SCAQMD Rules 1401 and 1401.1 – Why Do They Matter?

Author: Michael McHale

As discussed in our previous 4 the Record Article on SCAQMD Permitting 101, air quality permitting is often the critical path for facilities looking to implement capital projects in the South Coast Air Quality Management District’s (SCAQMD’s or the District’s) jurisdiction.

Air quality permitting in the South Coast Air Basin involves several independent approval tests including offset requirements, criteria air pollutant modeling, criteria pollutant best available control technology requirements, command-and-control limitations established under Regulations XI and XIV, and air toxics risk-based thresholds. The tests and requirements of Rule 1401 are critical for SCAQMD permitting and can directly affect whether a project is permissible or prohibited. This article will focus on the New Source Review requirements for Toxic Air Contaminants (TAC or air toxics) established under Rules 1401 and 1401.1.

Rule 1401 – New and Modified Permit Units

Rule 1401 establishes health risk thresholds for new, relocated, or modified permit units that emit TAC listed in Table I of Rule 1401 or otherwise identified by the District as TAC.

Applicability

Rule 1401 applies to new, relocated, and modified permit units subject to permitting requirements established under Rules 201 and 203. Under Rule 1401, a relocated permit unit is any device that has the potential to emit TAC that is removed from one parcel of land in the District and installed at another parcel where the two parcels do not touch and are not separated solely by a public roadway or other public right-of-way.

SCAQMD may not issue a Permit to Construct unless the applicant demonstrates, through the preconstruction health risk assessment, that the project’s calculated risk is below the applicable significance thresholds.

Core Risk Thresholds

Rule 1401 sets four risk thresholds that a project must meet to receive approval:

Maximum Individual Cancer Risk (MICR)

This value represents the highest estimated cancer risk that an individual could experience over a lifetime as a result of exposure to TAC. It represents the risk to the single most exposed person, assuming that the individual breathes the facility’s emissions continuously over a 70-year lifetime.

For example, a MICR of 1.0 x 10^-6 means that, at the location of highest exposure, one additional cancer case could occur in a population of 1,000,000 people exposed at that level over a lifetime. It is important to note that this is a modeled estimate, not a prediction that someone will actually develop cancer.

SCAQMD cannot issue a permit for a project if the MICR is greater than 1.0 x 10^-6 at any receptor if the permit unit is not constructed with Best Available Control Technology for Toxics (T-BACT) or if the MICR is greater than 10 x 10^-6 at any receptor for all TAC emitted from new, relocated, or modified permit units.

Cancer Burden

This value represents how many additional cancer cases could potentially occur in a population subject to a MICR greater than or equal to 1.0 x 10^-6  resulting from exposure to TAC. It represents the total expected increase in cancer caused across the exposed population, not just the risk for a single person.

SCAQMD cannot issue a permit for a project if the cancer burden is greater than 0.5 for all TAC emitted from new, relocated, or modified permit units.

Chronic Hazard Index (Chronic HI)

This value represents the potential long-term health risks from exposure to TAC. It compares the estimated long-term exposure level for a person to a health-based reference exposure level¹ (REL) set to protect people from chronic health effects. It is calculated as the ratio of the estimated exposure to the REL.

SCAQMD cannot issue a permit for a project if the cumulative increase in total Chronic HI due to total emissions from the new, relocated or modified permit unit is greater than 1.0 at any receptor for all TAC emitted.

Acute Hazard Index (Acute HI)

This value represents the potential short-term health risks from exposure to TAC. It compares the estimated one-hour exposure level for a person to an REL set to protect people from short term health effects. It is calculated as the ratio of the estimated exposure to the REL.

SCAQMD cannot issue a permit for a project if the cumulative increase in total Acute HI due to total emissions from the new, relocated or modified permit unit is greater than 1.0 at any receptor for all TAC emitted.

Receptor Location

For the purpose of calculating the Acute HI, a receptor is any location outside the boundary of the facility at which a person could experience acute exposure. For the purpose of calculating Chronic HI and MICR, a receptor is any location outside of the facility at which a person could experience long-term exposure (e.g., a residence).

