The key principles for creating and maintaining a clean indoor environment are straightforward and can be summarized in order of effectiveness as follows:
1. Source control: Keep contaminants and pollutants out of the building.
2. Spot ventilation: Remove indoor airborne contaminants by venting them to outdoors.
3. Whole-house ventilation: Dilute indoor contaminants by bringing fresh air into the building
4. Air cleaning: Filter indoor contaminants out of the air using filters and air handling equipment
Source Control Strategy to Reduce Indoor Contaminants
The most effective way to avoid a household hazard is not to bring it into the house in the first place. In the case of building materials, this typically requires a material substitution.
For example, one of the most common indoor air pollutants is formaldehyde, widely used in wood composites such as particleboard, hardwood plywood, and medium-density fiberboard (MDF). If acceptable substitutions can be found at an affordable price, the problem is solved. Another example is fiberglass duct board, which releases small amounts of fiberglass, a lung irritant, into the air stream. Use rigid metal ducts or flexible metal-lined duct instead.
In cases where there is no acceptable alternative, look for ways to seal the chemicals in. For example, medium-density fiberboard (MDF) that is sealed on all six sides by plastic laminate, as is the case on some laminated cabinets, emits only low levels of formaldehyde. In general, a material that is impervious to water vapor can effectively block formaldehyde emissions.
Combustion devices are another major source of both gases and particulates. To keep emissions to a minimum, avoid the use of fireplaces, woodstoves, and unvented combustion appliances, including gas stoves and heaters.
If gas cooking is desired, select a unit with a pilot-less ignition. Also, substitute sealed-combustion appliances for atmospherically vented heating, ventilating, and air- conditioning (HVAC) equipment. This eliminates the possibility of flue-gas spillage and usually has higher efficiency ratings as well.
Other steps that can have a big impact on indoor air involve lifestyle changes that are decidedly low-tech, including the following:
Close windows when outdoor air is full of pollen or other particulates.
Remove footwear at the entry to prevent spreading pesticides, lead, biological materials, and a wide range of pollutants around the house.
Avoid bringing strong chemicals into the home for cleaning or hobbies.
Keep smoking, pets, workshops, and other sources of pollutants and allergens out of the main living area.
Minimize the use of dust-collecting surfaces such as carpeting, open shelves, and upholstered furniture.
Vacuum frequently with an efficient vacuum cleaner. If allergens are a concern, use a HEPA vacuum or a central vacuum that vents to the exterior.
Spot Ventilation Strategy to Reduce Indoor Contaminants
Some pollutants are created by our daily living patterns. It is far more effective to exhaust these directly at the source than to try to remove them after they are distributed throughout the household air.
The most common examples are kitchens and bathrooms. Both produce large amounts of water vapor, not a pollutant in itself, but a contributor to other problems. Too much moisture in the air significantly increases formaldehyde emissions and can lead to mold and mildew growth.
An effective range hood also removes atomized grease, particulates, and, in the case of gas ranges and cooktops, combustion by-products.
Spot ventilation is also important for darkrooms and other hobby areas that can produce high concentrations of chemical fumes. Home offices with high-capacity laser printers or photocopiers can also generate enough pollutants to justify spot ventilation.
Whole-House Ventilation Strategy to Improve Indoor Air Quality
Whole-house ventilation is designed to provide a low level of fresh air to all habitable spaces, particularly bedrooms and main living areas, and to help flush out the low levels of pollutants generated by occupants, pets, and building materials.
Occupants and pets produce moisture, carbon dioxide, and odors. In addition, most homes have a certain amount of chemical and biological pollutants from pets, cleaning, and hobbies and from outgassing from paints, plastics, pressed wood products, fabrics, and other household materials.
Whole-house ventilation is not meant to take the place of spot ventilation, which is still required to exhaust concentrated pollutants from cooking, bathing, and hobby areas.
Although not yet required in most current building codes, whole-house ventilation is being incorporated into more and more new homes, and is recommended by model energy codes and standards organizations, such as the American Society of Heating, Refrigerating, and Air- Conditioning Engineers (ASHRAE) and the National Fire Protection Association (NFPA).
Operating costs. It is important to note that mechanical ventilation costs money both to operate the fans and to heat or cool the incoming fresh air. To provide the recommended ventilation levels, using an efficient fan in either an exhaust or supply system, the annual cost for a 1,500-square-foot house ranges from about $150 to $200 per year, depending on climate and fuel costs.
Heat-recovery ventilators (HRVs) have higher electrical costs with dual fans but save money through heat reclamation, so annual energy costs are similar.
New homes. Plan to run the ventilation system at high speed for at least the first few months of occupancy, since paints, plastics, pressed wood products, and many other materials will outgas at their greatest rate during this period. If the house still smells of fresh paint or new carpet, volatile organic compound (VOC) levels are still too high.
Air Cleaning Strategy for Improving Indoor Air Quality
Air cleaning is the least effective strategy for maintaining a healthy indoor environment, but it can play a role along with source control and ventilation. There are many different types and sizes of air filters on the market, both portable units and filters integrated into the home’s HVAC system. Situations that may call for air cleaning equipment are:
Where the outside air is polluted or full of pollen and needs to be filtered before bringing the air into the house.
Homes with high pollutant sources, such as tobacco smoke or certain hobbies.
Individuals with asthma, allergies, or chemical sensitivities.
Homes with high levels of indoor dust, for example pets shedding hair and dander, a building in a dusty environment, and where finding a new home for the pets or moving out of the dusty environment are not options.
Different approaches to filtering pollutants from indoor air are discussed under Air Cleaning Strategies.
— Adapted with permission from Best Practices Guide to Residential Construction.
Building Fresh Air Requirements – 15 cfm per person
The U.S. EPA recommends an outdoor fresh air supply of at least 15 cfm per person.
In occupied buildings, especially offices and other locations with multiple occupants, we like to take a look at the carbon dioxide level as one means to check for inadequate fresh air intake for the building.