When considering air pollution, smoke stacks or trucks often come to mind. Although these sources are certainly impact human health, indoor levels of air pollution are commonly two to five times more than outdoor levels, according to the U.S. Environmental Protection Agency. Many daily activities and possessions impact home indoor air quality. Cooking generates particles, while finishes, adhesives, carpets, furniture and cleaning products may off-gas toxins.
Because people spend 90 percent of their time indoors, air quality ranks as one of the top five environmental risks to human health. Most new homes have tighter building envelopes with less air infiltration, thus indoor air quality deteriorates without ample ventilation. This is especially noteworthy during winter months because less outdoor air typically enters the home through doors and windows, causing concentrations of indoor contaminants to rise.
“As homes get tighter, there is certainly a higher risk to indoor air quality, but we do have the tools for reducing exposure to indoor air contaminants, as well as designing proper ventilation systems, including tight ducts and filtration, that can mitigate that risk and improve indoor air quality,” says Iain Walker, a scientist for Lawrence Berkeley National Laboratory.
One of the primary strategies for ensuring fresh air indoors is through ventilation, therefore diluting the concentrations of pollutants. These are two of the most common forms of ventilation.
One of the main strategies for boosting indoor air quality is to reduce pollution at the source, preventing it from infiltrating the entire house. Exhaust-only ventilation systems use exhaust fans to remove moisture, odors, contaminants, and stale air. They are commonly found in bathrooms and kitchens (as range hoods). Although exhaust fans cannot effectively mitigate the impact of water leaks on indoor air quality (from mold and mildew), they can vent excess moisture from showering and particles from cooking. It’s imperative that these systems vent to the outside. These systems bring air into the home from leaks in the building envelope to replace the air vented from the home with the exhaust fan.
This is typically a lower-cost ventilation option that can be effective in removing contaminants from the source, but has some drawbacks. Conditioned air is vented out of the home, while unconditioned makeup air enters the home. This is not energy efficient and makes the heating or air conditioning system work harder to compensate. In more airtight homes, a negative pressure is created when air is vented out of the house, as makeup air is looking for a path into the home. This can cause air to enter through the flue of a furnace, woodstove, fireplace or gas hot water heater, causing unwanted emissions to back drafting into the home.
These systems include a variety of systems: single- or multiple-point supply; single- or multiple-point exhaust; heat recovery ventilation (HRV) systems; energy recovery ventilation (ERV) systems; and ventilation with a central air handler and partial HRV or ERV systems. Balanced ventilation systems result in more thorough ventilation, especially when considering the entire house, but they do typically have a higher upfront cost.
HRV and ERV systems transfer heat from the exhaust air to the intake air, saving money. Zehnder HRV systems are up to 95 percent efficient in transferring heat before the air leaves the home. A constant stream of fresh air is typically supplied to living spaces, while an equal amount of air exits the home, often from the bathrooms and kitchen. Such systems enhance air quality by removing contaminated air and replacing it with fresh, filtered air.
Most balanced ventilation systems filter the intake air. These filters however need to be cleaned or replaced regularly to be effective. Using filters with higher MERV rating enables finer particles such as dust and pollen to be filtered out, enhancing indoor air quality.
“If you are building a really great home and want high indoor air quality, a good filter [on the ventilation system] is really important,” Walker says. “Instead of using a cheap filter, which doesn’t do much for particles and only protects the [ventilation] equipment, use one with a filter with a MERV 13 rating, which removes a lot of what we are concerned about for health.”
Image Credit: Sarah Lozanova and Lindsay Grant/Flickr