Compliance · US EPA
EPA Hazardous Organic NESHAP (HON): fenceline monitoring (40 CFR 63.184)
Ambient air: chemical plant fenceline (USA)
The EPA's 2024 update to the Hazardous Organic NESHAP (HON) extended the refinery fenceline approach to synthetic organic chemical manufacturing and to polymers and resins producers. Under 40 CFR 63.184, affected facilities monitor six air toxics at the fenceline and compare the annual average Δc for each against an action level: benzene 9 µg/m³, 1,3-butadiene 3 µg/m³, vinyl chloride 3 µg/m³, ethylene dichloride 4 µg/m³, chloroprene 0.8 µg/m³, and ethylene oxide 0.2 µg/m³. Exceeding one triggers a root cause analysis and corrective action.
The standard
What the HON Fenceline Requirement Is
Benzene, 1,3-butadiene, chloroprene, and ethylene dichloride are sampled with passive tubes over 14-day periods under EPA Methods 325A and 325B; ethylene oxide and vinyl chloride are collected in canisters over 24 hours every five days under EPA Method 327.
Limit values
EPA HON Limit Values
Every value US EPA publishes for the parameter below, with its averaging period, as the standard states it.
Who it applies to
Who It Applies to
Facilities subject to the HON (40 CFR 63 Subparts F, G, and H) and the related polymers and resins rules. Existing sources began fenceline monitoring by July 15, 2026, with corrective action applying from July 15, 2027; chloroprene monitoring at neoprene plants started earlier, on October 15, 2024. Presidential exemptions granted in 2025 and 2026 give the named facilities two more years.
Monitoring
How Monitoring Demonstrates Compliance
Compliance is the laboratory-analyzed sampler and canister record, the Δc for each sampling period, and the rolling annual averages against the action levels. Continuous monitoring does not replace it, and of the six compounds only benzene is among the parameters our instruments measure. Continuous boundary monitoring with wind data supports the program around it: tracing the timing and direction of releases, which is what a root cause analysis needs.
Recommended
best match-
Thiamis connects 1 instrument
A gateway that connects the third-party instruments you choose to the Aethair Platform.
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Defiant Technologies Specialty Air Analyzers
BTEX / speciated VOCDefiant Technologies FROG portable gas chromatograph (GC-PID).
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Who measures each parameter
| Parameter |  Aethair PRO |  Aethair IAQ | External |
|---|---|---|---|
| BTEX / speciated VOC | — | — | 1 instrument via Thiamis |
Devices
Devices That Cover EPA HON
How much of this standard one Aethair monitor covers, followed by the third-party instruments in the sensor catalog that measure its parameters. Each connects to Environet through a Thiamis gateway, which adds whatever the monitor leaves out.
Covers it on its own
Solutions
Solutions Judged Against EPA HON
Oil & Gas / Petrochemical
Fence-line VOC, H₂S, and methane leak detection
Article Library
Articles on EPA HON
Fenceline Air Quality Monitoring: Equipment and Approaches
Compare fenceline air quality monitoring approaches: standalone stations, pelican-case kits, and connected 4G sensor networks.
Industrial Perimeter Air Quality Monitoring: Action Levels
What industrial perimeter monitoring requires at fenceline and construction sites: DER-10 CAMP action levels, EPA fenceline rules, and continuous records.
EHS Compliance Reporting Guide: Defensible, Audit-Ready Data
EHS compliance reporting needs defensible, audit-ready documentation. Build a workflow that holds up to OSHA and EPA audits and NAAQS benchmarks.
Questions
EPA HON FAQ
What are the HON fenceline action levels?
Annual average Δc action levels of 9 µg/m³ for benzene, 3 µg/m³ for 1,3-butadiene, 3 µg/m³ for vinyl chloride, 4 µg/m³ for ethylene dichloride, 0.8 µg/m³ for chloroprene, and 0.2 µg/m³ for ethylene oxide.
When does HON fenceline monitoring start?
Existing sources began monitoring by July 15, 2026, and corrective action applies from July 15, 2027. Chloroprene monitoring at neoprene production started on October 15, 2024.
How are the HON compounds sampled?
Benzene, 1,3-butadiene, chloroprene, and ethylene dichloride with passive tubes over 14-day periods under EPA Methods 325A and 325B; ethylene oxide and vinyl chloride with canisters over 24 hours every five days under EPA Method 327.
Monitor Against EPA HON
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