Indoor air quality is effected mostly by pollutants that come from materials such as wood, walls, insulation, and other household materials. Other particles can come form humans, pets, or outdoor substances like pollen. The most dangerous would be chemical substances like carbon monoxide, lead particles, and asbestos. The inhalation of mold can cause damage to ones health as well. One of the most impactful things you can do to keep the air as clean as possible is have a reliable air filtration system. HEPA, ionizers, and UV filters are some common examples of filters that will help keep the air in your home clean. In regards to how often they should be changed will depend on various factors such as the size of your home, whether you have pets, and the quality of your filters.