It employs a medium with a significant amount of thickness providing 'filtering in depth'. The path through the filter is much longer and random, providing greater possibility of both direct interception and dirt retention. Larger particles tend to be trapped in the surface layers and the finer particles trapped by succeeding layers. Generally the overall performance and dirt holding capacity of a depth filter is better than that of a surface filter.
This type of filter draws and captures particles inside the filter. Eventually the depth filter becomes clogged, reducing airflow through the filter, which can compromise its performance and potentially shorten the life of the vacuum cleaner.