South Coast Air Quality Management District

The South Coast Air Quality Management District covers Orange County and the urban parts of Los Angeles, Riverside, and San Bernardino counties, and its territory produces the largest share of California's CEQA documents. Its authority stack is the most fragmented of the five. The 1993 CEQA Air Quality Handbook remains the base document; the district's handbook web page says it is available only by request, that portions are no longer current, and that a replacement is being developed. The numbers consultants actually use sit on a one-page table, Air Quality Significance Thresholds, most recently revised March 2023, supplemented by a 2008 localized significance threshold methodology and a 2008 interim greenhouse gas threshold.

Mass daily thresholds

Pollutant	Construction (lb/day)	Operation (lb/day)
NOx	100	55
VOC	75	55
PM10	150	150
PM2.5	55	55
SOx	150	150
CO	550	550
Lead	3	3

The table cites the 1993 Handbook as the source. There are no annual thresholds and no exhaust versus fugitive split: the comparison is the project's daily emissions, total PM, against the number. The Handbook has carried a separate VOC threshold for the Coachella Valley, which San Diego County's 2007 guidelines borrow; the 2023 table prints a single column.

Localized significance thresholds

The district's Localized Significance Threshold Methodology, adopted 2003 and revised July 2008, is voluntary guidance for the effect of a project's on-site emissions on its nearest neighbors.

• Four pollutants only: NOx, CO, PM10, and PM2.5. Off-site mobile emissions are excluded.
• Mass rate look-up tables for one, two, and five acre sites, by source receptor area (the district is divided into 37 of them), at receptor distances of 25, 50, 100, 200, and 500 meters. A receptor closer than 25 meters uses the 25 meter value.
• The tables apply to projects of five acres or less. Larger projects are expected to run dispersion modeling.
• The derivation back-calculates from the most stringent ambient standard: the 1-hour state NO2 standard of 0.18 ppm, the CO standards of 20 ppm for 1 hour and 9 ppm for 8 hours, a construction PM10 and PM2.5 increment of 10.4 micrograms per cubic meter over 24 hours drawn from Rule 403, and an operational increment of 2.5 micrograms per cubic meter drawn from Rule 1303.

Ambient air quality thresholds on the 2023 table

The district is in attainment for NO2 and CO, so a project is significant if it causes or contributes to an exceedance of these standards. The other values are the allowable changes from Rule 1303, Table A-2, except the construction PM values from Rule 403.

Pollutant	Averaging time	Threshold
NO2	1 hour	0.18 ppm (state)
NO2	Annual mean	0.03 ppm (state), 0.0534 ppm (federal)
PM10	24 hour	10.4 micrograms per cubic meter (construction), 2.5 (operation)
PM10	Annual	1.0 micrograms per cubic meter
PM2.5	24 hour	10.4 micrograms per cubic meter (construction), 2.5 (operation)
SO2	1 hour	0.25 ppm (state), 0.075 ppm (federal, 99th percentile)
SO2	24 hour	0.04 ppm (state)
Sulfate	24 hour	25 micrograms per cubic meter (state)
CO	1 hour	20 ppm (state), 35 ppm (federal)
CO	8 hour	9.0 ppm (state and federal)
Lead	30 day average	1.5 micrograms per cubic meter (state)
Lead	Rolling 3 month average	0.15 micrograms per cubic meter (federal)

Greenhouse gases

Staff's October 2008 Draft Guidance Document proposed an interim GHG significance threshold for projects where the district itself is the lead agency, which in practice means permitted industrial and stationary source projects, and took it to the Governing Board's December 5, 2008 hearing; the district's GHG thresholds web page records the Board's adoption that day. The operative number is a Tier 3 screening level of 10,000 metric tons CO2e per year, and the 2023 table prints exactly that: 10,000 MT per year CO2e for industrial facilities.

The 2008 staff guidance is a five tier decision tree: Tier 1 CEQA exemption; Tier 2 consistency with a GHG reduction plan that meets AB 32 and specified criteria; Tier 3 the screening level; Tier 4 performance options such as 30 percent below business as usual or a sector efficiency standard; Tier 5 offsets for a 30 year project life. Staff derived a residential and commercial screening level of about 3,000 metric tons per year in the same document, as 10,000 scaled by those sectors' 9 percent share of the state inventory against industry's 30 percent. The 2023 table prints only the industrial value; the residential and commercial figures remain staff proposals. Lead agencies in the basin that use them do so on their own authority, and the report must say so.

Health risk, odor, and CO

Impact	Threshold
Excess cancer risk	10 in a million or greater
Cancer burden	More than 0.5 excess cases, in areas at 1 in a million or greater
Chronic and acute hazard index	1.0 or greater, project increment
Odor	An odor nuisance under Rule 402
CO	The district is in attainment; significant if the project causes or contributes to an exceedance of 20 ppm (state) or 35 ppm (federal) for 1 hour, or 9.0 ppm for 8 hours

Construction measures

There is no district list of construction practices that a CEQA finding depends on. Fugitive dust control is a regulatory requirement under Rule 403, and the construction PM localized thresholds are derived from that rule's allowances. Mitigation measures for a project that exceeds a threshold come from the Handbook's mitigation chapters and the lead agency's own conditions.

Model

CalEEMod, which the district hosts. The 2023 thresholds table does not pin a version.

Where the thresholds are published

South Coast Air Quality Management District, Air Quality Significance Thresholds, revision March 2023 (one page, with the 1993 CEQA Air Quality Handbook cited as the source of the mass daily values); Final Localized Significance Threshold Methodology, June 2003, revised July 2008, with Appendix C look-up tables; Draft Guidance Document, Interim CEQA Greenhouse Gas Significance Threshold, October 2008, considered at the Governing Board's December 5, 2008 hearing.