A HEPA filter can have additional features beyond those specifically required. One common feature is a layer of activated charcoal that removes odors and some chemicals the HEPA layers do not. Another feature is the ability to wash the filter so it can be reused several times before being replaced, although the effectiveness of the filter might degrade with each washing. 

Ionized air is air in which the oxygen content has been given an electrical charge where it either has a negative charge due to the presence of one or more extra electrons per oxygen molecule, or a positive charge due to the presence of less than the normal number of electrons per molecule. Most air purification systems that claim to ionize the air are imparting a negative charge to oxygen molecules. 

When looking at a furnace air filter, it is important to choose one that is sized to fit your particular heating system. Some heating systems may require a different type of filter made of a different material. The manufacturer's recommendations should be followed exactly. Otherwise, the effectiveness of the furnace may be diminished.

One concern when buying an air filter system is the amount of noise it makes during normal operation. Many people object to the interference caused by a noisy appliance, which can sometimes be so loud that it disrupts normal activities such as quiet conversation or watching television. Listen to the system running at both low and high speeds to see if it is going to be too noisy for the location in which you are planning on using it. If you can’t view the machine in person, read customer reviews and pay particular attention to comments about how loud it is.

Unlike ion and HEPA purifiers, there is some controversy surrounding the once popular ozone home air purifier. This type of purifier sends ozone into the air, which then eliminates debris from an environment. The problem is that ozone purifiers produce harmful chemicals once ozone reacts with air pollutants.

 

A whole house electronic air cleaner is a system that filters, cleans and otherwise frees the air of pollutants and allergens. These devices typically filter air throughout an entire home and often can be found in specialized units located within the HVAC system. There are various benefits to using these devices, the main one being that they can clean the air in larger areas than more traditional portable units.

Some whole house electronic air cleaner systems use filters. These act to trap and hold airborne allergens, pollutants, and even microorganisms. The contaminants that a system can capture depends on the type of filter being used, with smaller and more advanced filters costing much more than ones that allow some microscopic elements to pass through.

If the home being filtered is especially large or in an allergen-prone area, it is especially important to change filters often. Filters should be checked every few days to ensure that dust and dirt have not collected to the point of inhibited performance. To supplement the effectiveness of a whole house electronic air cleaner, portable units can be purchased separately.

Air conditioning units contain a special chemical called a refrigerant, which has the unique ability to change from a gas to a liquid in a short amount of time. A refrigerant called freon is commonly used in AC units, although there are other commercial refrigerants available. The refrigerant is pumped into the unit at the factory, along with a small amount of lubricating oil for the compressor.