Localized Significance Thresholds

Localized significance thresholds (LST’s) were developed in response to Governing Board’s Environmental Justice Enhancement Initiative I-4.  The LST methodology was provisionally adopted by the Governing Board in October 2003 (Appendix C of the LST methodology adopted by the Board is outdated, the user is referred to the latest version of Appendix C - Mass Rate LST Look-up Table below) and formally approved by SCAQMD's Mobile Source Committee in February 2005.On October 6, 2006, the SCAQMD Governing Board unanimously adopted a methodology to calculate PM2.5 and PM2.5 significance thresholds.  The documentation that was adopted in October 2006, also included PM2.5 LST look-up tables in support of the LST program.  Additional information, including final methodology and the PM2.5 LST look-up tables, can be found on the PM2.5 Significance Thresholds and Calculation Methodology webpage.

LST Methodology

LSTs are only applicable to the following criteria pollutants: oxides of nitrogen (NOX), carbon monoxide (CO), particulate matter less than 10 microns in aerodynamic diameter (PM10) and particulate matter less than 2.5 microns in aerodynamic diameter (PM2.5).  LSTs represent the maximum emissions from a project that are not expected to cause or contribute to an exceedance of the most stringent applicable federal or state ambient air quality standard, and are developed based on the ambient concentrations of that pollutant for each source receptor area and distance to the nearest sensitive receptor.  For PM10 LSTs were derived based on requirements in SCAQMD Rule 403 – Fugitive Dust.

The use of LSTs is voluntary, to be implemented at the discretion of local public agencies acting as a lead agency pursuant to the California Environmental Quality Act (CEQA).  LSTs would only apply to projects that must undergo an environmental analysis pursuant to CEQA or the National Environmental Policy Act (NEPA) and are five acres or less.  It is recommended that proposed projects larger than five acres in area undergo air dispersion modeling to determine localized air quality.  Projects that are statutorily or categorically exempt under CEQA would not be subject to LST analyses.  Projects exempt from CEQA also include infill projects that meet the H&S Code provisions or projects identified by lead agencies as ministerial.

Final LST Methodology Document (PDF, 422kb)  (Appendix C provided below)

PM2.5 Significance Thresholds and Calculation Methodology

Appendix C - Mass Rate LST Look-up Table

The mass rate look-up tables were developed for each source receptor area (SRA) and can be used on a voluntary basis by public agencies to determine whether or not a project may generate significant adverse localized air quality impacts.  The LST mass rate look-up tables only apply to projects that are less than or equal to five acres.  Lead agencies may use the LST mass rate look-up tables to determine localized air quality impacts or use the LST mass look-up tables as a screening analysis.  If the project exceeds any applicable LST when the mass rate look-up tables are used as a screening analysis, then project specific air quality modeling may be performed.  In the event that the project area exceeds five acres, it is recommended that lead agencies perform project-specific air quality modeling for these larger projects. If evaluating the project in CalEEMod, please refer to the suggested guidance (PDF, 40kb) in determining the maximum daily disturbed acreage.

 Appendix C - Mass Rate LST Look-up Tables (PDF, 63kb)

Source Receptor Area (SRA)/City Table

The following document can be used to identify the appropriate SRA in which the project is located.

SRA/City Table (XLS, 38kb)

Sample Construction Scenarios

Sample scenarios were designed to be used by local lead agencies as models or templates for analyzing construction air quality impacts for projects undergoing an environmental analysis under CEQA or the National Environmental Policy Act NEPA.  The sample construction scenarios have been developed to generically represent a broad range of project types that occur in the district, e.g., commercial, residential, educational, etc.).  Each sample construction scenario is divided into five non-overlapping phases: demolition, site preparation, grading, building, and architectural coatings and paving.  The construction scenarios can be used in their entirety to represent similar construction for the project proposed in the air quality analysis.  Alternatively, the lead agency can use the construction scenario spreadsheets and tailor them to fit the characteristics of the project analyzed.  The construction scenarios are based on actual numbers of construction equipment and activity (hours of operation, area disturbed, dirt and debris handled, etc.) obtained from construction site surveys.

        Appendix A - One Acre Site Example (XLS, 628kb)
        Appendix B - Two Acre Site Example (XLS, 614kb)

        Appendix C - Three Acre Site Example (XLS, 613kb)

        Appendix D - Four Acre Site Example (XLS, 615kb)

        Appendix E - Five Acre Site Example (XLS, 599kb)

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