If my home's air filters are clean when I change them, are they really working?

"A build-up of dust is usually apparent," Williams said. "There should be gray, ashy-looking material on the duct side of the filter."

If your filter looks clean after it's been in place for the recommended time, here are some things you should check:

Your home environment and how often you run the heat or air can also affect how quickly your air filter gets dirty. If your home is well sealed, you have no pets, no dust-prone furnishings like carpet and fabric-covered furniture, and you dust and vacuum every day, your air filters will have fewer air-born particles to collect. Also, the system only filters the air when it's running. If you install a new filter, but don't turn on the heat or air conditioning until a month or two later, the filter should still be relatively clean since the system hasn't been forcing air through it.