You should be cleaning your house often. I have to clean mine every week, considering how the kids run around, going inside and outside all the time. They track in a lot of dirt sometimes and a rug can only do so much when it comes to unwanted things in the air. There is so much pollen and dust outside that it can be a real pain when it comes to your allergies. Breathing in that pollen or dust can make them go haywire, so having an air filter is a good thing. You have to keep them clean, though.
Cleaning an air filter is pretty easy, especially when it comes to HEPA filters. They are both washable and permanent. The washable ones can be washed by hand and the permanent ones can be vacuumed. This should be done every two to three months, as said here: http://www.damagecontrol-911.com/how-to-clean-a-hepa-filter/
This, along with other things, can be learned there. It’s a great resource and teaches you how to clean your HEPA filter, how long they should last, how it works, and other things. There are many ways that you can use a HEPA air purifier, so be sure to get one for your home to make your breathing environment even better.