Dear Mr. Faggert,

Currently, Virginia regulations allow data centers to use older diesel generators (Tier II) only during “sudden and reasonably unforeseeable” emergencies. The new guidance proposed by DEQ (Dec 2025/Jan 2026) seeks to redefine “emergency” to include "planned outages" or grid stress events. Essentially, this would allow data centers to fire up thousands of diesel generators simultaneously during heatwaves or peak load times to relieve pressure on the grid, without being required to install the modern pollution controls (Tier IV) usually required for non-emergency power generation.

To the public, redefining an “emergency” sounds bureaucratic. Biologically and physically, it represents a significant shift in exposure risk. By classifying grid capacity issues as "emergencies," facilities can bypass Best Available Control Technology (BACT) standards which poses a significant contaminant threat. Tier II generators release significantly higher levels of Nitrogen Oxides (NOx) and Diesel Particulate Matter (DPM) than Tier IV units. Excess grid stress usually occurs during hot, stagnant summer days. Releasing high volumes of NOx during these specific windows acts as a catalyst for ground-level ozone formation, creating air quality events exactly when populations are most vulnerable to heat stress. Combined with the associated industrial-scale noise and land impacts, many experts have designated data centers to be “incompatible” with residential use (JLARC r. 598). 

The December 2024 JLARC report outlined how unregulated data center growth will increase energy demand beyond commercial enterprise. To maintain grid reliability, infrastructure costs would drive up residential energy bills upwards of $37 a month ($444 annually), and place a disproportionate economic burden on low-income Virginia families. JLARC’s report mostly considers projections of data center growth from 2024, which have already been exceeded in 2026  by +16% 10-year growth rate; +90% Dominion contracted capacity; +1,038% capacity market price; and +170-220% residential bill impact (PJM 2026, Dominion IRP 2025, SCC-PUR-2024-00144). 

We recommend that the Governor’s Office lead DEQ to reject the loosened definition of “emergency” in APG-576. If data centers are to be used as a part of the grid’s reliability strategy (demand response), they would essentially become power plants, and therefore be regulated under mandate Tier-4 controls and real-time monitoring. This would ensure that any generator used for “planned outages” must use selective catalytic reduction (SCR) to remove NOx emission, plan 2-hour battery buffered infrastructure, and establish fenceline air sensors at facilities to alert municipalities when PM2.5 levels spike (PMID: 37584085). 

As the Department of Environmental Quality evaluates the revision of DEQ Guidance Memo APG-576/578, it is instructive to consider how other states are pioneering alternative regulatory frameworks to manage the unprecedented resource demands of data centers. Across the country, state legislatures and utility commissions are actively working to protect residential ratepayers from absorbing the massive infrastructure costs and environmental burdens associated with this industry. For example, California recently advanced SB 57 and SB 886 to establish special tariffs and explicitly prevent data centers from shifting infrastructure and transmission costs onto residents and small businesses. Similarly, Indiana (HB 1007) and Ohio (PUCO  2025) now require large-load AI data centers to cover 80% to 85% of the upfront costs for new, dedicated energy infrastructure and projected electricity usage. Furthermore, Minnesota (HF 16) is tackling physical resource consumption by establishing a distinct utility customer subclass for facilities consuming over 100 million gallons of water annually, highlighting a growing national consensus on the need for targeted, data-center-specific regulations.

This national momentum is strongly reflected here in Virginia, where the General Assembly is currently weighing nearly 60 bills, like HBs 897 related to data centers—nearly double the volume of the previous session. Key legislation mirrors these nationwide efforts to balance economic growth with grid stability and environmental protection. For instance, HB 897 directly ties state tax exemptions to stringent sustainability standards, explicitly requiring operators to transition away from diesel backup generators to non-carbon-emitting power sources. Additionally, HB 284 mandates comprehensive transparency regarding energy, water usage, and emissions. Integrating these legislative priorities—particularly the shift away from diesel generation and enhanced emissions transparency, which V-SCI wholeheartedly endorses—directly into the framework of the DEQ Guidance Memo APG-576/578 Revision will be critical for ensuring Virginia maintains strong environmental stewardship while managing this rapid industrial expansion.

