What a CEQA air quality report is measured against
Every discretionary development project in California answers the six air quality and greenhouse gas questions in Appendix G of the CEQA Guidelines. The lead agency, usually the city or county approving the project, decides whether each impact is significant. The regional air district does not make that decision. It publishes recommended thresholds of significance and a method for comparing a project against them, and nearly every lead agency adopts what its district recommends. So the technical report is written to the district's numbers, and the same building gets a different analysis depending on which of the 35 districts it sits in.
The four kinds of threshold
| Kind | What is compared | Who uses it |
|---|---|---|
| Regional mass, daily | Pounds per day of ROG, NOx, PM10, PM2.5, sometimes CO and SOx. The Bay Area averages over working days or the year; South Coast and Sacramento compare daily values; San Diego lead agencies use permit trigger levels with no averaging convention | Bay Area, South Coast, Sacramento, San Diego lead agencies |
| Regional mass, annual | Tons per year of the same pollutants | San Joaquin Valley (annual only), Bay Area and Sacramento (alongside daily) |
| Localized or concentration | Whether emissions near the site cause or add to an exceedance of an ambient standard, by look-up table or dispersion model | South Coast (localized significance thresholds), San Joaquin Valley (ambient analysis above 100 lb/day), Sacramento (5 percent of a state standard) |
| Health risk and odor | Excess cancer risk per million, hazard index, PM2.5 concentration, odor complaint history or screening distance | All five, with different numbers |
The mass numbers often started life as permit thresholds, which is why they differ so much. San Joaquin Valley uses its New Source Review offset levels in tons per year. San Diego lead agencies borrow the district's permit trigger levels from Rule 20.2.
The three paths for greenhouse gases
| Path | How it works | Where |
|---|---|---|
| Design elements or plan consistency | The project either builds in a fixed set of features (no natural gas, VMT reduction, EV readiness) or shows consistency with a qualified local GHG reduction strategy under CEQA Guidelines 15183.5(b). No number to compare against. | Bay Area (2022), City of San Diego (Climate Action Plan consistency), Sacramento operations after tier 1 practices |
| Bright line | Emissions above a fixed tonnage are significant. | Sacramento: 1,100 MTCO2e per year for construction; 10,000 for stationary sources. South Coast: 10,000 for industrial projects where the district is lead agency. |
| Percent below business as usual | Quantify, then show a reduction against a no-action baseline. | San Joaquin Valley: 29 percent, or Best Performance Standards, under a 2009 policy still in force |
The five largest districts at a glance
Share of California's 2025 negative declarations and environmental impact reports, counted by whole county from State Clearinghouse filings, is in parentheses.
| District | Guidance in force | Mass basis | Construction dust rule | GHG path | Cancer risk trigger | Odor rule |
|---|---|---|---|---|---|---|
| South Coast AQMD (31%) | 1993 Handbook; thresholds table revised March 2023 | lb/day only | Rule 403 compliance; no CEQA list | 10,000 MT industrial, district as lead agency only | 10 in a million | Rule 402 nuisance |
| Bay Area AQMD (20%) | 2022 CEQA Guidelines, April 2022 | lb/day and tons/yr | All nine basic practices in Table 5-2 | Table 3-2 design elements or 15183.5(b) strategy | 10 in a million | Five confirmed complaints a year over three years |
| San Joaquin Valley APCD (20%) | GAMAQI, March 2015 | tons/yr only | Regulation VIII is law, not mitigation | Best Performance Standards or 29 percent below business as usual | 20 in a million | More than one confirmed complaint a year over three years |
| San Diego APCD (5.5%) | District has no land use thresholds; County 2007 and City 2022 documents | lb/day, with lb/hr and tons/yr | County design considerations; City 100 lb/day dust screen | City: Climate Action Plan consistency | County: 1 in a million without T-BACT | Nuisance rule; City complaint test |
| Sacramento Metro AQMD (3.3%) | CEQA Guide, thresholds table April 2020 | lb/day, with tons/yr for PM | Zero PM threshold unless all feasible practices applied | 1,100 MT construction; tier 1 and tier 2 practices for operations | 10 in a million, stationary sources only | Case by case under Rule 402 |
Why one CalEEMod run gets different answers
The model output is the same everywhere. The comparison is not.
- Averaging period. The Bay Area divides each construction year's tons by that year's working days. South Coast compares daily emissions. San Joaquin Valley compares tons per calendar year for operations and over a rolling twelve months for construction. A project can pass one test and fail another on identical emissions.
- What counts as PM. The Bay Area compares construction exhaust PM only and handles dust with practices. San Joaquin Valley sums exhaust and fugitive dust against one annual number. South Coast compares total PM10 and PM2.5 per day.
- Permitted equipment. San Joaquin Valley evaluates permitted stationary equipment separately from everything else, each against its own threshold. Other districts compare the project total.
- Mitigated runs. Where a threshold is exceeded, the analysis reruns the model with adopted measures. Which measures count as mitigation differs: Bay Area basic dust practices are a condition of a less than significant finding, while San Joaquin Valley says compliance with its dust rules is not mitigation at all because the law already requires it.
Each district page in this section carries the current numbers, the document they come from, and the revision date. The lead agency's own thresholds control where they differ.