Controls II: enclosures, negative air and decontamination

When asbestos work can release large numbers of fibres, source controls alone are not enough. High-risk work, such as Ontario's Type 3 operations, takes place inside a sealed enclosure kept under negative pressure, with HEPA-filtered exhaust and a decontamination facility 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 facilities operate, and how supervisors inspect them. It describes the principles so that supervisors and other trades understand the system; building and running enclosures is work for trained, certified abatement crews.

What an enclosure does

An enclosure creates a sealed room around the work so that fibres 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 with polyethylene, turned up the walls, and cover walls with polyethylene so that the room can be cleaned and the sheeting removed as waste. Many specifications call for two layers 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.

Ontario's regulation requires indoor Type 3 enclosures, where the enclosure is not inside a building that is being demolished, to be kept at a negative pressure of 0.02 inches of water (about 5 pascals) relative to the area outside the enclosure, with HEPA-filtered exhaust and replacement air drawn from outside the enclosure, and with the pressure difference measured at regular intervals. The regulation does not set an air change rate, but specifications commonly require at least four to six air changes per hour, and many consultants specify more.

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 per cent 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 fibres 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: fibres that never become airborne do not need to be filtered.

The decontamination facility

Every person leaving the enclosure is a possible carrier of fibres 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.

Ontario's regulation requires the decontamination facility for Type 3 operations to consist of a series of interconnected rooms in sequence, with curtained doorways: a room for changing out of protective clothing, a shower room with hot and cold water or water at a constant temperature between 40 and 50 °C and individual controls, and a clean room for street clothes. Every person leaving the work area must pass through each room, decontaminating clothing with a HEPA vacuum or damp wiping, placing disposable clothing in a waste container, showering and cleaning the respirator. Type 2 operations need no full decontamination facility, but workers must decontaminate their clothing and wash their hands and face 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, labelled 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

Ontario's regulation requires a competent worker to inspect the enclosure, barriers and decontamination facility at the beginning of each shift, at the end of a shift when no shift follows, and at least daily on days without shifts, and to repair defects immediately. Negative air units must be inspected before each use to make sure they do not leak, and damaged filters replaced.

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 fibres 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.