Respectfully,

The Virginia Scientist-Community Interface (V-SCI)

https://virginiasci.org/

virginiasci123@gmail.com

Dear Mr. Faggert,

Virginia has drafted amended guidance proposed by DEQ (March 9, 2026) including provisions related to the testing of source emissions. Revisions to DEQ Guidance Memo APG-576 address presumptive Best Available Control Technology (BACT) requirements for emergency and non-emergency diesel engine-generator sets located at data center stationary sources.

Additional revisions will ensure Virginia maintains adherence to the emissions compliance goals while managing more efficient and accurate emissions assessments.  EMA encourages compliance with the most updated standards as technically appropriate for the given application and mission. 

The following is provided in summary with more detailed proposals included below:

1. Section B – Definitions and Section C – Applicability
   • Update proposed language for BACT applicability to the appropriate section
2. Section D – Emission controls and BACT under Article 6

1. • Avoid suggestion of technology-forcing requirements with specific components (e.g., DOC, DPF); it is critical to achieve specific emissions reductions
     • EPA requirements do NOT specify technology for achieving emissions limits
     • BACT should specifically clarify emissions limits (PM, CO, etc.)

2. • Align NOx limits with federal standards (engines >30 L/cyl)

3. Section F – Emission Limits / Calculations
   • Update language to align with current EPA referenced sections (e.g., 40 CFR Part 1039)
4. Section I – Testing Requirements (data centers) and Section J – Testing Requirements (non-data center source categories)
   • Retire visual emissions evaluations (VEE)
     • Tier 4-certified or compliant systems already certify to ensure compliance with PM limits
     • PM testing follows EPA 40 CFR 1065; accurate emissions sampling procedures specific for measurement of low emissions engines
     • EMA is working with EPA on CI NSPS regulation amendments to alternatively accept 40 CFR 1065 methodologies for stationary source testing
   • Exempt initial stack testing requirements for certified units
     • Performance of initial stack testing for certified engines is redundant as the system was already approved under strict test procedures by the EPA; testing would create emissions without benefit
       • Include language to exempt Tier 4 certified products from any site testing like some AQMDs in California (e.g., SCAQMD)
     • Initial stack testing for uncertified engines is appropriate but should consider potential updates to in-use testing process
       • Update procedures with alternative stationary source requirements under review by EPA
       • Defined modal test points – 90 to 100 percent
       • Shortened minimum test intervals – 15 minutes

EMA is reviewing proposed regulatory amendments to the CI NSPS with the EPA to more effectively support operation of engine-generator sets that meet specific testing requirements while demonstrating emissions performance via on-site testing. Proposals submitted by EMA to Virginia DEQ are consistent with those submitted to EPA and we encourage alignment between federal and state agencies.

• Support proposed alternative of accurate and efficient field testing requirements and methods; essential for Tier 4 certified engines
• Align requirements that avoid redundant stack testing for Tier 4 certified engines; provides no value while increasing site emissions

Respectfully,

The Truck and Engine Manufacturers Association (EMA)

Specific Draft Comments

Section B. Definitions

• Propose to remove the applicability note from the definition of “Data Center”
• Move applicability to section “C. Applicability”

Section C. Applicability

• Propose addressing applicability in section “C. Applicability” with additional clarification of applicability dates

“The data center specific Best Available Control Technology (BACT) provisions of this guidance document will be applied to all air permit applications submitted to DEQ on or after July 1, 2026. The data center specific BACT provisions of this guidance document do not apply to engine-generator sets for which air permit applications have been submitted prior to July 1, 2026.”

Section D. Emission Controls and BACT under Article 6

1. Particulate Emissions
   1. Emergency and non-emergency engines (located in data centers)

• Propose language that better clarifies alignment with Tier 4 emission standards
• Propose language that focuses on specific limits; allow flexibility for the appropriate technology to comply

“Particulate emissions shall be controlled by a diesel particulate filter (DPF), or equivalent (as required to meet the applicable Tier 4 standard. If a diesel particulate filter (DPF) is required, it shall be equipped with a backpressure monitor that notifies the permittee when the high backpressure limit of the engine is approached.”

