The top 4 air pollutants in a home are : Carbon Monoxide, Mould & Radon. As scary as this sounds there are a few things that can be done about it.

Get Professional Duct Cleaning - The air ducts to your HVAC System are a critical. This is how the air and the heat travel throughout your home. Unlike the rooms in your home, you may have never had your ducts cleaned. Dust can build up quite fast in the ducts and this can cause allergy symptoms. Also if you have noticed that your energy bills are getting higher and your family is having more allergy symptoms if it probably a good idea to start with calling a professional and scheduling to have your ducts cleaned.
Using an Indoor Air Purifier - Another way to improve the air quality in your home is to purchase an Air Purifier. Having an air purifier appliance in your home is a good way to get rid of allergens and other pollutants from the air.
Change your Air Filters on your HVAC System - The Air filter on your HVAC system works like an air purifier. This will only be effective if you regularly change them roughly every three months. With that being said make sure you are checking the manufactures recommendation as to the regular maintenance of changing the air filter. If you have pets, smoke out have allergies you may need to replace the filter sooner as it will collect more allergens.
Let in the Fresh Air - As important as it is to filter out all the particles from your indoor air, you also need to remember to let the fresh air into your home. On low pollen days be sure to open the window and let the fresh air in. You can open the window in the winter months as well to let a little bit of fresh air in.