The big ten
Use of air filtration not only makes air clean, but also enhances energy, alertness and comfort. It can help individuals with respiratory ailments, such as allergy and asthma, enhancing their quality of life. With in-room air filtration recognised as a legitimate tool for improving personal and public health, air purifiers are being used in cleanroom environments in industries, homes, offices and schools. In selecting an air purifier, analyses of room size, the allergens and pollutants, and any existing health concerns should be conducted. This not only allows matching the need with the technology, but also saves time and money in the long run.
What's available? In-room air purification systems utilise two basic technologies: mechanical or electrostatic filtration, or a combination of both. Both technologies are based on four well-understood principles that describe airflow between obstructions (filter fibres) and the behaviour of particles as they ride the air stream through the filter media. Most air purifiers use high efficiency particulate air (HEPA) filters, a technology developed in the 1940s. These are glass fibre filters which are not friendly to the environment and have a very high pressure drop. They use all four principles to capture particles of varying sizes and are efficient at capturing the mid-range particles that pose the greatest threat. To increase the percentage of small particles intercepted, electrostatic filters use an electric charge to boost the force of attraction between particles and filter fibres. Passive electrostatic filtration charges the filter media, achieving high efficiency at first; as the charge decays, efficiency gradually diminishes. Active electrostatic systems continually charge either the filter fibres or the particles, maintaining consistent efficiency levels over time. Active electrostatic technology alone is ineffective in removing mid-range and large particles. Hybrid systems combine mechanical and electrostatic technologies, often achieving significantly better performance. At Blueair, the HEPASilent filter technology uses all four principles of mechanical filtration and adds ionisation to achieve higher efficiency in capturing the small particles trapped by interception. The following steps will hopefully lead to a better understanding of purification systems in a cleanroom environment.
1. Filter efficiency Before buying an air purifier, ensure you know the highest percentage of particles removed by unit and the smallest particle size captured. Filter efficiency may differ for particles of varying sizes. For example, the HEPASilent system is rated at 99.9% efficiency at 0.3µ and 99.97% efficiency at 0.1µ. Air cleaning performance is another criteria to consider. In the US, a rating called the Clean Air Delivery Rate (CADR) was developed to make meaningful comparisons between air purifiers in terms of actual performance. CADR measures the total volume of air that a particular air purification system cleanses of a specific pollutant in one minute. The higher the rating, the faster the unit filters the air (visit www.cadr.org for more information).
2. Capacity Next, consider air volume capacity. Make sure the unit is rated to handle the total volume of air in the room. Blueair units, for example, are designed for room capacities from 18.6m2 to 62.0m2. Room size recommendations are expressed in square metres and assume 2.4m high ceilings. Simply measure the room's length and width, and multiply to obtain the square metres required. Be sure to include ceiling height in calculating the unit's air processing capability. For example, if a room measures 3 x 6m with an 2.4m high ceiling, you need a unit recommended for 432m2. For the same room measuring 3 x 6m, but with a 3.3m high ceiling, you need a unit recommended for 59.4m2.
3. Specific health concerns Air cleaning alone cannot adequately remove all of the pollutants typically found in indoor air. However, when combined with source removal and adequate ventilation, air cleaning can be considered a viable means of improving indoor air quality. With the exception of allergens and airborne pathogens, the size of a given particle determines the degree of potential threat to human health. Generally, airborne particles are identified as small (0.1 to 0.3µ), mid-range (0.3 to 0.9µ) and large (1.0µ or larger). An air filter model's efficiency may differ for particles of different sizes. The addition of active electrostatic filtration to HEPA filtration allows certain units to achieve higher efficiency for small particles trapped by interception. Some HEPA filters are only effective for mid-range particles. Mid-range particles: Mid-range particles present the greatest health concern as they are small enough to get past the cilia in the nasal passages and too large to be easily exhaled. Particles in this size range are more likely to become lodged in lung tissue and are suspect in a wide range of related health problems, from headaches and dizziness to cardiovascular disease and cancer. These particles (0.3 to 0.9µ) include house and textile dust, pollen, pet dander, dust mites and their faeces, many bacteria, auto exhaust, mould spores, and particles from laser printers and copiers. Small particles: Small particles (0.1 to 0.3µ) include many viruses, bacteria, gases and chemical fumes. Although these small particles are inhaled and exhaled more easily than mid-range particles, even these minute particles may irritate already compromised breathing passages and lungs. Large particles: Large particles (1.0µ and larger) are usually trapped by cilia in the nasal passages before reaching the lungs. Large particles include house dust and some of the larger pollen and bacteria. Airborne pathogens: Airborne pathogens range in size from .018µ to as large as 1.325µ. While many bacteria and viruses fall within HEPA and electrostatic capture ranges, some pathogens capable of movement may be able to pass through the filter media, and the mucilaginous coatings of some bacteria may affect the filter's ability to trap them.
