Furnace filters are a screen that sites between the air intake for your furnace and the furnace itself.
  They stop any foreign matter like dust or anything else from entering the furnace and this can improve the life of your furnace and help reduce the need for expensive maintenance.
  They are also useful because they can draw dust, pet dander and even pollen out of the air which can often cause breathing difficulties for people with asthma and allergies.
  You can buy special furnace filters which are specially designed to pull contaminants from the air with maximum efficiency.
  Once you have a good filter in place, you will probably notice less dust in the air, see that it takes longer for a film of dust to build up on your furniture and even find that it's easier to breath in the house with less coughing and sneezing!  It's important that you replace your filter every once in a while as they become full.
  An older furnace filter may be clogged with dust and therefore be putting a strain on your furnace and not reducing the dust in the air efficiently.
  Some filters must be thrown away and replaced.
  A few others can simply be washed off, left to dry and then replaced in the furnace intake pipe.
  It's critical that you replace the filter when you take it out: a furnace without a filter is not a good situation to be in.
  Replacing the filter is much cheaper than replacing the furnace itself!