Airflow and air changes on site
Airflow is quoted in CFM (cubic feet per minute) or cubic metres an hour, and at full speed most portable units move roughly 500 to 800 CFM, or about 850 to 1,350 cubic metres an hour. As a rough guide, aim to change the air in the work area at least four times an hour: a 750 CFM unit does that for around 120 square metres of floor under a 2.5 metre ceiling. Larger or heavily contaminated areas need two or more units.
Free standing or negative air
Free standing, a scrubber pulls in the room's air, filters it and returns it clean, which suits general dust control while people work. Ducted to the outside of a sealed work area, it exhausts the filtered air out of that area, so air is drawn in through the gaps rather than dust and mould spores leaking out to the rest of the building. Models with a carbon stage also take out the smell of smoke after a fire.
Power, pre-filters and safe disposal
Each unit plugs in and draws a few hundred watts, with 110 volt versions for building sites. Change the pre-filter often so it keeps coarse dust off the HEPA filter behind it, and replace the HEPA when airflow falls or the filter indicator, where fitted, shows it is loaded. Bag used filters inside the work area before carrying them out, so what they have caught does not spread through the building.
