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Asthma Health Center

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Asthma and Air Filters


Types of Air Filters

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.

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