588 Applicability
AB 2588 applicability is determined by the air district based on facility type, operations, and potential toxic emissions. Several factors put a facility in scope:
Facility type. Certain categories are automatically included — refineries, chemical manufacturing, metal plating, dry cleaning, waste treatment, and other operations known to emit listed TACs.
Chemical use and emissions. Facilities that use, store, or process listed toxic substances above de minimis quantities may be included regardless of their SIC/NAICS code.
Proximity to sensitive receptors. Facilities near schools, daycare centers, hospitals, or dense residential areas face lower thresholds for inclusion and prioritization.
Permit conditions. Some SCAQMD permits include AB 2588 reporting as a specific condition, independent of the general applicability criteria.
The 2024-2025 CTR Expansion
CARB's expansion of the Criteria and Toxics Reporting regulation significantly broadened the universe of affected facilities. The expanded CTR added new facility types that previously weren't subject to toxic emission reporting requirements.
Key changes include:
New facility categories. Operations that weren't previously considered significant TAC sources — including certain commercial and institutional categories — were brought into the reporting framework.
Additional reportable substances. The list of substances requiring quantification grew. Facilities that were already reporting may need to add compounds they hadn't previously tracked.
Diesel engine facilities. A major expansion target. Many facilities with stationary or portable diesel engines now face reporting requirements for diesel particulate matter, a listed TAC and known carcinogen.
2026 Program Activities
CARB's planned activities for 2026 directly affect when and how facilities will be evaluated:
April 2026 — CARB will develop prioritization scores for diesel engine facilities. This is the first wave of prioritization for many newly included operations. Facilities with significant diesel engine emissions should have their inventories in order before scores are calculated.
August 2026 — CARB will prioritize additional core facilities beyond the diesel engine category. This second wave captures the broader set of newly included facility types from the CTR expansion.
If your facility was newly captured by the CTR expansion, the window between now and these prioritization dates is your opportunity to ensure your emissions data is accurate and defensible.
Why Emissions Data Accuracy Matters
In most regulatory programs, overestimating emissions is the conservative, safe approach. In AB 2588, it's not that simple.
The Cost of Overestimation
If your emissions inventory overstates TAC emissions — because you used conservative emission factors instead of actual test data, or because screening-level estimates replaced measured values — your prioritization score goes up. A higher score increases the likelihood of being pushed into the HRA process.
If the HRA then uses those same inflated emission rates, the modeled risk may exceed thresholds, triggering mandatory risk reduction. You could end up installing controls or making process changes to address a risk that was a data artifact, not an actual health concern.
HRAs cost $30,000 to $100,000+. Control equipment costs far more. Source testing to establish accurate baseline emissions is a fraction of those costs.
The Risk of Underestimation
Understating emissions creates a different problem. If your inventory doesn't capture actual TAC emissions — because sources were missed, emission factors were too low, or speciation was incomplete — and the community later identifies health concerns, the regulatory response is severe.
SCAQMD can require a facility to redo its inventory and HRA. If the corrected assessment shows significant risk, the facility faces risk reduction requirements plus enforcement action for the underreporting. Community trust, which is difficult to rebuild, takes a direct hit.
The Value of Measured Data
Source testing provides measured emission rates that replace assumptions. A properly conducted emissions test using appropriate methods — Method 29 for metals, Method 18 or TO-15 for speciated organics, Method 5 for particulate, modified Method 5 for semi-volatiles — gives you defensible numbers.
Those numbers may be higher or lower than emission factors predicted. Either way, they're real. And real data is what survives regulatory scrutiny, community challenge, and HRA peer review.
OEHHA Reference Exposure Level Updates
The Office of Environmental Health Hazard Assessment periodically updates the toxicity values used in health risk assessments. These Reference Exposure Levels (RELs) and cancer potency factors directly affect HRA outcomes.
In 2025, OEHHA added or updated RELs for several compounds:
Isocyanate compounds — New RELs for specific isocyanate species affect facilities using polyurethane coatings, adhesives, and foam products. Facilities that previously reported isocyanates as a group may need to speciate their emissions and re-evaluate risk.
1,4-Dichlorobenzene — Updated toxicity values for this commonly used solvent and deodorizer compound. Facilities using products containing 1,4-dichlorobenzene should review whether their current inventory reflects the updated toxicity.
