Inexpensive air filter for allergy relief.

We have found an inexpensive way to filter home air to remove dust, pollen, pet dander, and other particles. Many people with sinus problems, hay fever, or other conditions may find this helpful, especially now as the spring allergy season arrives.

The idea is to use a 20-inch box fan to pull air through a 20-inch square furnace filter. Everything needed (the fan and the filter) can be found at a local hardware store for under $30. No construction is required.

While this is not equivalent to the home air filters that cost hundreds of dollars, it does filter air rapidly and remove a considerable amount of foreign material from it. In my experience it greatly relieved hay-fever symptoms, and resulted in much less dust being deposited in the room where it is used.

We have not heard of this idea elsewhere. It should be considered for asthma and other conditions, because many people cannot afford the commercial home air purifiers--some of which seem unlikely to work well.

Notes:

* Good air conditioning will already filter the air, so the filter we suggest may not make much difference in addition.

* To avoid cutting or construction, the fan and filter should be the same size, and the fan should have a flat air-intake grate with no obstructions, allowing the filter to fit on it snugly. It is probably best to avoid an oscillating fan. When the fan is on, the force of the air alone will hold the filter against it. But it may be convenient to use a couple pieces of adhesive tape, for example, to hold the filter in place when the fan is turned off. This will still allow the filter to be easily detached, for using the fan separately.

* A 3-speed 20-inch box fan can be found for about $25, although some are more expensive. We used the Lasko model 3723, which we found for about $20 at a small hardware store; you can also find it on the Web--try a Google search for "fan Lasko 3723" (without the quotation marks). Disposable fiberglass furnace filters cost about $2 each, and will usually last for weeks or months. We prefer a better disposable furnace filter (such as the "Ultra Allergen" from 3M), which costs about $20 each at the hardware store, somewhat less on the Web.

* Fiberglass can be hazardous to children or pets. Make sure they cannot get into it, or use a different kind of filter.

* Most furnace filters have a printed arrow on the edge, showing the direction air should flow.

* Household fans are not intended for outdoor use. So if a fan is used near an open window to bring in outside air, make sure it will not get wet in case of a storm.

* Most household fans are not intended to be used with a filter, which restricts airflow to some degree. We have kept our fan on a low setting, as a precaution against the motor overheating. Most fiberglass filters reduce airflow very little.

* If you want to use the better filter but cost is an issue, consider using both together, so that only particles that get through the fiberglass are caught by the expensive filter, extending its useful life.

Let us know whether or not you have good results with this improvised air filter--and send us any suggestions that other readers should know about.