4. Manufacturer's claims Check the manufacturer's legitimacy and performance claims before purchasing a unit. Watch for uncorroborated filter efficiency and air volume capacity statements, no actual address on brochures or websites or lack of detailed information on filter composition or technology.
5. Indoor environmental factors From dust and humidity to the types of particulates likely to be present, indoor environmental conditions vary widely. Dust: Some units have a built-in pre-filter, which can help strain out large dust particles before they fill the spaces between HEPA filter fibres. You can also trap dust by installing vent filters over heating and air cooling registers. Humidity: Environments with high relative humidity levels (over 50%) are of concern when allergies or asthma are triggered by mould and mildew spores and when immune disorders are present. Absorbent HEPA filter media retain ambient moisture and create an ideal internal environment for mould and bacterial growth. To combat this, some manufacturers recommend periodic filter sterilisation or adding chemical-based bacteriostats. Hydrophobic filter media, such as the polypropylene employed in Blueair's HEPASilent filter, counter mould and bacterial growth by eliminating moisture retention to start with. Water-repellent fibres perform as well as absorbent fibres in the context of HEPA filtration, while remaining dry and free from water-related mould and bacteria colonisation. Tobacco smoke: Due to its pervasiveness and its complex composition, tobacco smoke is a particular challenge for current air filtration technology, as are other gaseous pollutants. While HEPA filters can remove some tobacco smoke particles, none can remove all of the 4,000 chemicals that comprise tobacco smoke, according to the U.S. Environmental Protection Agency. VOCs: Volatile organic compounds (VOCs) are gas particles introduced into room air through the indoor use of pesticides, glues, solvents and cleaning agents. They include a variety of chemicals emitted as gases from petroleum-based solids and liquids. To combat this, look for a HEPA filter with activated carbon compounds. Filter replacement is critical to avoid the eventual release of VOCs from the saturated filter. Filter changing is recommended by the manufacturer to reduce the risk of outgassing.
6. Operating cost Check the manufacturer's recommended replacement interval and the filter cost. Also check the energy efficiency of the unit. Usually, the unit's energy use, expressed as watts, is included with the technical specifications on the packaging or operating instructions. Bear in mind that filter efficiency claims are based on constant, rather than intermittent, operation.
7. Construction quality Look for a system with quality construction, especially internal moving components such as fans and blowers. The durability of materials used for the cabinet is a strong secondary consideration. An air cleaner housed in a metal versus a plastic cabinet is likely to produce less operating noise.
8. Ease of use Filter change can be as simple as lifting the unit's lid and sliding filters in and out, or a complicated operation involving hand tools. Look for units for easy filter change, not needing heavy lifting or tools. Internal cleaning is a particular issue with electrostatic precipitators, in which filters are cleaned and sterilised rather than replaced.
9. Warranty Look at the warranty in terms of length and coverage. Most manufacturers offer a limited warranty for a specific period of time. A limited warranty covers only those components specified and may not cover the cost of diagnosis, labour and/or shipping in the event of product failure.
10. Noise, air flow and space Operating noise can be a significant factor when the unit is used in a quiet environment. Request specific operating noise values, expressed in decibels. Draft from the unit's air processing system is not as easy to quantify. This can be evaluated by seeing (or feeling) the unit in operation. Some units direct a strong stream of air from one or two exit points, while others diffuse air through many exit points to mitigate uncomfortable drafts. Select a unit that is efficient and has the required air volume capacity and consider space requirements. Check operating instructions for placement advice. Most units require specific clearance from walls and windows to operate effectively. In conclusion, in-room air filtration is recognised as a legitimate tool for improving personal and public health. In selecting a unit, make sure the unit's capacity matches your needs.
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