The incoming air goes through the ventilation unit filter, which stops most of the particles that exasperate respiratory problems like asthma or hay fever, this filter become black (shiny black if near a busy road) in time as it traps more particles. Outgoing air is also filtered this is to protect the ventilation unit heat exchanger from dust build up enabling it to continue to work efficiently. The ventilation unit motors draw the air through these filters so the more the filters clog up the less air passes through them resulting in a less efficient system. It is therefore important that the filters are cleaned regularly and replaced annually.
We can supply finer filters that block out more particles, however these require the motor to work harder. We can discuss the advantages and disadvantages of both. As the filters are stopping the particles entering your home your are no longer breathing these particles into your lungs, so the dirtier the filter the cleaner your lungs.