There are several ways to minimize indoor air pollution, but each has limitations. To determine what works best in your situation, it helps to organize pollutants into three categories:
- Pollutants originating outside the living space,
- Pollutants released by "things" inside the living space, and
- Pollutants resulting from human metabolism and activities.
Pollutants from Outside
Pollutants originating outside the living space include airborne mold spores (though these have indoor sources too) and pollen, radon in the soil, automobile exhaust, lawn chemicals, outgassing and particulates from insulation, etc. The best way to prevent these pollutants from contaminating the living space is to build a tight structure, to separate them completely from the living space. Both gaseous and particulate pollutants travel into the living space primarily by moving on air currents through random holes and gaps in the structure. (To a much lesser degree, gaseous pollutants also diffuse through solid surfaces, so installing a diffusion retarder can also help minimize entry.) Air movement and diffusion are also the primary ways water vapor moves through building assemblies.
Pollutants from Inside
Pollutants that originate from "things" inside the living space include outgassing or particulates released from home furnishings or building materials directly exposed to the interior living space (paints, wall paneling, cabinetry, etc.), evaporation from cleaning and home-maintenance products, mold spores from colonies growing within the house, pollen from house plants, etc. The best way to minimize pollution from many of these sources is to simply eliminate them, mainly by using less polluting alternatives. For example, choose low-outgassing paints and finishes, furniture without long-term formaldehyde emissions, less-noxious cleaning products etc. Cacti and succulents, which require less water than other house plants and release less pollen, can be substituted for plants requiring more damp conditions. When mold is a problem indoors, the source of the moisture that led to the fungal growth must be eliminated. Without adequate moisture, the mold will become dormant and will no longer produce spores or the metabolic VOCs (volatile organic compounds) that give it its characteristic odor.
Keep in mind that the air inside HVAC ducts will be breathed by the occupants. Therefore, a duct's interior should be considered living space. So, a tight structure should also mean tight ductwork to prevent pollutants from outside the ducts (e.g. those found in the attic and crawl space) from entering them and ultimately affecting the occupants. It's also important that the interior surface of the ducts not be a source of particles or contamination, so don't use ducts lined with fiberglass that could enter the airstream.
Dealing with People Pollution
Mechanical ventilation is best used for pollutants resulting from human metabolism (e.g., carbon dioxide, ammonia, methane, water vapor, etc.) and for occupant activities generating moisture (e.g., bathing, laundering clothes, and dish washing). Most houses need two types of ventilation. Local ventilation (e.g., exhaust fans) should be used intermittently to reduce humidity levels in kitchens and bathrooms quickly, while general ventilation should be ongoing, to change the air in the entire house. Certain activities, such as hobbies that generate their own pollutants, need to be dealt with as well. Sometimes less-toxic materials can be substituted, as when an artist switches from oil paints to water colors, but it's often necessary to use additional local ventilation in a hobby room.
Separation, Elimination, and Ventilation
Although it's usually more effective to pick the right pollution-reduction method for the job, you can certainly use one of the principles of separation, elimination, and ventilation to deal with pollutants originating from any source. For example, you could deal with formaldehyde outgassing from kitchen cabinets by coating them with a sealant. This would be a form of separating the source (the cabinets) from the living space by placing a barrier (the sealant) between the occupants and the source. But, in most cases, eliminating the source is more effective. You could also try to rely on ventilation to deal with all your indoor pollution problems. However, you'll probably need a more powerful ventilation system - one more costly to install, more expensive to operate, and probably noisier - than if you used the principles of separate and eliminate first. Overall, a three-pronged approach is best:
- Build a tight structure,
- Use low-tox alternatives for materials directly exposed to the living space, and
- Use a modest amount of ventilation to meet the needs of the occupants themselves.
What, you might ask, about filtration? Well, filtration can be combined with the ventilation system to remove airborne pollutants such as mold spores and pollen from the incoming air. Or, it can be combined with a forced-air heating/cooling system to filter out particles released by interior materials into the recirculated air (e.g. lint from cotton upholstery or drapery materials). To depend on filtration to do everything - without using the principles of eliminate, separate and ventilate - is difficult to do. Such a system will need to be very effective and very powerful; meaning costly-to-install, expensive-to-operate, and noisy. While affordable, room-sized filters are available, they are just that, room sized. They aren't designed to handle more than an average-sized room with the door closed. And even these work much better if you've implemented the other principles first. The bottom line is this: Filtration is most effective at removing the pollutants that remain after you've implemented the other three principles.
