What is the HON Rule?
The 2024 amendments to the USEPA’s Hazardous Organic NESHAP (HON Rule) revise standards under 40CFR Part 63 for facilities in the Synthetic Organic Chemical Manufacturing Industry (SOCMI) and related Polymers and Resins I & II sectors. The rule applies to any facility that uses, produces, stores, or emits one or more of six targeted hazardous air pollutants (HAPs). Together, these changes affect more than 600 chemical plants nationwide, including over 125 facilities required to conduct fenceline monitoring and nearly 300 with updated flare provisions.
Key revisions include:
HON Fenceline Monitoring Requirements
Fenceline Monitoring programs for six air toxics: ethylene oxide (EtO), benzene, 1,3-butadiene, chloroprene, ethylene dichloride, and vinyl chloride.
Revised Flare Requirements
New combustion and monitoring standards for flares, including compliance with MACT CC requirements for Net Heating Value (NHV), visible emissions, pilot monitoring, and updated Flare Management Plans (FMPs).
Removal of Startup, Shutdown, and Malfunction (SSM) Exemptions
All emission sources are now required to meet applicable control and performance standards at all times, including during events that were previously exempt.
Compound-Specific Rules
- EtO: Max 20 tons/year to flares, PRD releases considered violations, control to 1 ppmv monthly records required.
- Chloroprene: Max 1 tpy to flares (3.8 tpy for neoprene production), vent to non-flare controls
- Dioxins & Furans: Must be reduced to 0.054 ng/scm (dry basis at 3% O2)
Mandatory Public Reporting
Quarterly requirement of fenceline data to be submitted to EPA and made public via CEDRI one year after the program starts.
Recent HON Updates
On July 17, 2025, a presidential declaration granted two-year deferrals to 52 chemical facilities, citing national security and manufacturing supply chain impacts. For all other facilities, the original compliance schedule remains in effect.
How to Prepare for HON Rule Compliance
Whether your site received an exemption or not, compliance requirements are imminent. The facilities that utilize this window to prepare will have more flexibility to refine their system design, allocate budgets, and avoid last-minute implementation hurdles.
Here are some steps you can take now to position your facility for success as the compliance deadline approaches.
Run Pilot Studies to Gather Baseline Data
Establishing a pre-compliance emissions baseline helps your team understand where high concentrations are most likely to occur. Pilot studies can be used to determine site-specific factors, such as wind direction, source proximity, and facility layout. These variables can significantly influence the accuracy and representativeness of your fenceline monitoring program. With this data, your facility can optimize monitor placement and address high-priority areas before investing in permanent fenceline monitoring systems.
Identify Root Causes Before Public Reporting Begins
Once fenceline monitoring compliance deadlines are reached, any exceedance of the annual average action levels will require a root cause analysis (RCA), with results submitted to the EPA and made publicly available through CEDRI. If your facility cannot determine the cause of an exceedance within 30 days, the rule requires you to initiate real-time monitoring using technologies like mobile gas chromatographs or optical spectroscopy. If the source still cannot be identified after 48 hours, additional monitors must be added or relocated until the emissions source is confirmed.
Short-term deployments such as mobile GC emission studies can give your team space to identify unusual concentrations and hotspots early, allowing time to respond internally before formal CEDRI reporting begins. These studies can be especially useful for facilities that share a fenceline, as they provide the data needed to identify potential emission sources that may be crossing the perimeter from another facility.
Assess Your Flare System and Flare Management Plan
The final rule amendments reference MACT CC requirements for flares, introducing tighter combustion efficiency standards and expanded monitoring expectations. Facilities must comply with updated thresholds for Net Heating Value (NHV), flare tip velocity (Vtip), and combustion zone parameters (NHVcz), all verified through Continuous Parameter Monitoring Systems (CPMS).
Additionally, many existing Flare Management Plans (FMPs) were developed prior to these new requirements and do not reflect current operating standards. Updating your FMP now allows your team to align documentation with actual operations, assess whether instrumentation or control upgrades are necessary, and prevent rushed capital expenditures and production shutdowns later.
Clarify State vs Federal Requirements
Even with a federal delay, your state agency may still require emissions controls, fenceline monitoring, or early action. Some states, such as California and Colorado, have their own ambient air monitoring programs and air toxics thresholds that may apply even if a federal exemption is in place. Collecting defensible data can help clarify where requirements apply and give your team evidence for negotiating timelines, permits, or variance requests.
Learn More
Access our presentation, Advanced Fenceline Monitoring: EPA HON Requirements and the Role of Real-Time Technologies, for the latest in HON monitoring strategies can support compliance and public reporting requirements.
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