If you live with a smoker, an air filter or room air cleaner is likely to be helpful. Secondhand smoke always worsens asthma symptoms. Secondhand smoke also causes nasal congestion for small children. Almost all room air cleaners efficiently remove smoke from the room (as long as the air filter is large enough, the fan turned on, and the air filter is maintained).
Can air filters help prevent asthma symptoms? Maybe. Room air cleaners remove small particles that are in the air near the air cleaner. However, room air cleaners don't remove small allergen particles that are caused by local disturbances, such as the microscopic house dust mite feces that surround a pillow when your head hits it (or you turn over in bed). You inhale these allergens before they ever get near the room air cleaner. Room air cleaners take 5 to 15 minutes to remove such temporary local sources of dust and allergens.
Wall-to-wall carpets also provide a massive source of allergens, which simply cannot be removed by vacuuming or the use of a room air cleaner. However, these accumulated allergens remain in the carpet until disturbed. Vacuuming carpets, which uses a beater brush, causes very large amounts of allergens to be thrown into the room, even if a HEPA vacuum cleaner is used. Room air cleaners are also unlikely to be adequately effective if the source of the allergens remains in the home, such as a cat, dog, bird, or hamster.
Room air cleaners only work for the room in which they are placed. Since you spend eight hours in your bedroom every night, this should be the first room in which to locate a room air cleaner. However, you may also spend several hours each day in your kitchen, TV room or office, so you may need a separate room air cleaner for each of these rooms.