Air conditioners cannot purify air, but they do filter dust, pollen, and other allergens. The air filter is part of the entire HVAC system, and all air conditioners have a dust filter in the indoor unit. This traps dust in the air as it circulates, mainly to prevent it from obstructing the inner workings. Most central air conditioning units have two or more filters that clean the air and ensure good air quality as the unit circulates between indoor and outdoor air.
HVAC air filters do exactly the job their name suggests. The best way to filter very fine particles such as smoke, mold spores, pollen and viruses is with a HEPA (High Efficiency Particulate Air) filter, which is found in many vacuum cleaners and most air purification units, but not in air conditioners. If you've never changed your air conditioning filter, the easiest way is to have an HVAC technician do it the first time. Your air ducts may also have deposited dust and pollen there, waiting for the air conditioner to send it to circulate in your home.
As a general rule, the best way to determine the need to change is to simply check the air filter on your own. The air filter prevents these residues from entering your home and from inhaling them by you and your family. Some of these factors are the type of filter you have, the size of the house, the quality of the air, the number of pets in the house, how often you use your unit, and if you or someone in your family has allergies or certain medical conditions. Before air is dispersed in your home, it passes through an air filter that removes particles such as dirt, dust, bacteria, pet hair, and human hair.
Contrary to the common belief that dirt helps filters work better, dirt-free filters are what keep the air clean. The air conditioner works hard to keep the heat out, but it won't keep contaminants and debris out of the air at least, not without the aid of a filter. It's logical then that if you're in a dusty area like the desert, you'll need a better quality air filter to keep all that dust out, and you'll need to change it more often. Depending on the type of filter you have, six months is the longest you should go without replacing or cleaning the filter.
The indoor air may not even be bad to begin with, but the unit is throwing polluted air everywhere. Air filters will get dirty over time, so you should replace them every two months to keep your air conditioner running efficiently.