• Propose alternative language to ‘DPF-level control’ that is a non-specific limit; language should focus on meeting Tier 4 emission standards

“DEQ’s experience with the data center source category does support limiting the application of DPF-level control to T4 emissions standards to gen-sets that do not serve as secondary or tertiary backup power sources (i.e., backup other emergency gen-sets) or do not provide power to non-server resources at the site. Therefore, presumptive BACT...”

• Propose alternative language that focuses BACT to comply with Tier 4 emission standards rather than a specific technology

“However, any new or replacement gen-sets included in a project would be subject to the DPF (or equivalent) applicable Tier 4 PM emission standard presumptive BACT described in this section.”

“As defined in 9VAC5 Chapter 50, a BACT limitation is always a case-by-case determination… The evaluation would need to demonstrate that the site specific factors are so significantly different from the rest of the data center industry that the cost of DPF (or equivalent) control equipment required to meet T4 PM standards for the gen-sets covered by the application is substantially greater than the cost of DPF (or equivalent) control the equipment required to meet T4 PM standards for gen-sets included in the data center source category as a whole. A showing that simply indicates a “high” cost or average cost-effectiveness for DPF (or equivalent) systems emissions reduction systems should not be expected to succeed.”

3. For non-emergency engines (source categories other than data centers)

• Propose language that better clarifies alignment with Tier 4 emission standards
• Propose language that focuses on specific limits; allow flexibility for the appropriate technology to comply

“Particulate emissions shall be controlled by a diesel particulate filter (DPF), or equivalent (as required to meet the applicable Tier 4 standard. If a diesel particulate filter (DPF) is required, it shall be equipped with a backpressure monitor that notifies the permittee when the high backpressure limit of the engine is approached.”

• Propose language that removes reference to ‘cost prohibitive’; language should focus on meeting Tier 4 emission standards

“If a DPF is too cost prohibitive or is not needed to meet the applicable Tier standard, then PM emissions shall be controlled by the use of good operating practices and performing appropriate maintenance in accordance with the manufacturer recommendations.”

2. Carbon Monoxide Emissions
   2. Emergency and non-emergency engines (located in data centers)

• Propose language that better clarifies alignment with Tier 4 emission standards
• Propose language that focuses on specific limits; allow flexibility for the appropriate technology to comply

“Carbon monoxide (CO) emissions shall be controlled by a diesel oxidation catalyst (DOC), or equivalent (as required to meet the applicable Tier 4 standards.)”

• Propose alternative language to ‘oxidation catalyst-level control’ that is a non-specific limit; language should focus on meeting Tier 4 emission standards

“DEQ’s experience with the data center source category does support limiting the application of oxidation catalyst-level control Tier 4 emissions standards to gen-sets that do not serve as secondary or tertiary backup power sources (i.e., backup other emergency gen-sets) or do not provide power to non-server resources at the site. Therefore, presumptive BACT...”

• Propose alternative language that focuses BACT to comply with Tier 4 emission standards rather than a specific technology

“In general… gen-sets that were previously permitted as emergency units and that will remain permitted as emergency units continue to have a presumptive BACT equivalent to that specified for emergency gen-sets for non-data center source categories. However, any new or replacement gen-sets included in a project would be subject to the DOC (or equivalent) Tier 4 CO emission standards presumptive BACT described in this section.”

“As defined in 9VAC5 Chapter 50, a BACT limitation is always a case-by-case determination… The evaluation would need to demonstrate that the site specific factors are so significantly different from the rest of the data center industry that the cost of DOC (or equivalent) control equipment required to meet T4 CO standards for the gen-sets covered by the application is substantially greater than the cost of DOC (or equivalent) control equipment required to meet T4 CO standards for gen-sets included in the data center source category as a whole. A showing that simply indicates a “high” cost or average cost-effectiveness for DOC emissions reduction systems should not be expected to succeed.”

