It is very important for people who suffer allergies and those with babies or elderly invalids at home to invest in an air purifier. For an air purifier to work well it has to a high clean air delivery rating (CADR). It is expressed in terms of cubic feet per minute; so that a 500 CADR means that the filter can release 500ft3 of clean air per minute. Air purifiers use different means to clean the air. Some techniques are more effective against some pollutants than others. Here are some of them. Air purifiers commonly use filters to remove particles in the air. A purifier with an HEPA (high efficiency particle air filter) sign means that it can remove even the smallest particles. Be aware that HEPA enabled devices need to remove 99.97 % of particles and nothing less otherwise it may not be an accurate representation of what you are looking for. If the ad says HEPA-like or HEPA-type then it may not be hat you should be buying. Other filters use carbon. Carbon is highly porous and can absorb chemical in the smallest forms that is why it is good for filtering water. It can be used together with an HEPA filter. Some other filters use ultra violet light. In sterilization, ultra-violet light is used to kill bacteria and can eliminate 99.9 % of bacteria and viruses. Another alternative is to use ultra-violet light bulbs that will also sterilize the air indoors. They need to be replaced yearly. Ionizers are another breed of air purifiers. They produce ions that attach themselves to air borne particles which are then attracted to an electronically charges collector plate. Ozone is another way to clean the air in homes but can only be used in unoccupied rooms. Ozone is a strong oxidant and can oxidize other chemicals and can remove odour and smoke. Other concerns for customers of air purifiers are the noise levels and any by-products produced by air purifiers. This noise level can easily be obtained by trying the air purifier in the store or asking customer service. The cost of operating the air purifier will of course be an interest and this relates to the cost of replacing the filters if any are used or the amount of power needed to operate it. For power consumption look for an air purifier with the energy star symbol as this indicates that it is energy efficient so it will use less than other brands that do not carry the sign.