By Paul Ehrlich • Clean Air Issues Coordinator
Several years ago, I wrote an article for the Raritan Valley Group webpage on using residential air filters to help treat several respiratory diseases. Since that time new filters have been developed that can remove significantly smaller particles than earlier filters, a major improvement in their efficacy. Research studies have also contributed to development of best practices for using air filters—for example, where to place the filters for maximum efficacy. Below I’ve described several characteristics of residential air filters, including recent improvements.
Particles and Gases
There are two major types of air pollutants: particles and gases. High-efficiency particulate air (HEPA) filters are very effective for filtration of particles, and these filters can remove smaller particles than HEPA filters manufactured several years ago, but they have little or no effect on the concentration of gas molecules. A different type of filter, typically activated charcoal, is necessary for a large effect on gas molecules. However, while activated charcoal is very effective at removing hydrocarbons and other gaseous chemicals, it is not effective in trapping some gaseous molecules, such as carbon dioxide, ammonia, and oxides of sulfur.
Assembling the Residential Air Filter Device
Residential air filters can produce cleaner air if the air is filtered by at least two types of filters: HEPA filters to remove particles and activated charcoal to remove gases. Activated charcoal traps pollutants that are sometimes released back to ambient indoor air. In addition, charcoal has a relatively small capacity for trapping pollutants (which is why charcoal filters have to be replaced more often than HEPA filters). Therefore, activated charcoal should be the last filter before clean air is ejected from the filtering device. HEPA-filtered air should be immediately directed to the charcoal filter. The charcoal layer should be about 0.125 inches to 2 inches thick. While thicker layers are better for filtration, excessively thick layers can cause a significant drop in air pressure that could damage the device.
In my experience, charcoal filters that are included with new residential air filters are not thick enough to provide significant filtration. If only two charcoal filters can fit into the filter chamber, an extra-large (length x width) HEPA filter can be placed above the air intake grid plus the surrounding area of the device. Thus, only air that has passed through the HEPA filter will continue to the charcoal filter. This is important in several areas of the filtering device—even a small amount of air bypassing any filter will significantly reduce air quality.
Placement of Filtering Device
It is best for the filtering device to be used in a room with doors and windows closed. If it is necessary to keep a door open, or partly open (for example, for small children), filtering can still retain some efficacy. Placing a filtering device in some parts of a room can be advantageous in delivering air as clean as possible to individuals in the room. For residential air filters that clean air from diffuse sources of pollutants, the device should be positioned close to individuals for the benefit of the cleanest air in the room that is not being mixed with relatively dirty air.
That is, individuals are close to the filtering device’s area where the clean air first leaves the device. The device will create a breeze that could be uncomfortable so adjustments in speed of air flow and distance from the device may be required. However, for cleaning air that originates from a point source, such as a printer, it is best to position the device close to the source to maximize filtering of the air, even if the filtration unit must be placed relatively far from the person to be helped by filtering. This will help prevent turbulence that causes significant dirty air to mix with the clean air exiting the filtering device.
Additional Advantages and Disadvantages of HEPA/Charcoal Filters
If the HEPA and charcoal filters are not subject to significant air movement when removed from the device, or the filters are not subject to abrupt movements, it is very unlikely that pollutants or pollutant byproducts will detach from the filters.
It is sometimes difficult to determine when to change filters. Manufacturers usually print advice on the packaging or include directions in the package. In my experience this advice is not always relevant. There are many factors that determine filter efficacy, so a good method is to determine whether the filter(s) has lost some ability to mitigate pollutant effects. Charcoal filters can be tested by determining whether a gaseous, nontoxic chemical’s smell is removed quickly. You could also test to see if the device successfully removes food odors after cooking.
A fan should be included in all air filters, or the air will not move fast enough to remove a significant amount of pollutants. The fan can cause enough noise to annoy a few people, especially if they are trying to fall asleep. The speed of the fan could be lowered so there is less noise. In addition, the filtering device could be moved farther away from individuals bothered by the noise. Both options have drawbacks, but it is likely that significant filtering will still occur.