It doesn't matter if you take off your shoes the moment you step in the door. It doesn't matter if you keep your house spotless -- some allergens will make their way inside. (After all, they are nearly invisible.) Once those allergens -- dust, pollen, dander, microscopic mold and other organisms -- find their way in, gravity pulls them to the floor. There, they become trapped in and around carpet fibers. Granted, this reduces the opportunity for those nasty little things to circulate in the air, where you can breathe them in and suffer a perpetually runny nose, but every step on that carpet sends tiny clouds of particles back into the air.
The good news is that people with allergies and asthma can have carpet. Most fibers used in mass-produced rugs and carpets today are harmless materials like polyester and nylon, the same stuff clothes and bags are made out of. These synthetic blends are constructed out of lab-developed fibers that repel allergens, in part because they are nonorganic and offer an inhospitable climate. For example, mold has nothing to eat and pollen dries out. Nylon is the most effective allergy-controlling carpet fiber. Wool, conversely, should be avoided because allergens and mold can thrive in it. Regardless of material, avoid shag -- the shorter the strands, the fewer places the pollen can go. Also choose carpet with tightly woven strands for the same reason.
When choosing a carpet, remember that they're often treated with special chemicals to neutralize or repel offending particles. Buy carpet labeled low VOC (volatile organic compound), which limits the use of substances such as formaldehyde and benzene. These common carpet treatments convert to gas over time and enter the air, affecting indoor air quality and exacerbating allergies. You'll also want to ask carpet retailers about other "green" extras, such as carpet padding and glue.
But no matter which carpet type you choose, dust, pollen and other allergy-aggravators are still in there, so get rid of them. Vacuum regularly, shampoo, and/or steam-clean carpets. Use a deep-cleaning vacuum, regular professional carpet cleaning sessions or invest in a HEPA (high-efficiency particulate air) filter, which traps allergens; some vacuums are even equipped with them.
Allergens sink into the carpet and out of the air, but clean as often as you like, they're still down there and they circulate. Hard floor coverings offer no haven for allergies -- they just sit on the surface. They're easier to clean than carpets -- a simple sweep and vacuum quickly gets rid of allergens. Materials used to make hard floors (organic and synthetic) include cork, tile, linoleum, bamboo, wood and woodlike laminate. If you opt out of carpet, however, note that hard surfaces require more frequent cleaning. If not maintained, the result will be a dusty, sneezy environment that is inferior to the allergen-trapping properties of a carpet-lined room.