Simply defined, a combination filter is one that does at least one other processing job at the same time as filtering a suspension. The classic example, of course, is the packed bed of activated charcoal, which acts as a remover of chemical impurities from the suspension, as well as removing dust particles - although this is actually not a very good example, because it is primarily an adsorber, which also acts as a filter. Another example would be the canister filter of a respirator, which has a piece of filter medium, with a separate layer of adsorbent or reactive material.