Controls II: enclosures, negative air and decontamination

When asbestos work can release large numbers of fibers, source controls alone are not enough. Class I work, and some Class II work, takes place inside a regulated area with critical barriers, usually a negative pressure enclosure with HEPA-filtered exhaust and a decontamination area through which every person and every bag of waste must pass. This lesson explains how enclosures are built, how negative air works and is sized, how decontamination areas operate, and how competent persons inspect them. It describes the principles so that supervisors and other trades understand the system; building and running enclosures is work for trained, accredited abatement crews.

What an enclosure does

An enclosure creates a sealed room around the work so that fibers released inside cannot escape into the rest of the building. It does three things. It contains the dust physically with plastic sheeting and sealed openings. It holds the inside at lower air pressure than the outside, so that any leak draws clean air in rather than letting dusty air out. And it controls the only ways in and out, through a decontamination facility for people and a waste load-out route for materials.

Building the enclosure

Enclosure construction follows a standard sequence, set out in more detail in regulations and project specifications:

• Prepare the area. Remove movable items, or clean and cover items that cannot be moved. Shut down, lock out and seal the building ventilation serving the area, and isolate electrical circuits that are not needed.

• Seal openings with critical barriers. Cover windows, doors, vents, diffusers, pipe penetrations and other openings with polyethylene sheeting sealed with tape or spray adhesive.

• Line floors and walls. Cover floors and walls with polyethylene so that the room can be cleaned and the sheeting removed as waste. OSHA requires impermeable drop cloths on surfaces beneath all Class I removal activity, and many project specifications call for two layers of 6-mil sheeting on floors and walls.

• Build entry and exit points. Attach the decontamination facility to the enclosure and provide a separate waste load-out where the project is large enough. Provide emergency exits that can be opened quickly, marked so that they are visible.

• Install negative air units and run exhaust ducts outdoors, away from air intakes, windows and people, or into a filtered return where outdoor exhaust is impossible and the specification allows it.

• Test and inspect. Start the negative air, check the pressure differential with a manometer, look for leaks with smoke tubes, and inspect the enclosure before work begins.

Enclosures must also allow safe work: lighting, emergency exits, fire extinguishers, ground-fault protected power, and planning for heat stress, because workers inside an enclosure in disposable coveralls and respirators can overheat quickly.

Negative air

A negative air unit, sometimes called a negative air machine or air filtration device, is a fan with a series of filters ending in a HEPA filter. It draws air out of the enclosure, filters it and exhausts it. Because more air leaves than enters by direct openings, the enclosure is held at a slightly lower pressure than its surroundings. Make-up air enters through the decontamination facility and any small leaks, flowing from clean to dirty areas.

OSHA's construction standard requires negative pressure enclosures for Class I work to maintain a pressure differential of at least 0.02 inches of water column relative to adjacent areas and at least four air changes per hour, with HEPA-filtered exhaust. The pressure differential must be checked before work begins and monitored during the work, often continuously with a recording manometer. Project specifications commonly require more air changes, such as six or eight per hour.

Sizing negative air

The number of negative air units needed depends on the enclosure volume, the required air changes per hour, and the delivered airflow of each unit. Delivered airflow is always lower than the rated airflow, because filters, ducts and bends add resistance, and it falls further as filters load. Specifications often derate units to about 70 to 80 percent of their rated capacity for planning purposes.

import math

def units_needed(volume_m3, ach, unit_rating_m3h, derate=0.75):
    required = volume_m3 * ach                  # m3 per hour
    per_unit = unit_rating_m3h * derate
    return required, math.ceil(required / per_unit)

# Enclosure 12 m x 9 m x 3 m, a unit rated at 3,400 m3/h (about 2,000 cfm)
volume = 12 * 9 * 3
for ach in (4, 6, 8):
    req, n = units_needed(volume, ach, 3400)
    print(f"{ach} ACH: need {req:5.0f} m3/h ({req / 1.699:5.0f} cfm) -> {n} unit(s)")

# Air changes delivered by two units as filters load
for derate in (0.9, 0.75, 0.5):
    ach = 2 * 3400 * derate / volume
    print(f"two units at {derate:.0%} of rating: {ach:4.1f} air changes per hour")

# Pressure differential conversion
in_wc = 0.02
print(f"{in_wc} in w.c. = {in_wc * 249.09:.1f} Pa")

4 ACH: need  1296 m3/h (  763 cfm) -> 1 unit(s)
6 ACH: need  1944 m3/h ( 1144 cfm) -> 1 unit(s)
8 ACH: need  2592 m3/h ( 1526 cfm) -> 2 unit(s)
two units at 90% of rating: 18.9 air changes per hour
two units at 75% of rating: 15.7 air changes per hour
two units at 50% of rating: 10.5 air changes per hour
0.02 in w.c. = 5.0 Pa

The script shows two practical points. First, the number of units depends heavily on the air change rate chosen, so the specification matters. Second, as filters load, delivered airflow can fall below the required rate even though nothing visibly changes. This is why pressure must be monitored, pre-filters changed often, and spare units kept on site.

What negative air does not do

Negative air keeps fibers from escaping the enclosure; it does not make the air inside the enclosure safe to breathe. Concentrations inside an active enclosure can be very high, so workers inside must wear the respirators required for the task. Negative air also does not replace wetting: fibers that never become airborne do not need to be filtered.

The decontamination facility

Every person leaving the enclosure is a possible carrier of fibers on skin, hair, clothing and equipment. The decontamination facility removes the contamination before the person reaches clean areas. A full facility has three rooms in sequence, each separated by curtained doorways made of overlapping polyethylene sheets that act as air locks.

OSHA requires a decontamination area for Class I work involving more than 25 linear feet or 10 square feet of TSI or surfacing material, consisting of an equipment room, shower area and clean room in series, adjacent to and connected with the regulated area. Workers must enter and exit the regulated area through the decontamination area, shower before leaving, and must not leave the equipment room wearing contaminated clothing. For smaller Class I jobs and for Class II and III work, the employer must provide an equipment room or area where workers decontaminate clothing and equipment with HEPA vacuums or wet methods before leaving.

Entry works in reverse: workers change into clean protective clothing in the clean room, pass through the shower room, and enter the work area through the equipment room. Respirators are put on and checked before entering the work area and stay on until the person is in the shower.

Waste load-out

Bags of waste must leave the enclosure without contaminating the decontamination facility or the route outside. On larger projects, a separate waste load-out has a washing area, where the outside of each bag is wet-wiped or HEPA vacuumed and placed inside a second clean bag, and a holding area. Waste is then carried directly to a sealed, labeled container or vehicle. On smaller projects without a load-out, waste passes through the decontamination facility only when no one is showering, and the bags are cleaned and double-bagged first. Lesson 11 covers waste handling in detail.

Inspecting enclosures

OSHA requires the competent person to inspect Class I enclosures and areas at least once per shift and to check the integrity of barriers, the pressure differential and the decontamination area. Leaks, breaches and pressure losses must be corrected before work continues.

When things go wrong

Power failures, fan breakdowns or torn barriers can lead to loss of negative pressure. If negative pressure is lost, stop removal work, keep material wet, close the enclosure, restore negative air as quickly as possible (spare units should be on site), and do not open the enclosure to the outside until pressure is restored. If a barrier is breached and fibers may have escaped, the area outside should be assessed and cleaned under controlled conditions, and air monitoring may be needed. In a medical emergency, life safety comes first: injured workers are removed through the quickest route, and contamination is managed afterwards.