The 2024 HON rewrite didn’t set one deadline — it set several, and a 2025 Presidential Proclamation added another. Here’s how to tell which clock is actually yours.
The baseline
The 2024 Hazardous Organic NESHAP (HON) amendments (40 CFR Part 63) became effective July 15, 2024 and, for the first time, require fenceline monitoring at chemical plants for six air toxics: ethylene oxide, chloroprene, benzene, 1,3-butadiene, ethylene dichloride, and vinyl chloride. For most facilities, fenceline monitoring begins July 15, 2026.
The three (or four) clocks
| Situation | The date to track |
|---|---|
| Chloroprene at neoprene production facilities | Began 90 days after effective — Aug 2024 (already live) |
| General fenceline monitoring, most facilities | July 15, 2026 |
| Certain requirements (per 2025 revisions) | Revised to July 5, 2027 |
| Facilities on the 2025 Proclamation list | ~2-year extension from the original date |
What to do now
- Confirm which of the six pollutants you use, produce, store, or emit
- Determine your facility’s actual compliance date — baseline, 2027 revision, or proclamation list
- If your date is 2026, start siting samplers and building the data workflow now — that year goes fast under audit scrutiny
Frequently asked
When does HON fenceline monitoring start?
For most facilities, fenceline monitoring for the covered air toxics begins July 15, 2026. Chloroprene at neoprene facilities started earlier (August 2024), certain requirements were revised to July 5, 2027, and a facility-specific list received a further extension — so your exact date depends on your facility.
Does the 2025 HON extension apply to my plant?
Not automatically. The July 2025 Presidential Proclamation extended deadlines for a specific, named list of facilities — reported as about a quarter of HON-affected sites. The majority remain on the original schedule. You have to confirm your facility is actually on the list rather than assume it.
Which pollutants require HON fenceline monitoring?
Six air toxics: ethylene oxide, chloroprene, benzene, 1,3-butadiene, ethylene dichloride, and vinyl chloride. Whether you’re covered depends on which of these your process units use, produce, store, or emit.
This is general information, not a regulatory applicability determination or legal advice. Rules, deadlines, and how they apply change and depend on your specific equipment, permit, and facility. Confirm your obligations with a qualified professional or your permitting authority.
Two tools that keep chemical plants ahead of the HON
The same regulatory engine we run for clients, put in your hands.
EHS Compliance Hub
Track every LDAR rule, Federal Register update, and compliance deadline that hits chemical plants — in one place, updated live. Free to use.
QuickScope™
Database-driven field inspections powered by the same rules engine as the Hub — built to make LDAR surveys faster and audit-ready.