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Why does licensed work use a negative pressure enclosure?.

CONTAINED

The work area is completely sealed in heavy-gauge polythene over a timber or scaffold frame, with viewing panels where practical. Before any asbestos work starts, the enclosure is smoke-tested: it is filled with smoke while the outside is checked for leaks. Any leak is sealed and the test repeated until the enclosure holds.

NPUs draw air out of the enclosure through high-efficiency HEPA filters, running continuously — including overnight — so the enclosure sits at a lower pressure than its surroundings. If a small breach ever occurred, air would flow inwards, not out. The filtered exhaust is cleaner than the ambient air around it.

Airflow is calculated to give a minimum number of air changes per hour, and the units are monitored throughout the job.

Operatives enter and leave through a three-stage airlock and use a dedicated decontamination unit (DCU) with dirty and clean ends separated by showers. Waste leaves through its own bag-lock, double-bagged and surface-cleaned. Nothing crosses the boundary uncontrolled — people, tools or waste.

With a properly built enclosure under negative pressure, the rest of your building can usually remain occupied and operational while removal takes place — a major factor for schools, healthcare and commercial clients. It is also why licensed work passes independent air testing at the end.

The enclosure is a sealed box built inside your building from timber or aluminium framing and heavy polythene, taped and sealed at every joint, with viewing panels so the work can be observed from outside. Services running through it are isolated and sealed. Before a single sheet of asbestos is touched, the enclosure is smoke tested: smoke is generated inside and the whole structure is watched for leaks, then any that show are sealed and the test is repeated.

That test is recorded. If an enclosure will not hold smoke, it will not hold fibre, and work does not start.

A negative pressure unit is a fan drawing air out of the enclosure through an H-type HEPA filter and discharging it outside. Because more air is being pulled out than can leak in, the inside of the enclosure sits at a slightly lower pressure than the rooms around it, so any air movement through a small breach travels inwards rather than outwards.

Units are sized to give the enclosure a continuous air change rate and are left running through the whole job, including breaks and overnight, until clearance is granted.

Nobody walks straight in or out. Operatives pass through a three-stage airlock, and waste leaves through a separate baglock where each bag is wiped and double-wrapped before it crosses into clean space. A defined transit route runs from the baglock to the skip or waste vehicle and is kept clear of everyone else on site.

Personal decontamination happens in a decontamination unit — a three-chamber shower unit with dirty, shower and clean ends — so nothing leaves the site on clothing or skin.

A UKAS-accredited analyst who is independent of us carries out the four-stage clearance: a check of the paperwork and site condition, a thorough visual inspection inside the enclosure, air monitoring, and a final assessment after the enclosure comes down. Only when all four stages pass is a Certificate of Reoccupation issued.

That certificate, the air test results and the waste consignment notes are what you keep. They are the evidence that the building was handed back clean.

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