When you are using your HVAC system, it sucks air from inside your home before it can return conditioned air through the vents. While this system can be remarkably effective, it does mean that any contaminants inside your indoor air can also be sucked up. Although your HVAC system will contain at least one air filter, it may not be effective at eliminating some of the most common yet harmful contaminants that may be inside your home. These particles are often tiny, and the particles are simply too small to be collected by a typical home filter. Before you race out to change your filter, you should be aware of the factors that can impact your indoor air quality. 
Dirty Filters:
You can have the best filter in the world installed on your HVAC system, but it will be ineffective if it is dirty and needs to be replaced or cleaned. When your air filters are dirty, it reduces the airflow in your ducts, which can cause several problems. Your air handler has been designed to handle a particular level of airflow, and when this is impaired, it can trigger electrical or mechanical problems. Additionally, clogged filters will be ineffective at eliminating further airborne contaminants. This means that these contaminants will either be drawn into the system, potentially causing more problems or released into your air. Most manufacturers recommend changing air filters every two to three months. It is important to choose a high MERV filter that is specifically designed for your system for optimum performance.
Poor Ventilation:
When you are looking to heat or cool your home, the first thing you are likely to do before running your HVAC system is to shut all the windows and doors. While this is important to maintain the temperature, it does mean that the air can become stale. Your HVAC system will continue to circulate this stale air around your home. Fortunately, it is possible to use an energy recovery ventilation to add ventilation in your home without wasting energy.
High or Low Humidity:
When the humidity or moisture level in your air becomes too high or too low, it can impact your air quality. High humidity levels can make the room feel unpleasantly warm and creates the perfect conditions for mold development. Unfortunately, too little moisture can also be a problem. Low humidity or dry air can lead to a number of issues including irritation of bronchial issues, difficulty in recovering from illnesses and damage to furniture.
Leaking Ducts:
If your ducts are leaking, then the levels of heating and cooling will be compromised. Leaky ducts allow conditioned air to be released into areas of the home where it may not be beneficial. Additionally, the leak could allow contaminants to be introduced into the air. This can be particularly problematic if the leak is in an unoccupied area of the home, such as a crawlspace or attic. With even a small leak, you may unwittingly be allowing insulation material, dust, and other debris to enter the air in your living space. Therefore, it is important to check that all your ducts are sealed and insulated to ensure that there are no leaks.