The EPA’s Hazardous Organic NESHAP (HON) rule — updated in 2024 — significantly expanded fenceline monitoring requirements for chemical manufacturing facilities. If your facility handles any of the 200+ regulated hazardous air pollutants (HAPs), you likely face new obligations. This guide explains what the rule requires and what monitoring technology actually meets those requirements.

What Does the HON Rule Require for Fenceline Monitoring?

The updated HON rule requires continuous or periodic fenceline monitoring for specific HAPs including benzene, 1,3-butadiene, chloroprene, ethylene dichloride, and vinyl chloride. Key requirements include:

  • Perimeter coverage: Monitoring must cover the full fence line of the facility, with no gaps in detection.
  • Real-time or near-real-time data: Facilities must be able to detect and respond to elevated concentrations promptly.
  • Data defensibility: Readings must be logged, archived, and retrievable — particularly important if used in enforcement actions.
  • Multi-compound capability: Because multiple HAPs are regulated simultaneously, single-gas monitors often fall short.

Why Single-Gas Electrochemical Monitors Often Fall Short

Many facilities initially consider conventional point-source electrochemical sensors. While lower in upfront cost, these have significant limitations for HON compliance:

  • They measure only one compound per sensor, requiring multiple units for multi-HAP compliance.
  • Electrochemical sensors require frequent calibration and have limited lifespan in harsh field conditions.
  • They provide point measurements, not path-integrated readings — meaning gaps in perimeter coverage are common.
  • They generally cannot provide the defensible spectroscopic data that regulators increasingly expect.

Open Path FTIR and UV-DOAS: The Compliance-Grade Solution

Open path analyzers using FTIR (Fourier-Transform Infrared) or UV-DOAS (Ultraviolet Differential Optical Absorption Spectroscopy) technology address all of these limitations:

  • Path-integrated measurement: A single transmitter-receiver pair monitors the entire air column across a path up to 1 kilometer — providing true gapless perimeter coverage.
  • Multi-compound simultaneous detection: One instrument can monitor benzene, 1,3-butadiene, chloroprene, vinyl chloride, and dozens of other compounds at the same time.
  • Parts-per-billion sensitivity: Detection limits reach parts-per-trillion for some compounds — far exceeding HON action level thresholds.
  • Archived spectral data: Raw spectrographic data is stored for up to one year, providing defensible records that have held up in legal proceedings.

Cerex Solutions for HON Rule Compliance

Cerex open path analyzers were specifically engineered to meet fenceline monitoring regulations, including the HON rule, EPA Method 325 A/B, SCAQMD Rule 1180, and BAAQMD Refinery Regulation 12 Rule 15.

  • Air Sentry FTIR: Open path FTIR for simultaneous monitoring of dozens of HAPs and VOCs across long perimeter paths.
  • UV Sentry: UV-DOAS open path analyzer optimized for BTEX, 1,3-butadiene, SO₂, and other UV-absorbing compounds with exceptional sensitivity.
  • Aeres: An integrated fenceline monitoring system purpose-built for HON and similar regulations, with turnkey hardware and software.

What to Ask When Evaluating Fenceline Monitoring Equipment

When evaluating options, ask vendors the following:

  • Can the system simultaneously monitor all the HAPs listed in my HON permit?
  • What is the detection limit for benzene and 1,3-butadiene specifically?
  • How is raw data archived and for how long?
  • Has the system’s data been used in regulatory or legal proceedings?
  • What is the calibration frequency and maintenance burden?
  • Is the system compliant with EPA Method 325 A/B?

Cerex application engineers are available to review your specific facility layout and HON obligations, and recommend the right configuration. Contact us to discuss your project.