Enclosure ventilation when removing ceiling tiles dependent upon void volume
When removing high-risk asbestos-containing materials the work should be carried out inside a specially constructed ventilated enclosure to prevent the spread of asbestos outside the work area.
HSE commissioned research to investigate the factors that affect the containment potential of temporary ventilated enclosures. This work entitled RR988 – Ventilation of enclosures for removal of asbestos containing materials considered:
- the way that air moves within ventilated enclosures;
- how the construction of enclosures and airlocks affects air movement and containment;
- how the positioning of extraction points and air inlets affect air movement and containment;
- relationship between air flow and negative pressure, and containment; and
- factors which affect the ability of enclosures to contain asbestos, such as unplanned openings.
In addition, the effect of removing ceiling tiles and the size of ceiling void relative to the enclosure was investigated.
Celing voids may need to be considered contaminated
The effect of removing ceiling tiles was found to be dependent upon the volume of the void above the false ceiling compared to the enclosure. The researchers concluded that:
“When the volume of the void was much larger than the enclosure, for instance a small enclosure in a large room where the void is unsealed, removing a ceiling tile caused ΔP and the airlock door deflection to fall to zero.
Tracer gas and smoke tests showed that once the tile was removed air was able to enter the void and therefore potentially spread contamination to surrounding areas.
When the volume of the void was less than the enclosure, removal of a ceiling tile caused a reduction of ΔP but the airlock door deflection was unchanged. Tracer gas and smoke tests showed that once the pressure between the two spaces had equalised air could move freely between the two. This means that in this situation the whole void would potentially be contaminated and would require cleaning after completion of the work.
These results were interesting and should be considered, especially in cases where the void is small compared to the enclosure. The results have shown that once the tile is removed the void becomes part of the enclosure and contaminated air can move freely between the spaces. This means that upon completion of the work the whole void should be considered to be contaminated and be cleaned. ((ΔP) is pressure difference between the interior of the enclosure and the surrounding space)”