The longer a filter is left in a water system, the more particles block its filtering ability. If the filter is not changed for a long period of time or is exposed to a large number of contaminants in a short period of time, it can become clogged. This lowers water pressure, since the water takes longer to pass through, and inhibits the filter's straining abilities. For reverse osmosis filters, for instance, the water must flow at a certain rate of speed. Too slow or too fast, and contaminants will make it through.
Carbon or charcoal filters also develop efficiency problems if they are left in too long. These filters become blocked and let water flow through without purifying it.
Bacteria grow in colonies and can develop more easily in the presence of other bacteria and contaminants in a water system. When a filter is left unchanged, it develops small bacteria colonies. This means that the bacteria already present in the water and the bacteria growing on the filter can enter the water, increasing the likelihood of bacterial infection.
When filters are left unused for long periods of time, the standing water in them can become a breeding ground for mold. Mold develops from spores that love moist, dark areas. While few molds are hazardous to the health, some can cause allergic reactions, and all will discolor the water and make it smell or taste unpleasant. Filters should be used regularly to avoid this problem; the water will wash away spores before they can successfully root. Replacing filters on time also helps avoid mold problems.