3. 3. For non-emergency engines (source categories other than data centers)

• Propose language that better clarifies alignment with Tier 4 emission standards
• Propose language that focuses on specific limits; allow flexibility for the appropriate technology to comply

" Carbon monoxide (CO) emissions shall be controlled by a diesel oxidation catalyst (DOC). A DOC may be required for this type of engine in order to meet Tier 4 standards."

• Propose language that removes reference to being ‘cost prohibitive’; language should focus on meeting Tier 4 emission standards

“If a DOC is too cost prohibitive or is not needed to meet the applicable Tier standard, then CO emissions shall be controlled by the use of good operating practices and performing appropriate maintenance in accordance with the manufacturer recommendations.”

4. Nitrogen Oxide Emissions
   4. Emergency and non-emergency engines (located in data centers), Emission limit – 0.60 g/hp-hr

• Propose language that clearly specifies the load point(s) at which emissions limits apply; 90% engine-generator set load is proposed based on Section I: Testing Requirements
• Accurate measurement of this emissions limit is not achievable in the field with current reference methods; acceptance of 40 CRF Part 1065 compliant equipment – including PEMS – would support these detection limits

“a. Emergency and non-emergency engines (located at data centers): Emissions limit = 0.60g/hp-hr at 90% engine-generator set load.”

• Provide flexibility for stable catalyst bed temperature
• Propose clarifying language for load point to reflect the generator load; engine loads will be difficult to assess in the field
  • Propose load point to be changed from “…when the engine(s) are operating at or above 20% load…” to 25% of generator load

“(b)Engine exhaust gas shall be treated with urea or an ammonia solution when the engine(s) engine-generator set(s) are operating at or above 20% 25% load and the catalyst bed exhaust temperature of 570°F is achieved, except for periods of start-up or shutdown. The temperature is the preferred metric, but minimum load is available applicable if for some reason (e.g., certain Tier 4-certified units), the catalyst bed temperature is unavailable. Note that both the minimum temperature and load values are general estimates that should be revised if data specific to the SCR and/or gen-manufacturer, which may be higher or lower, is available.”

• Remove explicit catalyst bed exhaust temperature not otherwise specified by the manufacturer; retain limit (930 deg F) if not specified
• Define limit based on ‘exhaust temperature’

“(c)The permittee shall operate the gen-sets and SCR systems such that the catalyst bed exhaust temperature specified by the manufacturer is not exceeded. does not exceed 930°. 930°F may be used in the absence of more specific data from the SCR and/or gen-set manufacturer.”

5. 5. Emergency engines (source categories other than data centers), Emission limit – 6.00 g/hp-hr

• Propose language that clearly specifies the load point(s) at which emissions limits apply; 90% engine-generator set load is proposed based on Section I: Testing Requirements

“b. Emergency engines (source categories other than data centers): Emission limit = 6.0 g/hp-hr at 90% engine-generator set load.”

6. 6. Non-emergency engines (source categories other than data centers), Emission limit – 0.60 g/hp-hr

• Provide flexibility for stable catalyst bed temperature
• Propose clarifying language for load point to reflect the generator load; engine loads will be difficult to assess in the field
  • Propose load point to be changed from “…when the engine(s) are operating at or above 20% load…” to 25% of generator load

“(b)Engine exhaust gas shall be treated with urea or an ammonia solution when the engine(s) engine-generator set(s) are operating at or above 20% 25% load and the catalyst bed exhaust temperature of 570°F is achieved, except for periods of start-up or shutdown. The temperature is the preferred metric, but minimum load is available applicable if for some reason (e.g., certain Tier 4-certified units), the catalyst bed temperature is unavailable. Note that both the minimum temperature and load values are general estimates that should be revised if data specific to the SCR and/or gen-manufacturer, which may be higher or lower, is available.”

• Remove explicit catalyst bed exhaust temperature not otherwise specified by the manufacturer; retain limit (930 deg F) if not specified
• Define limit based on ‘exhaust temperature’

“(c)The permittee shall operate the gen-sets and SCR systems such that the catalyst bed exhaust temperature specified by the manufacturer is not exceeded. does not exceed 930°. 930°F may be used in the absence of more specific data from the SCR and/or gen-set manufacturer.”

