EPA Method 204: Controlling Fugitive VOC Emissions

This article was updated in July 2026 to reflect current EPA Method 204 guidance.

Fugitive VOC emissions are the uncontrolled release of gases to the atmosphere — vapors that never make it into a control device at all. A well-built thermal oxidizer, RTO, or catalytic oxidizer routinely destroys 95–99%+ of the VOCs that reach it, but that destruction rate only counts toward your permit if the emissions actually get there.

Overall control efficiency is: Overall Control (%) = Capture Efficiency (%) × Destruction Efficiency (%) ÷ 100

If your capture system is only 70% effective and your oxidizer destroys 98% of what reaches it, your overall compliance number is 68.6% — often not enough to satisfy a permit limit. This is why fugitive emissions are frequently the single largest source of uncontrolled VOCs at a facility, and why regulators require a documented, tested capture strategy before crediting your oxidizer's destruction rate at all.

What is EPA Method 204?

EPA Method 204 is the promulgated test method that sets the design and verification criteria for the enclosures used to demonstrate VOC capture efficiency ahead of a control device. It defines two enclosure types:

Permanent Total Enclosure (PTE) — a fixed structure, built into the process, that contains fugitive VOC emissions and routes them to an oxidizer on an ongoing basis.
Temporary Total Enclosure (TTE) — a structure built specifically to support a compliance test, then removed or opened afterward.

If your enclosure meets Method 204's criteria, regulators generally allow capture efficiency to be assumed at or near 100%, simplifying both testing and ongoing recordkeeping. If it doesn't meet the criteria, actual capture efficiency testing (Methods 204A–204F) is typically required to quantify what's really reaching the oxidizer.

The full method text and EPA's own FAQ are useful references if you're working through a design or permit application:

There are three general approaches to capturing fugitive VOCs ahead of a control device — close capture hooding, a Permanent Total Enclosure, or a Temporary Total Enclosure for testing purposes. Enclosures fall into four categories, shown below.

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Table of Contents

The three types of VOC capture strategies we will be discussing in this series are:

Close Capture Hooding
Permanent Total Enclosures (PTE)
Temporary Total Enclosures (TTE)

Close Capture Hooding

What is a Close Capture Hood?

Close capture hooding is a means to collect emissions from a variety of point sources. As a standalone method it's the least desirable of the three approaches, but in some cases it's the only appropriate option - for example, sources that can't practically be enclosed.

Proper hood design is based on the following factors:

Height – Y
Distance – X
Capture velocity – v1

To achieve good capture, the hood must sit as close to the source as possible without impeding access to it.

Potential hazardous and pollution applications require special solutions. Always check local regulations.

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Permanent Total Enclosure (PTE)

What are Permanent Total Enclosures?

A PTE is a fixed installation that contains all fugitive VOC emissions so they can be collected and directed to a control device — a Thermal Oxidizer (TO), Regenerative Thermal Oxidizer (RTO), or Catalytic Oxidizer (CatOx).

A total enclosure is a permanent containment structure — completely enclosed with a floor, walls, and roof to prevent exposure to the elements — with limited openings for access/egress, free of breaks, cracks, gaps, or deterioration.
A total enclosure with negative air vented to pollution control equipment must meet industrial ventilation guidelines at every opening, with enclosure air vented to the control device.

The Five PTE Design Criteria (per EPA Method 204)

Natural Draft Openings (NDOs) Total NDO area cannot exceed 5% of total enclosure surface area (walls, ceiling, floor combined). Normally-closed access doors don't count as NDOs.
Distance to NDO from emission point. At least 4 NDO diameters between the nearest fugitive VOC emission point and any NDO (e.g., a 2 ft. NDO requires 8 ft. of separation).
Distance to NDO from capture hoot/duct. At least 4 duct diameters between the nearest capture hood or duct opening and any NDO.
Face Velocity. Average inward face velocity of at least 200 fpm at every NDO.
Access doors / panels. Any doors or panels not counted as NDOs must remain closed during normal operation.

CPI PTE Worker Exposure Considerations

Beyond the five EPA criteria, a PTE should minimize worker exposure to VOC fumes if employees will regularly work inside it. Each solvent has a Permissible Exposure Limit (PEL) that must factor into the design. For example, if toluene is emitted, the enclosure's air turnover needs to keep concentrations below the OSHA PEL of 200 ppmv.

The typical design sequence: first size the enclosure to meet EPA's five criteria, then model worst-case emission concentrations against OSHA PELs. If a PEL may be exceeded, the enclosure may need to be redesigned and/or exhaust rates increased.

A PTE can be designed to contain fugitive emissions from a storage and mixing operation.

If workers are expected to enter for tasks or inspection, worker exposure limits must be considered - in some cases via supplied-air systems.

PTE Design Tips

Enclose only what is necessary to meet the five criteria - oversized enclosures mean more exhaust air, which means a larger, costlier oxidizer.
Follow good engineering practices - Consider access and workability from the start.
Seal all connections between the VOC source and the enclosure.
Do not underestimate access needs for cleaning, make-ready, and repairs - dismantling an enclosure costs and and production.
Use wire-reinforced glass or solvent resistant plastic for visual inspection windows.
Use the oven, coater, or equipment itself as an exhausting mechanism where possible, rather than adding a separate fugitive blower - this lowers air volume at the oxidizer and reduces overall cost.
Get operator and maintenance input during design, not after - it improves long-term compliance and buy-in.
Account for electrical hazard ratings (Class 1, Zone1 or 11, depending on design) for instruments and devices inside the enclosure.

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Temporary Total Enclosure (TTE)

What is a Temporary Total Enclosure?

A TTE is a temporary installation used for testing purposes only, to quantify fugitive VOC emissions that would otherwise escape through doors, windows, or a facility's HVAC system.

TTEs generally take one of two forms:

A truly temporary structure built around the emission unit - often framing covered in a plastic sheeting material such as Visqueen.
A "Building Enclosure" - an existing room or building where doors are closed, passageways sealed, and HVAC turned off for the test period. Method 204 sets strict design criteria for this option as well.

What a TTE Test Involves

A TTE test is long and, relative to other emissions tests, expensive — typically at least four 10-hour days, including three eight-hour capture test runs plus supporting tests. It can be disruptive to production, though the "Building Enclosure" option is usually less so.

Given that time, expense, and disruption, it's worth confirming your local capture systems are already performing well before formal testing begins. For most processes, VOC capture efficiency can be reasonably estimated beforehand — comparing VOC usage rates to ductwork emission rates, and measuring face velocities at hoods with anemometers, pitot tubes, or smoke tubes — at a fraction of the cost of a formal test.

TTE Tips

Use this approach to avoid improperly sized rooms: Too big = too much air = excessive compliance costs.

Size the room correctly — too large means too much air and excessive compliance costs.
Account for traffic patterns — rooms with heavy foot/vehicle traffic may struggle to meet requirements during routine operation.
Seal everything — windows, doors, roof vents, and room-to-room ventilation may all need modification.
Consider close capture hooding to improve the enclosed environment, following OSHA ventilation guidelines.
Balance the process — an unbalanced process makes for a costly, inefficient capture system.

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Choosing the Right Approach for Your Oxidizer

Most facilities planning a new oxidizer install — or looking to avoid repeat TTE testing cycles — invest in a properly designed PTE from the start. Enclosure geometry, NDO placement, and exhaust rate all affect the size, fuel use, and heat recovery design of the oxidizer itself, so it's worth coordinating enclosure design with your oxidizer supplier early rather than treating them as separate projects.

We are happy to answer any questions you may have in the meantime; please contact us.

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