I just replaced the furnace filter - every six months, as directed by the professional who visits to fine-tune it periodically.

Looking at the accumulated dirt made me glad it was the filter, not my lungs, that looked so sooty, but it made me wonder about indoor air quality.

We have central air-conditioning, so our windows stay closed in the summer. When I do open the windows in the spring and mid-fall, there is a lot of sneezing because of pollen and leaf mold.

Winter is when much of the world locks down, however, and the experts tell me that indoor air quality can suffer as a result.

Space heaters, ranges, ovens, furnaces, fireplaces, water heaters, and building materials in the home can seriously affect air quality, as can dust mites, pet dander, and smoking.

Keeping fumes, dander, mites, and other pollutants and allergens to a minimum helps prevent respiratory problems and allergic reactions, including coughing, headaches, itchy and watery eyes, shortness of breath, even asthma attacks.

Here are three suggestions from air-filter manufacturer Permatron Corp., based in Elk Village, Ill.:

Clean it up. While most people ascribe to spring cleaning, getting your home ready for winter is equally important. Use a damp cloth to dust tops of doors, window frames, cabinets, artwork, and ceiling fans. Dust and vacuum air vents, under furniture, and behind the refrigerator.

Don't track outside dirt, sidewalk and road chemicals, and other pollutants into your home. Simply add grass or rubber mats outside, so people can wipe their feet before entering. Ask your family as well as guests to remove their shoes and store them on a rubber mat next to the front door to keep from tracking dirt and pollutants throughout the house.

Keep mites away. Combat them by keeping the humidity about 35 percent; drier air discourages mites. Clean drapes and bedding frequently in hot water.

Turn your recirculating fan to "on" instead of "auto." The "auto" setting causes the fan to shut off with the rest of the system as soon as the desired temperature is reached.

aheavens@phillynews.com or write him at The Inquirer, Box 8263, Philadelphia 19101. Volume prohibits individual replies.