7. Visible Emissions (Opacity)

• EPA Method 9 is a subjective test, more prone to variation and error. Suggest a more quantitative test or removal of opacity test requirements.

• The opacity limit of 5% is within the error of an observer; ref., Appendix A-4 to Part 60, Title 40, Method 9  

• • Alternatively, replace with applicable PM measurements methods defined in 40 CFR 1065

• Site Testing requirements for EPA Tier 4 certified units are redundant; remove requirement if maintained according to manufacturer instructions
  • Additional Stack Testing will only increase site emissions
  • EPA is reviewing language to support planned amendments to the CI NSPS
  • Other regional districts have aligned (e.g., SCAQMD)

Section F. Emission Limits / Calculations

• Propose regulatory alignment; EPA migrated regulatory requirements for engines under 40 CFR Part 89 to 40 CFR part 1039 in 2021

“NSPS standards should not be used or compared to the manufacturer specifications… The NSPS standards specified in 40 CFR Part 1039 Table 2 to Appendix I Table 1 (40 CFR 89.112) are measured using the procedures in 40 CFR 1039 Subpart C Subpart E of 40 CFR Part 89. Looking closely into Subpart F (Part 1039)E (89.404), the test cycles consist of various steady state operating modes)…”

1. Uncontrolled Emissions for Permit Applicability

• The definition references “manufacturer Not to Exceed”
  • Request clarification of the NTE reference definition; 40 CFR 1039 or 40 CFR 60.4212(c)
  • It is a common error to use the terms “Not to Exceed” and “Maximum Potential to Exceed” interchangeably

• Calculation should establish emissions limits from the exhaust of the system including the aftertreatment
• The definition for “Uncontrolled Emissions” requires an update to clarify that emissions from Tier 4 engine-generator sets be taken at the exhaust of the system
• Air pollution control equipment is considered as part of the engine system; separating the two is not representative

“Uncontrolled emissions for permit applicability should be calculated using the manufacturer Not To Exceed (NTE) (or equivalent) emission factors at the worst case load for that specific engine and pollutant. Load is pollutant specific. For example, NOX will have the worst case emission factor at 100% load while PM may have a worse case emission factor at 25% load. Uncontrolled emissions measurements for engine-generator sets which include either a 3^rd party emissions control unit, or include an emission control system as part of an EPA T4 certification, must be measured at the system exhaust.”

Section I. Testing Requirements (data centers)

AND

Section J. Testing Requirements (non-data center source categories)

• Accept alternative PM measurements methods including those defined in 40 CFR 1065
  • Include additional language that each of the three test runs must be at least 15 minutes; aligns with current proposal to EPA for planned amendments to the CI NSPS

• Site Testing requirements for EPA Tier 4 certified units are redundant; remove requirements if maintained according to manufacturer instructions
  • Additional Stack Testing will increase site emissions
  • EPA is reviewing language to support planned amendments to the CI NSPS
  • Other regional districts have aligned (SCAQMD)

“Initial VEE PM  Stack Testing: This boilerplate does contain a requirement for PM emissions an initial visible emissions evaluation for emergency (and non-emergency) gen-sets located at data center stationary sources. PM measurement An initial visible emissions evaluation shall be conducted according to the requirements listed in 40 CFR 60.4212. to show that the unit is in compliance with the appropriate opacity standard. Each test shall consist of 30 sets of 24 consecutive observations (at 15 second intervals) to yield a six minute average. Testing shall be conducted with the engine(s) operating at greater than 90% capacity, unless multiple load band testing is approved by DEQ. Engine generator-sets containing engines certified to EPA Tier 4 are exempt from these stack testing requirements.”

“Initial Stack Testing:

Emissions testing of each pollutant for each selected engine-generator set shall consist of three one-hour test runs under load. Each test run must last at least 15 minutes. The average of the three runs shall be reported as the short-term emission rate for that engine-generator set. Testing shall be conducted with the engine(s) operating at greater than 90% capacity, unless multiple load band testing is approved by DEQ. Engine generator-sets containing engines certified to EPA Tier 4 are exempt from these stack testing requirements.”

