Special Sorbents
To remove specific gaseous pollutants such as formaldehyde, special sorbents have been developed. Also, chemisorbents should not be expected to efficiently reduce pollutants for which these are not specifically designed, as these are specific for a limited number of reactive pollutants.
Removing Radon and its Derivatives
When source control techniques do not result in acceptable radon levels, or, are not possible, air-purifying techniques are used to reduce levels of radon gas and its derivatives.
Some radon derivatives are removed from air by filtration or electrostatic precipitation. However, the types of radon derivatives not cleaned may be of greater concern from a health point of view. Uncertainty still exists concerning the effectiveness of air purifiers in reducing the health risks associated with radon.
All pollutants cannot be effectively removed from indoor air by any air-purifying system that is presently available. When the use of other methods to reduce indoor air pollutants are not successful in reducing pollutants to acceptable levels, only then should the use of air purifiers be considered.
Some air purifying systems can remove certain particles effectively, under proper conditions, although the particles have to be suspended in the air. Some of the air purifiers containing sorbents may help eliminate some of the hazards from gaseous pollutants and may also remove a portion of these pollutants present in indoor air, at least temporarily. However, air-purifiers are not likely to completely remove all of the hazards from gaseous pollutants. In addition, gaseous pollutant removal systems may require periodic replacement of the sorbent and have a limited lifetime before replacement is necessary. Many of the gaseous pollutants from tobacco smoke are not effectively removed, even though some air-purifiers may be effective at reducing tobacco smoke particles. Some gases, as a matter of fact, may be emitted again from tobacco smoke particles captured by the air purifier.
The typical air purifier, which does not contain a specialized carbon-regenerating device, is not likely to be effective in removing radon gas and there are insufficient data to quantify the impact of air purifying on reducing the risks of lung cancer caused by radon derivatives.
How effectively air purifiers lessen allergic reactions produced by larger particles such as pollen, house dust allergens, some moulds, and animal dander, is not known with absolute certainty.
In February 1987, an ad hoc committee convened at the request of the Food and Drug Administration and several manufacturers of air-cleaning devices, to determine whether standards could be recommended for portable air purifiers, concluded "the data presently available are inadequate to establish the utility of these devices in the prevention and treatment of allergic respiratory disease."
All air filters are not equal and the filter used in most home ventilation systems is a flat filter with a thickness ranging from half an inch to one inch. It is efficient in collecting large particles that might hurt equipment. However, it is not likely to trap the particles that can be breathed deep into the lungs and cause health problems. High efficiency filters are needed to trap these small particles.
If not disturbed and suspended in the air again, pollen and house dust allergens settle down quickly from the air. Air purifiers may be relatively less effective in their removal, as only a small proportion of these allergens are generally suspended in the air.
Other allergen particles, such as animal dander, do not settle as rapidly as pollen and house dust allergens. However, the amount of allergen associated with surfaces either due to direct deposition or due to settling is generally far in excess to that in air. Air cleaning may be more effective in reducing these particles under some circumstances. However, use of an air purifier may disturb allergen that has settled on surfaces, leading to a decrease in overall allergen removal from the air.
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