The Environmental Protection Agency’s (EPA) 2024 updates to the Hazardous Organic National Emission Standards for Hazardous Air Pollutants (HON MACT) expand fenceline monitoring requirements across chemical manufacturing facilities, placing greater emphasis on continuous emissions awareness and rapid response.
The updated rule strengthens monitoring of hazardous air pollutants (HAPs) including benzene, ethylene oxide and vinyl chloride compounds linked to elevated cancer risk and community exposure concerns. According to the EPA’s HON Rule Fact Sheet, the regulation will significantly reduce emissions of toxic air pollutants from chemical manufacturing facilities.
These changes mark a shift from periodic compliance testing to continuous emissions intelligence.
Regulatory relief creates strategic uncertaintyOn July 17, 2025, a federal order titled “Regulatory Relief for Certain Stationary Sources to Promote American Chemical Manufacturing Security” granted temporary exemptions from several EPA rules, including the HON MACT rule, for selected facilities deemed essential to national security and economic resilience.
Covered sectors include facilities producing components for semiconductor manufacturing, defense systems, advanced electronics and medical device sterilization.
The order provides two years of regulatory relief for certain sources. However, the exemption does not eliminate the underlying regulatory expectations. Facilities that delay preparation face significant operational challenges if enforcement resumes or exemptions are narrowed.
The current regulatory landscape creates a high degree of complexity. Organizations can treat these changes or delays as a pause in compliance activities, or they can use them as an opportunity to strengthen operations from minimum compliance into a comprehensive program that drives insights and builds a future-proofed approach.
Expanded fenceline monitoring requirementsThe EPA first introduced fenceline monitoring requirements under the 2018 Refinery MACT rule, which established continuous benzene monitoring using EPA Method 325A/325B passive sampling.
The 2024 HON MACT updates extended similar monitoring obligations to additional facilities that use, produce, store or emit six hazardous air pollutants. Most affected facilities must begin monitoring within two years of the rule’s publication in the Federal Register.
An exception applies to Polymer and Resins I (P&R I) facilities producing neoprene, which must begin chloroprene fenceline monitoring within 150 days of publication.
The rule requires two primary methods:
Monitoring frequency and coverage are defined at the facility fenceline:
Together, these requirements create a monitoring network that operates continuously rather than periodically and offer insight into the future regulatory path ahead.
From monitoring to emissions managementThe most significant operational change introduced by the rule is the requirement for rapid response when emissions exceed action levels. Facilities must now identify emissions sources quickly and document corrective actions with defensible data.
If monitoring results exceed the threshold for any of the six regulated pollutants, facilities must begin a root cause investigation within five days. Initial investigations may include leak detection surveys using EPA Method 21, optical gas imaging (OGI) inspections, visual inspections and operational records and process data review.
If facilities cannot identify the cause of the exceedance within 30 days, the rule requires facilities to deploy real-time monitoring technologies to locate the emission source. These technologies may include mobile gas chromatographs, sensor networks, ultraviolet differential optical absorption spectroscopy (UV-DOAS) and Fourier transform infrared spectroscopy (FTIR).
The result is a monitoring framework that requires both continuous measurement and rapid diagnostics.
Why pilot studies are more important than everEarly implementation efforts suggest that pilot monitoring programs provide a practical pathway for facilities preparing for HON MACT compliance and a forward-looking monitoring strategy.
During Information Collection Request (ICR) monitoring campaigns, several facilities discovered that passive monitoring alone did not capture short-duration emission events. Passive samplers measure 14-day average concentrations, which can dilute intermittent releases such as tank vent emissions or maintenance events, while real-time monitoring reveals short-duration emission spikes and operational events.
Facilities that complemented passive monitoring with real-time gas chromatographs and on-site meteorological towers gained several advantages:
The key lesson from early monitoring programs is clear. Passive monitoring establishes baseline compliance, but real-time monitoring reveals operational risk. Facilities that rely solely on passive sampling may struggle to identify intermittent emissions before exceedance thresholds are triggered.
Designing an effective monitoring strategySuccessful implementation requires more than selecting monitoring equipment. Facilities must design integrated programs that combine monitoring technology, logistics and data systems.
Based on multiple HON MACT pilot studies, several implementation factors consistently determine program success:
Monitoring programs begin with careful site planning and operational coordination. Key components include:
Site mapping and sampling locations: Accurate site maps are essential for determining monitoring locations and ensuring representative fenceline coverage.
Equipment and supply lead times: Monitoring infrastructure such as shelters, canisters, flow controllers and meteorological stations often have procurement lead times exceeding four weeks.
Shipping and field staff coordination: Samples must reach laboratories within required hold times. Facilities should maintain backup staffing to account for holidays, weather disruptions and operational constraints.
Data management and visualization: Fenceline monitoring generates large datasets. Facilities need systems that automatically track sampling results, identify exceedances and visualize emissions trends.
Facilities must balance coverage and operational practicality when designing monitoring networks. Key components include:
Laboratory capabilities play a central role in defensible monitoring programs. Important considerations include:
Many facilities are now supplementing passive monitoring with real-time monitoring technologies. These systems can provide:
No single monitoring configuration works for every facility. Effective programs must reflect site layout, emission sources and operational priorities.
Actions facilities should take nowWith the July 2026 compliance deadline approaching, non-exempt facilities should have implementation plans well under way, as designing and validating a monitoring program typically requires 12 to 18 months.
Facilities preparing for HON MACT compliance should evaluate progress on these five actions:
Pilot monitoring programs: Test passive and real-time monitoring technologies before compliance deadlines.
Site-specific monitoring strategies: Tailor monitoring networks to facility layout, emission sources and operational constraints.
Integrated monitoring data systems: Establish centralized platforms that track monitoring results, identify exceedances and support reporting.
Rapid-response investigation procedures: Develop protocols for root cause analysis and corrective action.
Engagement with communities and stakeholders: Transparent monitoring data to strengthen trust with surrounding communities and regulators.
Facilities that complete this work now will enter the compliance window with tested systems, trained teams and a clear understanding of their emissions profile.
Preparing for the next generation of air monitoringThe HON MACT rule signals a broader shift in environmental oversight. Regulators and communities now expect continuous emissions awareness rather than periodic compliance testing.
The regulatory landscape will continue to evolve, but the need for reliable emissions intelligence will not. Facilities that integrate passive monitoring, real-time technologies and advanced data systems will gain faster diagnostics, stronger regulatory defensibility and improved operational visibility.
Organizations that treat the current transition as an opportunity to modernize monitoring programs gain a competitive advantage and are best positioned to navigate future regulatory requirements.
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