Use of purifiers

Dust, pollen, pet dander, mold spores, and dust mite feces can act as allergens, triggering allergies in sensitive people. Smoke particles and volatile organic compounds (VOCs) can pose a risk to health. Exposure to various components such as VOCs increases the likelihood of experiencing symptoms of sick building syndrome.H.M. Ang, M Tade, S Wang. (2007). "Volatile organic compounds in the indoor environment and photocatalytic oxidation: state of the art". Environmental International 33: 694-705. Air purifiers are used to reduce the concentration of these airborne contaminants and are especially useful for people who suffer from allergies and asthma. They also reduce the need for frequent household cleaning. Air purifiers use a small amount of electrical energy, causing a small amount of expense and environmental effect.

Purifying techniques

Several different processes of varying effectiveness can be used to purify air. Different processes may remove different contaminants, so there is advantage in using more than one process in a purifier.

• Filter-based purification traps airborne particles by size exclusion. Air is forced through a filter and particles are physically captured by the filter.

HEPA filters can, by definition, remove at least 99.97% of 0.3-micrometer particles, and are usually more effective for particles which are larger or slightly smaller. They are effective down to 0.01 micrometers in many cases, but become very ineffective for particles smaller than 0.01 micrometer. HEPA purifiers which filter all the air going into a clean room must be arranged so that no air bypasses the HEPA filter. In dusty environments, a HEPA filter may follow an easily cleaned conventional filter (prefilter) which removes coarser impurities so that the HEPA filter does not need to be changed or cleaned frequently. HEPA filters do not generate ozone or other harmful byproducts.

Filter for HVAC at MERV 13 or above can remove airborne particles of 0.3 micrometers or larger. A medium efficiency MERV 13 has a capture rate of 75% for particles between 0.3 to 1.0 micrometers. Although the capture rate of a MERV filter is lower than that of a HEPA filter, a central air system can move significantly more air in the same period of time. Using a high-grade MERV filter can be more effective than using a high-powered HEPA machine at a fraction of the initial capital expenditure. Unfortunately, most furnace filters are slid in place without an airtight seal, which allows air to pass around the filters. This problem is worse for the higher-efficiency MERV filters because of the increase in air resistance. Higher-efficiency MERV filters are usually denser and increase air resistance in the central system, requiring a greater air pressure drop and consequently increasing energy costs.