Allergies and Air Filters
by Arnie Katz
Q: My cousin Loretta, who likes to poke her nose into my business, has a sneezing attack every time she comes to my house. She says the air in my house is polluted, and that I should get an air filter for the heating system. There's already a filter on the system, and I change it once a month like they say, even though it never gets dirty. I figure if the air in the house was dirty, the filter would get dirty, so I can't have much of a problem.
A: My cousin Shelley came to visit shortly after we moved into our new house. By the third day, she was sneezing and wheezing, her eyes were puffy, and she was miserable within minutes every time she came into the house. Lest there be any doubt that something in the house was making her sick, her symptoms stopped within minutes of leaving the house.
I write and consult and give seminars about building "Healthy Houses," and thought I had done an excellent job on mine. Yet my house was making my cousin sick. It turned out that my cousin is very allergic to pine terpenes — chemical vapors given off by pine — and my house is filled with pine — pine ceilings, pine window trim, pine floors, even some pine walls.
To make matters worse, most of this pine, except the floors and the trim in the kitchen and bathrooms, is unfinished. I wanted to minimize the use of chemicals that might give off unhealthy fumes. Thankfully, the amount of terpenes given off decreases over time and no one else seems to be allergic to my house.
Obviously, a better air filter would not have helped Shelley, who, by the way, never knew she was allergic to pine. So Rule #1: try to make sure the proposed solution has something to do with the problem. Loretta may be allergic to something in your house, but a better air filter may not be the answer. Could she, for instance, be allergic to the cat that always sleeps in the chair she like to sit on?
I do have concerns about your very clean filters. First of all, if the filter is clean, don't change it--that's a waste of money. Second, in almost every case I've seen where the filters were really clean, there was not much air moving through the filter. I've seen return ducts totally disconnected, so the fan on the furnace or heat pump was pulling air directly from the crawl space or attic. I've seen parts of the building, such as the space between floor joists or wall studs, or the space under the stairs, used as a return duct. These spaces are almost never sealed properly, so the fan simply pulls in air from all over the house through all the little cracks. The filter stays clean because very little air is flowing through it.
Finally, these filters are called furnace filters, or air conditioner filters, because they are designed to filter the air going to the equipment. They are known in the trade as "leaf and twig" filters, because their purpose is to protect the equipment from large particles of stuff. They do virtually nothing to filter the air of contaminants that affect people.
To filter the air for people, there are filters that trap gasses, such as activated-charcoal or activated-alumina filters. There are also filters that trap particles, such as extended surface (pleated) filters, HEPA filters, electrostatic filters, and electronic filters. There are whole house filters, room filters, and portable filters.
One of the best discussions of the different types and their uses is in the book The Healthy Household by Lynn Marie Bower of the Healthy House Institute.