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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 pollutants released by low-tox 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, but 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.

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 be inert; so, for example, don’t use ducts lined with raw fiberglass.

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 should be used intermittently to reduce humidity levels in kitchens and bathrooms quickly, while general ventilation should be ongoing in order to change the air in the entire house. Certain activities, such as hobbies which 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.

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 that are directly exposed to the living space, and use a modest amount of ventilation to meet the needs of the occupants themselves.