• Propose a reduction of the Initial VEE and Stack Testing application
  • Reduce from 20-25% to 10% for each type of generator-set
  • May alternatively align with the same provision in section J. Testing Requirements
  • Excessive Stack Testing increases site emissions without additional benefit

“Initial VEE PM Stack Testing and Stack Testing: The initial compliance determination testing described above will typically be applied to 10% 20-25% of the units for each type of gen-set permitted at a data center.”

• Add language to clarify field measurement procedures aligned with EPA for engines with a displacement ≥ 30 liters per cylinder; reference 40 CFR Subpart

April 8, 2026

Stanley Faggert 

Air and Renewable Energy Division 

Virginia Department of Environmental Quality 

1111 E. Main Street, Suite 1400 

Richmond, VA 23219

Re: Diesel Engine-Generator Set Procedure for Writing New and Modified Permits – DEQ Guidance Memo APG-576

Dear Mr. Faggert:

On behalf of the Sierra Club Virginia Chapter, we write to express our strong support with caveats for the Department of Environmental Quality's proposed revision to DEQ Guidance Memo APG-576. The Sierra Club Virginia Chapter urges DEQ to finalize and rigorously implement this revision, which represents a critical step toward protecting the health of Virginia's communities — especially those already bearing a disproportionate burden of air pollution — from the growing environmental threat posed by data center diesel generator emissions.

The Sierra Club is the nation's oldest and largest grassroots environmental organization, with more than 3.8 million members and supporters nationwide. The Sierra Club Virginia Chapter has long championed clean air, public health, and environmental justice across the Commonwealth. 

Air Pollution and Community Health: The Urgent Case for Action

The Sierra Club's foremost concern regarding data center diesel generators is their direct impact on air quality and community health. These facilities routinely rely on diesel generators and the emissions they produce carry serious public health consequences.

Diesel exhaust is a toxic mixture containing harmful concentrations of particulate matter (PM2.5), nitrogen oxides (NOx), sulfur dioxide (SO?), and carbon dioxide (CO?). These pollutants do not stay at the facility fence line — they travel into surrounding neighborhoods, schools, and homes, with particularly acute effects on:

• Children, whose developing lungs are especially vulnerable to particulate matter and ozone formed by NOx emissions;

• Older adults, who face heightened risk of cardiovascular and respiratory complications from diesel exhaust exposure;

• Individuals with pre-existing conditions such as asthma, COPD, or heart disease, for whom diesel pollution can trigger acute health crises; and

• Environmental justice communities — often lower-income and communities of color — that already bear disproportionate pollution burdens and are frequently located in close proximity to industrial development.

Research has firmly established that diesel exhaust exposure contributes to the development and aggravation of chronic conditions including asthma and other respiratory diseases, cardiovascular disease, and premature death. 

Beyond direct health harms, diesel exhaust exacerbates climate change, contributes to acid rain, and degrades regional air quality through smog formation — impacts that fall hardest on those with the least capacity to protect themselves.

This Guidance Memo represents an important first regulatory response to these documented harms. The Sierra Club strongly supports measures that meaningfully reduce diesel exhaust emissions from data center operations, and we urge DEQ to treat this rule as a baseline — not a ceiling — for future protective action.

APG-578 and the Expanded Use of Emergency Generators

The urgency of the Guidance Memo is compounded by recent changes to DEQ's related Guidance Memo APG-578. As of December 2025, APG-578 broadly interprets the definition of "emergency" under the Virginia Administrative Code, permitting facilities to operate backup generators during planned electrical outages — events for which facilities may have had up to fourteen days' advance notice.

The Sierra Club has serious concerns about this interpretation. The Virginia Administrative Code requires that an "emergency" be "reasonably unforeseeable." Permitting diesel generators to run during knowingly planned outages stretches this definition beyond its plain meaning and, critically, significantly increases the total hours of diesel exhaust exposure for communities surrounding data centers. More frequent generator operation means more pollution, more health impacts, and greater cumulative harm to communities.

In this context, APG-576's requirement that generators meet Tier 4 equivalence — for both emergency and non-emergency use — is essential. If generators will run more frequently, including during events that are plainly not emergencies, then the strictest available pollution controls must apply. The Sierra Club fully endorses this approach.

