MAINTENANCE : Air Filters Can Suck Up Some of the Dangers That Lurk Within
Many of today’s tightly built, energy-efficient homes don’t allow natural indoor pollutants to escape. In fact, recent government studies suggest that indoor air can be 10 times more polluting than outdoor air.
Many homeowners are focusing their attention on a new family of high-efficiency air filters to get the job done. Combined with a forced-air heating system or central air conditioning, these units are becoming increasingly popular in new installations.
When considering air filters, it’s important to note that the filters only catch particulates. They cannot trap noxious gases, such as radon and those emitted by synthetic materials.
Air filter claims as to effectiveness can be confusing. The problem has to do with the variety of testing formats in use. What the rating does not tell you is that a filtering device might be nearly 100% efficient with a smaller size. What’s more, actual efficiencies change throughout the period of operation and vary with the age of the filter and the level of maintenance.
To keep things manageable, we’ll only consider overall efficiency, as defined by the ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers) Atmospheric Dust Spot Test.
Do these ratings mean that 90%-efficient filters will catch and hold 90% of the dust in your home? Not at all. They can only filter the dust that is pulled through your furnace. Not all dust particles, however, remain suspended in the air. The larger, heavier particles settle out unless disturbed. Many of these heavy particles include the most common allergy triggers.
Looking at specific types of filters, electrostatic replacement filters are designed to fit standard return-air carriages made for disposable, fiberglass filters. They are also a good deal less expensive (and less effective) than paper cartridge and two-stage ionization filters.
Lab tests rate electrostatic filters at up to 20% efficiency, as contrasted with the 3% to 4% offered by disposable fiberglass filters. The difference lies in the filtration media, which is a densely woven, petroleum-based material such as polyester.
Paper cartridge filters boast enormous surface area, and the dirtier the filter gets, the more effective it becomes--up to a point. The reason is that the dust accumulation actually increases the surface area of the filter.
While overall efficiency for this type of filter starts in the 25% to 30% range, it can be nearly 100% efficient in arresting particles in the 6 to 10 micron range--a size that includes many of the common allergy triggers such as pollen.
The most efficient--and expensive--alternative is the electronic ionization filter. These devices must be spliced into the return-air duct by professionals. A two-stage ionization filter consists of a coarse metal mesh that ensnares only the largest particles. Particles that make it through the mesh move past a series of light-gauge electrified wires that ionize, or charge, the full-size range of particles.
The particles are then attracted to, and trapped by, a network of metal plates mounted just behind the wires. A clean filter cell can have an overall efficiency of 90% to 95%, but the efficiency ratings fall off dramatically when the filter is not kept clean.
