One of the ways it does this is to collect dust and debris from the air which can lead to a build-up in the unit's filters. How quikcly this happens may depend on where you are located with city locations collecting more particulates more quickly than rural areas. In most cases all that needs to be done is to change or clean the filters.
OA simple look will be enough for most people to identify a filter change is mecessary as, there is a collection of dirt and debris that accumulate and cannot be removed with a light vaccum. Most MVHR manufacturers recommend filters are changed at intervals between 6-12 months.