Zooming out, Sierra Club is deeply concerned about the scope of and reliance on “non-emergency” generators and believes this guidance inadequately addresses the potential impacts of generators that lack constraints. These generators have historically been installed to meet emergency back-up functions, and not to serve as primary power or to invite more intense usage. We urge the Department of Environmental Quality to issue further guidance that accomplishes the following:  

1. Specificity in guidance about how and when generators can be used by placing 500 hour limits on all diesel generators.

2. Public notification when generators run for 4 or more hours. 

   1. The public, and especially those residents compromised by conditions that can be exacerbated by diesel pollution and emissions, need to know when particulate matter (PM2.5), nitrogen oxides (NOx), sulfur dioxide (SO?), and carbon dioxide (CO?) levels may be elevated. 

   2. Special notification standards should be developed to notify infant or early childhood daycares, preschools, elementary schools, secondary schools, hospitals and senior centers or living facilities.

3. The Department of Environmental Quality should define “cumulative impact.” 

   1. In areas where data centers are clustered and where cumulative emissions and pollutants can be far greater than the limits of one stand-alone permit, impacted communities are experiencing “cumulative impacts.” Air quality permits should consider “cumulative impact” on both local ecology and communities.    

Tier 4 Generators as Best Available Control Technology

The Guidance Memo appropriately designates Tier 4-equivalent generators as Best Available Control Technology ("BACT") for new data center stationary sources. The Virginia Administrative Code defines BACT as an emissions limitation based on the maximum degree of emission reduction achievable through the application of available methods, systems, and techniques, taking into account energy, environmental, and economic impacts. The Air Pollution Control Board is further instructed to consider the nature and amount of emissions, control efficiencies achieved in the industry, and total cost effectiveness.

Tier 4-equivalent generators satisfy each of these criteria:

• Emissions reductions: Tier 4 generators emit approximately 90% fewer pollutants than lower-tier alternatives, dramatically reducing nitrogen oxide, particulate matter, and other harmful emissions.

• Technological availability: Tier 4 generators incorporating selective catalytic reduction systems, diesel oxidation catalysts, and diesel particulate filters are commercially available and widely deployed in the data center industry.

• Cost effectiveness: The Guidance Memo establishes a rigorous standard for bypassing the Tier 4 presumption, requiring applicants to demonstrate site-specific factors of such uniqueness that the associated costs are substantially greater than those for the broader source category. Merely citing high costs will not suffice. This framework appropriately places the burden on industry to justify any deviation.

The Sierra Club firmly agrees with DEQ's determination that Tier 4-equivalent technology represents BACT for data center generators. The public health and environmental benefits of this designation far outweigh the incremental compliance costs for an industry known for its substantial capital investment.

H.B. 507 and the Importance of Regulatory Alignment

The Guidance Memo complements H.B. 507, which passed both chambers of the 2026 Virginia General Assembly and currently awaits Governor Spanberger's signature. H.B. 507 would amend the Code of Virginia to require that data center permits issued after July 1, 2026, reflect emission limits at or below those achievable by a Tier 4 generator.

By defining presumptive BACT for data centers to include the use of selective catalytic reduction systems, diesel oxidation catalysts, and diesel particulate filters, the Guidance Memo operationalizes the legislative intent behind H.B. 507. This alignment of regulatory guidance with statutory direction reflects sound policy and ensures that the Commonwealth's environmental commitments are implemented consistently and effectively.

The Sierra Club strongly encourages DEQ to finalize the Guidance Memo promptly and to ensure its effective enforcement so that data centers receiving permits on or after July 1, 2026, are held to the Tier 4-equivalent standard in both letter and spirit.

Virginians deserve to breathe clean air. As data centers multiply across the Commonwealth, the cumulative pollution burden on surrounding communities will only grow unless strong, enforceable standards are in place. Guidance Memo APG-576 is exactly the kind of protective action DEQ must take to meet its obligation to safeguard public health and the environment. 

Respectfully submitted,

Corrina Beall, Legislative and Political Director 

Sierra Club Virginia Chapter