Determining Risk Characteristics

Rule 1401 uses a tiered health risk assessment (HRA) to establish the risk characteristics for comparison to the four thresholds discussed above. As of December 1, 2024, SCAQMD expected Tier 1, 2, and 3 HRAs to be conducted using their HRA web-based Tool.

Tier 1 HRA – Screening Emission Levels

A Tier 1 HRA is an initial screening evaluation used to determine whether TAC emissions are low enough that a more refined health risk assessment is not needed. The method applies conservative assumptions, including worst case dispersion and continuous exposure, which intentionally overstate potential risk. If the estimated risk characteristics exceed the applicable thresholds, the analysis proceeds to a Tier 2 HRA.

Tier 2 HRA – Screening Risk Assessment

A Tier 2 HRA is a more refined evaluation that incorporates source-specific information such as source type, equipment characteristics (e.g., building area, equipment rating, and stack height), local meteorological conditions, and the distance to nearby receptors.

Tier 3 HRA – Screening Dispersion Model

A Tier 3 HRA is a more refined evaluation that uses the U.S. EPA’s AERSCREEN dispersion modeling tool to model how TAC emissions disperse in the atmosphere. Unlike Tier 1 and Tier 2 analyses, this modeling is conducted outside of the SCAQMD HRA web-based tool and provides a more detailed estimate of potential health risk. It allows users to refine dispersion characteristics by incorporating stack gas temperature, exit velocity, and terrain effects.

Tier 4 HRA – Detailed Health Risk Assessment

A Tier 4 HRA is the most refined evaluation and involves full air dispersion modeling using the U.S. EPA’s AERMOD model and the California Air Resources Board HARP2 program. This analysis replaces screening assumptions with site specific dispersion modeling and detailed exposure analysis. Earlier tiers intentionally use conservative simplifications to allow for rapid screening.

Toxic Best Available Control Technology (T-BACT)

If the MICR is greater than 1.0 x 10^-6 and less than 10 x 10^-6, a project may be permitted if T-BACT is applied. T-BACT is the most stringent emissions limit or control technique that has been demonstrated in practice or found to be technologically feasible for a class of source. T-BACT is established on a source specific basis and is typically determined in consultation with the permit engineer during the permitting process. When evaluating T-BACT, you should consider the following sources:

• SCAQMD BACT Guidelines for criteria pollutants (Volatile organic compounds are often used as a surrogate for organic TAC emissions, and particulate matter is commonly used as a surrogate for metal TAC emissions.)
• CARB Technology Clearinghouse
• Previous SCAQMD permits for similar equipment

Rule 1401.1 – Additional Requirements Near Schools

Rule 1401.1 establishes additional requirements for new or relocated facilities located within 1,000 feet of a school.² The rule divides this area into three distance categories, each with different permitting requirements.

Facilities Located Less Than 500 Feet From a School

This is the most restrictive category because emissions sources are very close to students.

In this case, the facility must conduct a risk demonstration at the school receptor showing that the health risk from toxic air contaminant (TAC) emissions meets strict limits:

• MICR < 1.0 × 10⁻⁶
• Chronic HI  < 1.0
• Acute HI < 1.0

If these limits cannot be met, the project generally cannot be permitted at that location.

Facilities Located 500–1,000 Feet From a School With Another Sensitive Receptor Within 150 Feet

In this situation, a different sensitive receptor is closer to the emissions source than the school. Examples of sensitive receptors include residences, daycare centers, hospitals, or senior housing.

Because the nearby receptor may experience higher exposure than the school, the rule shifts the focus of the permitting review. Instead of requiring a school-specific risk demonstration, the applicant must provide additional information under Rule 212, which supports the broader permitting evaluation of impacts on nearby receptors.

The facility must still comply with Rule 1401 risk thresholds and permitting requirements.

Facilities Located 500–1,000 Feet From a School With No Sensitive Receptors Within 150 Feet

If the school is the closest sensitive receptor, the facility must conduct a risk demonstration at the school.

