---
title: "EPA Method 325A/B: Passive Fenceline VOC Sampling | Aethair"
description: "EPA Methods 325A and 325B, in Appendix A to 40 CFR Part 63, are the reference method for measuring volatile organic compounds at a facility fenceline with…"
url: https://aethair.io/standards/epa_method_325/
---

Measurement method · US EPA

# EPA Method 325A/B: passive sorbent-tube sampling of fenceline VOCs

EPA Methods 325A and 325B, in Appendix A to 40 CFR Part 63, are the reference method for measuring volatile organic compounds at a facility fenceline with **passive diffusive sorbent tubes**. Method 325A covers where and how the samplers are deployed around the property boundary; Method 325B covers preparing the tubes and analyzing them by thermal desorption and gas chromatography in a laboratory. The tubes are exposed for a sampling period, typically **14 days**, and the result is an average concentration for that period at each location.

[Schedule a call](https://calendly.com/d/ctdd-mkr-v9v/aethair-demo-website) [All standards](https://aethair.io/standards/)

At a glance

**US EPA** Measurement method

1 parameter

[BTEX / speciated VOC](https://aethair.io/parameters/btex/)

[Official text from US EPA](https://www.ecfr.gov/current/title-40/chapter-I/subchapter-C/part-63/appendix-Appendix%20A%20to%20Part%2063)

Who uses it

## Who Uses It

Petroleum refineries under the Refinery Sector Rule ([40 CFR 63.658](https://aethair.io/standards/epa_refinery_fenceline/)) for benzene, and chemical plants under the 2024 HON fenceline requirement ([40 CFR 63.184](https://aethair.io/standards/epa_hon_fenceline/)) for benzene, 1,3-butadiene, chloroprene, and ethylene dichloride. Both rules compute Δc (the highest minus the lowest fenceline result per period) from Method 325 samples, and both let an approved alternative, such as open-path monitoring, replace samplers if it gives equal or better spatial coverage.

Monitoring

## How It Relates to Continuous Monitoring

Method 325 gives a laboratory-quality average over two weeks, not a time series: it shows that concentrations were elevated, not when or from which direction. Continuous boundary monitoring with wind data fills that gap, which is why the fenceline rules point facilities to real-time techniques when a root cause cannot be found from the passive samples alone.

Solutions

## Solutions That Rely on EPA Method 325

### [Oil & Gas / Petrochemical](https://aethair.io/solutions/oil-gas/)

Fence-line VOC, H₂S, and methane leak detection

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Article Library

## Articles on EPA Method 325

### [How to Choose an Air Quality Monitor: PRO, IAQ, or Thiamis](https://aethair.io/resources/choosing-aethair-monitoring-device/)

Compare Aethair PRO, IAQ, and Thiamis to match the right device to your site, from commercial interiors to industrial and fenceline monitoring.

### [Industrial Perimeter Air Quality Monitoring: Action Levels](https://aethair.io/resources/perimeter-air-quality-monitoring-guide/)

What industrial perimeter monitoring requires at fenceline and construction sites: DER-10 CAMP action levels, EPA fenceline rules, and continuous records.

[All articles](https://aethair.io/resources/)

Questions

## EPA Method 325 FAQ

What is EPA Method 325A/B?

The reference method for fenceline VOCs with passive diffusive sorbent tubes: 325A for deploying the samplers around the boundary, 325B for preparing and analyzing them in a laboratory.

How long are Method 325 samplers exposed?

Under the refinery and HON fenceline rules, for contiguous 14-day sampling periods.

Which rules use Method 325?

The EPA Refinery Sector Rule for benzene at petroleum refineries, and the 2024 HON fenceline requirement for benzene, 1,3-butadiene, chloroprene, and ethylene dichloride at chemical plants.