The regulatory landscape governing hazardous air pollutants (HAPs) continues to evolve, with the recent finalization of the Hazardous Organic NESHAP (HON) Maximum Achievable Control Technology (MACT) rule. This landmark development affects Synthetic Organic Chemical Manufacturing Industry (SOCMI) facilities and Group I and II Polymers and Resins Industry facilities.

The regulatory burden for measurement and reporting of HAPs within the facilities industrial processes as well as the fenceline that surrounds the facility will be substantially increased.  Many of these facilities will be required to perform ongoing fenceline monitoring using EPA Methods 325A/B and/or EPA Method 327. Let’s explore the key aspects of this final rule and what it means for industry stakeholders.

Implementation Timeline:

One notable provision of the final HON MACT rule is the introduction of a two-year implementation period before fenceline monitoring (FLM) becomes mandatory. This grace period offers facilities valuable time to prepare and adapt to the new requirements. The implementation date will be two years plus 60 days after the rule is published in the Code of Federal Regulations (CFR). We strongly recommend that facilities conduct FLM “pilot studies” to determine where they stand with regards to the regulatory limits and to allow for remedial action prior to the rules becoming enforceable.

Applicability and Action Limits:

Facilities with HON units that handle benzene, 1,3-butadiene, 1,2-dichloroethane, chloroprene, vinyl chloride, and/or ethylene oxide are subject to FLM requirements. These compounds are considered primary inputs, products, or waste/emissions streams within the scope of the rule.

Action limits, which determine compliance thresholds, are defined as the difference between the highest and lowest concentrations measured at all sampling locations averaged over a rolling 12-month period (ΔC). Notably, these action limits vary for each compound, with benzene at 9µg/m³, 1,3-butadiene at 3µg/m³, 1,2-dichloroethane at 4µg/m³, chloroprene at 0.8µg/m³, vinyl chloride at 3µg/m³, and ethylene oxide at 0.2µg/m³. Additionally, specific action limits apply for chloroprene in the neoprene production source category.

Fenceline Monitoring Methods:

FLM will be conducted using EPA Methods 325A/B for benzene, 1,3-butadiene, 1,2-dichloroethane, and chloroprene. These methods employ passive sampling of solid-adsorbent tubes over 14-day periods, a technique already familiar in the petroleum refinery sector.

For vinyl chloride and ethylene oxide, newly promulgated EPA Method 327 will be utilized. This method employs evacuated summa canisters to collect whole air samples over a 24-hour period every 5 days. Notably, Method 327 presents logistical challenges, particularly the requirement for sampling at all 8 sample points to commence within one hour.

Root Cause Analyses and Real-Time Monitoring:

Facilities exceeding actionable ΔC thresholds for any of the specified compounds must initiate root cause analyses (RCAs) within 5 days. RCAs may involve various investigative techniques, including leak inspections and operator assessments. If the root cause cannot be determined within 30 days, real-time monitoring techniques become mandatory to identify the source of the exceedance.

Site-Specific Monitoring Plans and Burden Reduction:

The final rule provides an option for facilities to implement site-specific monitoring plans (SSMPs) to address offsite sources impacting fenceline compound concentrations. Additionally, a burden reduction provision allows facilities with two years of continuous measurements below 10% of the action limit to skip fenceline measurement periods for specific monitors. However, this exemption does not apply to ethylene oxide and chloroprene.


As the final HON MACT rule ushers in a new era of regulatory compliance, affected facilities must navigate these changes diligently. From implementing FLM methodologies to conducting RCAs and potentially deploying real-time monitoring, proactive engagement with the requirements is paramount. By understanding and adhering to the stipulations outlined in the rule, industry stakeholders can uphold environmental stewardship while ensuring operational continuity in an evolving regulatory landscape. The HON MACT regulatory burden on facilities is significant, as is the introduction of fenceline HAPs monitoring with new analytical methods.

At Enthalpy Analytical our scientists, engineers and project managers are working alongside industry and regulators to develop robust sampling and analytical approaches to the fenceline monitoring requirements.  For more information, reach out to our experts on this page.

Clint Thrasher

Laboratory Director, Enthalpy Analytical
Clint Thrasher is the Laboratory Director at Enthalpy Analytical in Deer Park, Texas. He has nearly 20 years of experience in the environmental laboratory industry, specializing in the analysis of gas-phase samples. Clint has been employed with Enthalpy since 2005 and in that time has had four different job titles: FTIR/GC analyst, field testing team leader, staff engineer, and, finally, laboratory director. He is extremely well-versed in all of Enthalpy’s laboratory processes. Outside of work, Clint is involved in the Air and Waste Management Association (AWMA) Gulf Coast.
Clint received a BA in chemistry and a BS in chemical engineering from North Carolina State University and is a licensed Professional Engineer (North Carolina).