The same risk limits apply:

• MICR < 1.0 × 10⁻⁶
• Chronic HI <1.0
• Acute HI < 1.0

This ensures that emissions from new or relocated facilities do not create unacceptable health risks for students even when the facility is farther than 500 feet away.

Exemptions

The following sources are exempt from inclusion in the Facility-wide MICR and HIs calculation under 1401.1:

• Emergency internal combustion engines that are exempt from modeling and offset requirements under Rule 1304
• Engines subject to Rule 1470
• Equipment permitted solely for in-situ remediation of contaminated soil and/or groundwater
• Equipment permitted for use at various locations that do not remain onsite for more than 12 consecutive months
• Experimental research operations permitted under Rule 411 operating for one year or less
• Equipment exempt from permitting under Rule 219.

Common Fatal Flaws

A missed or failed HRA can add months to a project’s timeline or even prevent the project from being permitted. Some of the most common fatal flaws related to HRAs include the following:

Incorrect or Overly Conservative TAC Emissions Calculations

Errors in emissions calculations are one of the most common issues during the permitting process. Health risk scales directly with emissions, so mistakes such as the use of non-representative emissions factors, unit conversion errors, or inappropriate calculation methodologies can result in a failed HRA. Similarly, overly conservative assumptions can also inflate emissions estimates. These may occur when calculations assume continuous operation, ignore enforceable permit limits, or rely on generic emissions factors when more representative data are available. Because health risk scales directly with emissions, these assumptions can overstate emissions and lead to an unnecessary HRA failure. Early review of emissions calculations by experienced air permitting professionals can help identify these issues before they affect the HRA or permitting timeline.

Failure to Consider Nearby Sensitive Receptors

Pollutant concentrations generally increase as receptors get closer to the emission source, although the maximum ground-level concentration may occur some distance downwind depending on source characteristics and atmospheric conditions. Failing to identify nearby sensitive receptors such as schools, hospitals, daycare centers, detention centers, or residences during the permitting process can result in project delays or revised HRAs. If a closer receptor is later identified, the recalculated risk may exceed regulatory thresholds. Early review of surrounding land uses and nearby receptors can help identify potential risk issues before they affect the permitting timeline.

Conducting the HRA too Late in the Process

As previously stated, a missed or failed HRA can add months to the permitting process and, in some cases, prevent a project from being permitted. Many of the issues that lead to failed HRAs can be resolved if they are identified early in the planning process. For example, increasing stack height can improve dispersion and reduce ground-level concentrations of TACs. Similarly, it is often easier to incorporate T-BACT during the project design phase than after capital has already been allocated. Evaluating potential HRA challenges early in project planning can help avoid costly redesigns and permitting delays.

Closing Thoughts

Rule 1401 HRAs are one of the most critical components of SCAQMD permitting, and mistakes or late-stage analysis can significantly delay projects. Identifying potential HRA challenges early in the project planning process can help facilities evaluate design options, refine emissions calculations, and avoid unnecessary permitting hurdles.

ALL4 regularly works with facilities throughout the project lifecycle to identify environmental permitting challenges early and develop practical strategies. Our team routinely supports facilities with emissions evaluation, screening analyses, air dispersion modeling, and T-BACT evaluations. By integrating air permitting considerations into project planning and design, we help clients avoid unnecessary delays and costs.

If you are planning a capital project in the South Coast Air Basin, evaluating Rule 1401 applicability and impacts early in the process can help stave off budget and timeline challenges. If you are evaluating installing new equipment or modifying existing equipment in the South Coast Air Basin, ALL4 can help navigate the permitting requirements and support a smooth path forward. Reach out to Michael McHale (mmchale@all4inc.com) for assistance.

 

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¹RELs are health-based exposure limits developed by the California Office of Environmental Heath Hazard Assessment (OEHHA). They are used to evaluate potential non-cancer health risks from TAC.

²Rule 1401.1 defines a school as “any public or private school, including juvenile detention facilities with classrooms, used for the education of more than 12 children at the school in kindergarten through grade 12 and early learning and development programs such as pre-schools, Early Head Start, Head Start, First Five, and Child Developmental Centers.”  Private residences where education is conducted are not included in the definition.