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When the weather outside is frightful and the fire inside delightful, it's so tempting to curl up on the couch and avoid the nasty winter air. But after a few hours, you may feel the beginnings of a dull headache, perhaps a raspy throat or a stuffy nose, maybe a little nausea. You might think it's just another bout of flu or common cold, but if the symptoms persist, they may actually be due to allergens or irritants you're inhaling indoors.

Pesky yet omnipresent biological matter—mold, mildew, animal dander, dust mites, cockroaches and pollen, to name just a few—can spell misery to the more than 50 million Americans suffering from asthma and allergic diseases, according to the American Academy of Allergy, Asthma and Immunology. Furthermore, untreated allergies can lead to other illnesses like sinusitis and ear infections, and it's important to treat the symptoms while ridding your space of these triggers.

Where You'll Find Them

The most common biological pollutants include mold, mildew, pet dander, dust mites, cockroaches, pollen. Often, these are found in areas that are moist, places that have food residues and areas where dust collects.

What You Can Do

If you have recurrent cold or flu-like symptoms, see a doctor. "Having a test done to find out what you're allergic to is a critical first step," says Janice Nolen, M.S., director of national policy for the American Lung Association.

Still, the easiest way to improve the quality of your indoor air is to clean well and often.

*Dust and damp mop frequently, wash bedding in hot water biweekly to kill dust mites and vacuum at least once a week using a machine with a high-efficiency particulate air (HEPA) filter. Find one in our Vacuum Cleaners Buying Guide.

*If you can afford it, replace your wall-to-wall carpeting with bare washable floors and area rugs.

*Keep matter from getting tracked in by using a washable doormat and leaving shoes in the entryway.

*Open windows or run ventilation fans in high-humidity areas like bathrooms and kitchens to prevent mold and mildew growth.

*Air purifiers can help by trapping some biological and particle pollutants.