context: If you have asthma symptoms, an air filter or room air cleaner may help you to breathe better. The same is true for those with hay fever (allergic rhinosinusitis) or COPD (emphysema or chronic bronchitis). 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. There are many types of air filters available, including: Mechanical air filters use a fan to force air through a special screen that traps particles such as smoke, pollens, and other airborne allergens. The high-efficiency particulate air ( HEPA) filter is the best-known air filter. HEPA (which is a type of filter, not a brand name) was developed during World War II to prevent radioactive particles from escaping from laboratories. To qualify as a true HEPA filter, the air filter must be able to capture at least 90% of all particles 0.3 microns or larger in diameter that enter it. A disadvantage of these units is the noise of their fan and the cost of electricity to operate the fan motor. The noise can be reduced by using a large unit designed for a larger room, and running the fan motor at a lower speed. The fan should not be directed at a carpet or drapes, since this is likely to raise dust from the carpet or drapes. " Ultra- HEPA" room air cleaners are available, which remove even higher proportions of very small particles. There is no convincing evidence that they are more likely to improve asthma control when compared to conventional HEPA air cleaners (with a similar CADR, see below). Electronic air filters use electrical charges to attract and deposit allergens and irritants. If the device contains collecting plates, the particles are captured within the system; otherwise, they stick to room surfaces and have to be cleared away. A disadvantage of these units is that almost all of them create small amounts of ozone. Ozone irritates airways, causing temporary bronchospasm in those with asthma, and nasal congestion in those with hay fever or rhinitis. Hybrid air filters contain elements of both mechanical and electrostatic filters. Gas phase air filters use activated carbon granules to remove odors (volatile organic compounds or VOCs) and non-particulate pollution such as cooking gas, gases emitted from paint or building materials (such as formaldehyde), and perfume. These thin black filters are often placed in front of HEPA filters. A disadvantage of these filters is that they quickly become ineffective as they absorb a load of fumes, so they must be replaced as often as every month. However, they are relatively inexpensive. An advantage is that they remove large particles (acting as a pre-filter), thereby increasing the life span of the expensive HEPA filter. Germicidal air cleaners use ultraviolet ( UV) lights to kill bacteria, viruses, and molds that pass through the area with the UV light. Such UV lights can be included with other air cleaner devices, which use a fan. We are unaware of any research demonstrating that the addition of germicidal UV lights improves respiratory symptoms in people with lung disease. However, they have been used for decades to reduce the risk of infection with tuberculosis among staff and visitors in hospitals who are actively treating such patients. Ozone generators are devices that intentionally produce high concentrations of ozone to clean the air in a room. They are often used to decontaminate rooms after smoke exposure following a fire. Ozone causes bronchospasm in people with asthma, even in low concentrations, and thus should be avoided. Whole-house air cleaners may be used if your home is heated or air-conditioned through ducts. With a whole-house air cleaner, HVAC system includes air filters designed to reduce the accumulation of dust and dirt in the ducts and coils of the system. These simple filters cost less than a dollar each to replace every month or two, but they only remove large particles, not the small particles in the house that are inhaled into the lungs. You can buy more efficient replacement filters (usually for 6 to 20 dollars each) which will remove many smaller particles. These are often pleated or coated with an electrostatic charge. However, these replacement filters clog quickly in dusty environments, reducing airflow through the system, causing a reduction in the heating or cooling efficiency. Permanent whole-house air cleaners can be added to an HVAC system, but the cost is several hundred to a few thousand dollars for the unit and the installation. Disadvantages include the ozone byproduct of electrostatic air cleaners; the need for frequent cleaning of the plates; the need to keep the fan running continuously (24/7) to clean the air, and the electricity cost and noise associated with the large blower fan running continuously. Ask the following questions before purchasing an air filter: Is it large enough? Will the unit clean the air in a room the size of my bedroom every four to six minutes at a fan speed I can tolerate? An adequate size is determined by the device's clean air delivery rate ( CADR). The Association of Home Appliance Manufacturers ( AHAM) rates air cleaners according to their clean air delivery rates; see the AHAM web site for details. How difficult is it to change (or clean) the filter? Ask for a demonstration. How often does the filter (or the two different filters) have to be changed (in an average home with a smoker, which is the worst case)? How much do the replacement filters cost? ( Also check the cost when buying them online) What is the estimated cost to run the unit continuously for a year? How much noise does the unit make? This is very important for the HEPA units that include a fan. The CADR rating is only when you run the fan on the high speed, which is always the noisiest. Is it quiet enough to run while you sleep? ( Turn it on and try it, even though you will probably be in a store and may not get a true sense of just how noisy it is in the quiet bedroom environment.) If you or a loved one has asthma symptoms and there's a smoker in your home, an air filter is likely to improve your asthma control. If there is no secondhand smoke in your home, air cleaners may not help your asthma. If you are the one smoking, a room air cleaner will only help the nonsmokers in your home, not you. The bottom line on using air filters to prevent an asthma attack? If you're a healthy individual living in a relatively unpolluted environment, there's probably no need to spend the money.