We can tell you all about how amazing air filters are and how crucial they are for the health of your family, but once you get it installed, how the heck are you supposed to know if it is really doing the job it was built to do? That’s easy:

Do You Feel Better?

If you were feeling a lot of allergy or cold symptoms (scratchy throat, runny nose, itchy eyes, headaches, etc.) on a pretty consistent basis before you put in a new filter, then you should absolutely be able to feel a difference within a few days. Once all of the harmful substances are removed from your air supply, your body will finally be able to heal.

Does It Look Dirty?

However, if you weren’t feeling any of the above symptoms, you still want to know how effective this new handy dandy filter is, right? Of course you do, and the easiest way to do so is to see how dirty it is when it’s time to switch it out. According to How Stuff Works, if it is working properly, it should be covered in gray, ashy grossness once the three months have passed. Just think: all that would have been in your air supply if your filter had not been there.

But What If Your Filter Isn’t Dirty?

If it has been three months and your filter still doesn’t look dirty, then there are a few things you need to check out, pronto—because you definitely want to make sure your filter is working properly:

Does it fit snugly into the vent? If it is too little for the space, the air might be squeezing in around the edges instead of going through it. The same goes for if the filter is too big. If you have to fold it to get it to fit, then it is certainly letting unclean air through.

Is it installed the right way? The direction that you place the filter into the frame is important. The arrow should be pointing to the fan—if it’s not, then your air filters are not doing their job.

Is the filter dense enough? If you are using the cheapest filter you could find (like the ones under a dollar at the home improvement store), then it’s probably not removing very much from the air, so it won’t be that dirty. You need to upgrade to a MERV 8 filter, or higher, to notice a difference.

There is one more thing to be aware of:

You need to think about how much you have actually used your system while the filter has been in place. It is only filtering the air when your HVAC system is on, so if you are in the time of year where you don’t really use the A/C or the heater, then the filter probably won’t be very dirty—but that doesn’t mean it’s not working, it just means it hasn’t had the chance to.