It is generally accepted that there is a requirement for negative pressure units and type H vacuum cleaners to be serviced and tested every six months. As part of that process the efficiency of the filtration should be measured to determine the suitability of the equipment for use with asbestos.

Filter efficiency

The dispersed oil particulate test (DOP) is commonly used to determine filter efficiencies. The method requires the use of an aerosol photometer which has a built in pump which draws a sample of air through a flexible tube into the instrument. The sample is then drawn across an area of intensive white light in the optical chamber and any particles of significance present deflect the light source on to the sensitive area of a photomultiplier tube. Each individual burst of light is converted into a pulse of electrical energy which is proportional to the amount of light scattered by the particle. The amount of electrical energy generated by the pulse is displayed either as a digital readout or by a needle on a sensitive gauge.

In order for the aerosol photometer to measure the efficiency of a filter system an aerosol generator is required to provide particles in the form of a dense cloud of oil droplets in a stream of carbon dioxide or nitrogen (DOP).

To test an appliance the oil droplets are mixed with air to provide a homogenous sample with a concentration in the region of 80 to 100 micro grams per litre. The filter efficiency is purely a mathematical calculation whereby the amount of particles upstream of the filter system or appliance is compared with the number downstream therefore indicating whether the required standard has been met. Many aerosol photometers are capable of providing an efficiency reading directly and no actual calculation has to be carried out.

The filter efficiency requirements are laid down in BS8520-2:2009 Equipment used in the removal of asbestos containing materials, clause 4.6.1.

When tested for filtration efficiency in accordance with BS EN 60335-2:69, the HEPA filter shall allow a maximum penetration of 0.005% when fully assembled in an NPU.