When OEHHA tightens a REL, a facility's calculated risk goes up even if emissions haven't changed. This makes previously acceptable risk levels potentially significant — and makes accurate emissions quantification even more important as the margin between compliance and non-compliance narrows.
What Source Testing Provides for AB 2588
Source testing supports AB 2588 compliance at multiple points in the process:
Inventory development. Measured emission rates replace emission factors and engineering estimates. This is particularly valuable for metals, speciated organics, and other compounds where generic emission factors may not reflect your specific fuel, feedstock, or process conditions.
HRA inputs. If your facility is prioritized for a health risk assessment, the emission rates used in air dispersion modeling should be based on the best available data. Source test data is the gold standard.
Risk reduction verification. If you install controls to reduce risk, source testing demonstrates that those controls achieve the required emission reductions.
Ongoing updates. AB 2588 inventories are updated quadrennially or as directed by the district. Each update is an opportunity to replace older estimates with current test data.
Common Test Methods for AB 2588
- CARB Method 436 — Multi-metals (arsenic, beryllium, cadmium, chromium, lead, manganese, mercury, nickel, selenium).
- EPA Method 18 / EPA TO-15 — Speciated volatile organic compounds including benzene, toluene, ethylbenzene, xylenes, chlorinated solvents.
- CARB Method 428 — Dioxins and furans.
- CARB Method 429 — Semi-volatile organic compounds, PAHs.
- CARB Method 421 — Hydrogen halides (HCl, HF).
- CARB Method 430 — Formaldehydes and ketones (formaldehyde, acetaldehyde).
The specific methods required depend on your emission sources and the TACs identified in your inventory. A testing plan should be developed in coordination with your consultant and the district.
Should You Act Now?
If your facility falls into any of these categories, the answer is yes:
- Newly subject to CTR reporting — Get your first inventory right. Base it on measured data where feasible, not just emission factors.
- Diesel engine operator facing April 2026 prioritization — Quantify your diesel PM emissions accurately before CARB assigns your score.
- Existing AB 2588 facility approaching inventory update — Replace outdated emission factors with current source test data.
- Facility near sensitive receptors — Proximity amplifies prioritization scores. Accurate data is your best defense against unnecessarily conservative risk calculations.
- Facility using newly listed or updated-REL substances — If OEHHA changed the toxicity value for a compound you emit, recheck whether your current emissions data supports an acceptable risk level.
Frequently Asked Questions
Check with SCAQMD directly or review your facility's permit conditions. If you received a notification from the district or CARB regarding CTR reporting, you're in scope. Facility types automatically included are refineries, chemical plants, metal plating operations, and other categories listed in the regulation. The 2024 CTR expansion added new categories — if your operation involves diesel engines, certain commercial processes, or chemical use above de minimis thresholds, you may have been newly captured.
The Annual Emissions Report covers criteria pollutants — NOx, CO, SOx, PM, VOC. AB 2588 covers toxic air contaminants — a separate list of over 700 substances including metals, specific organic compounds, dioxins, and other toxics. Some pollutants overlap (e.g., PM can contain toxic metals), but the two programs have different reporting requirements, deadlines, and regulatory consequences. A facility can be subject to one, both, or neither.
Yes, emission factors are an accepted method for estimating TAC emissions in the inventory. However, emission factors are generic averages that may not represent your specific equipment, fuel, or operating conditions. If your facility is prioritized for an HRA, regulators and risk assessors will scrutinize the quality of your emission data. Source test data is more defensible and may result in lower (and more accurate) risk estimates than conservative emission factors would produce.
If the HRA shows a maximum individual cancer risk above 10 in a million or a chronic hazard index above 1.0 at any receptor location, the facility must conduct a risk reduction audit. This audit identifies feasible measures to reduce risk — emission controls, process changes, material substitutions, or operational modifications. The facility then implements the measures on a schedule approved by SCAQMD. Progress is tracked, and the facility may be required to conduct follow-up testing and a revised HRA to demonstrate that risk has been reduced below thresholds.
Accurate Environmental Services performs the full range of source testing methods used in AB 2588 emissions inventory development and HRA support. We hold LAP approval for CARB Method 436 (metals), SCAQMD Methods 5.1-5.3 (PM and semi-volatiles), EPA Method 18 (speciated VOCs), and other methods required for toxic air contaminant quantification.
Call (714) 379-9200, email wallymoe@accuenviro.com, or request a quote at accuenviro.com