How are air filter samples tested for asbestos?

Air filter samples for asbestos are typically analysed using microscopy techniques in a controlled laboratory setting. The most common method in the UK is phase contrast microscopy (PCM), which counts fibres on the filter.

In simple terms, the filter—where airborne fibres were collected—is prepared and examined under a microscope. A trained analyst counts the number of fibres visible within a defined area. This gives a concentration, usually reported as fibres per cubic centimetre of air.

If more detailed identification is needed, scanning electron microscopy (SEM) may be used. This can confirm whether the fibres are actually asbestos, rather than something that just looks similar under a basic microscope.

It’s a careful, quite manual process. Accuracy depends heavily on the analyst’s training and the lab’s quality controls, so using a reputable lab really matters here.