All of us face a variety of risks to our health as we go about our day-to-day lives. Driving in cars, flying in planes, engaging in recreational activities, and being exposed to environmental pollutants all pose varying degrees of risk. Some risks are simply unavoidable. Some we choose to accept because to do otherwise would restrict our ability to lead our lives the way we want. And some are risks we might decide to avoid if we had the opportunity to make informed choices. Indoor air pollution is one risk that you can do something about.

In the last several years, a growing body of scientific evidence has indicated that the air within homes and other buildings can be more seriously polluted than the outdoor air in even the largest and most industrialized cities. Other research indicates that people spend approximately 90 percent of their time indoors. Thus, for many people, the risks to health may be greater due to exposure to air pollution indoors than outdoors.

Causes of Indoor Air Problems

Indoor pollution sources that release gases or particles into the air are the primary cause of indoor air quality problems in homes. Inadequate ventilation can increase indoor pollutant levels by not bringing in enough outdoor air to dilute emissions from indoor sources and by not carrying indoor air pollutants out of the home. High temperature and humidity levels can also increase concentrations of some pollutants.

There are many sources of indoor air pollution in any home. These include:

• Combustion sources such as oil, gas, kerosene, coal, wood, and tobacco products
• Building materials and furnishings as diverse as deteriorated, asbestos-containing insulation, wet or damp carpet, and cabinetry or furniture made of certain pressed wood products
• Products for household cleaning and maintenance, personal care, or hobbies
• Central heating and cooling systems and humidification devices
• Outdoor sources such as radon, pesticides, and outdoor air pollution.

The relative importance of any single source depends on how much of a given pollutant it emits and how hazardous those emissions are. In some cases, factors such, as how old the source is and whether it is properly maintained are significant. For example, an improperly adjusted gas stove can emit significantly more carbon monoxide than one that is properly adjusted.

Some sources, such as building materials, furnishings, and household products like air fresheners, release pollutants more or less continuously. Other sources, related to activities carried out in the home, release pollutants intermittently. These include smoking, the use of un-vented or malfunctioning stoves, furnaces, or space heaters, the use of solvents in cleaning and hobby activities, the use of paint strippers in redecorating activities, and the use of cleaning products and pesticides in housekeeping. High pollutant concentrations can remain in the air for long periods after some of these activities.

If too little outdoor air enters a home, pollutants can accumulate to levels that can pose health and comfort problems. Unless they are built with special mechanical means of ventilation, homes that are designed and constructed to minimize the amount of outdoor air that can "leak" into and out of the home may have higher pollutant levels than other homes. However, because some weather conditions can drastically reduce the amount of outdoor air that enters a home, pollutants can build up even in homes that are normally considered "leaky."

Outdoor air enters and leaves a house by: infiltration, natural ventilation, and mechanical ventilation. In a process known as infiltration, outdoor air flows into the house through openings, joints, and cracks in walls, floors, and ceilings, and around windows and doors. In natural ventilation, air moves through opened windows and doors. Air movement associated with infiltration and natural ventilation is caused by air temperature differences between indoors and outdoors and by wind. Finally, there are a number of mechanical ventilation devices, from outdoor-vented fans that intermittently remove air from a single room, such as bathrooms and kitchen, to air handling systems that use fans and duct work to continuously remove indoor air and distribute filtered and conditioned outdoor air to strategic points throughout the house. The rate at which outdoor air replaces indoor air is described as the air exchange rate. When there is little infiltration, natural ventilation, or mechanical ventilation, the air exchange rate is low and pollutant levels can increase.

Identifying Air Quality Problems

A way to judge whether your home could develop indoor air problems is to identify potential sources of indoor air pollution. Although the presence of such sources does not necessarily mean that you will have an indoor air quality problem, being aware of the type and number of potential sources is an important step toward assessing the air quality in your new home. Another way to decide whether your home may have poor indoor air quality is to look at your lifestyle and activities. Human activities can be significant sources of indoor air pollution. Finally, look for signs of problems with the ventilation in your home. Signs that can indicate your home may not have enough ventilation include moisture condensation on windows or walls, smelly or stuffy air, dirty central heating and air cooling equipment, and areas where books, shoes, or other items become moldy.

Moisture in Your House

Cooking, bathing, washing clothes, even breathing can lead to damaging moisture buildup in your house that can produce mold.

In these times of tighter construction methods, water vapor that once escaped now becomes trapped inside. Over time, a moisture-laden house is more than just uncomfortable. There can be extensive damage to the structure.

Your house suffers from excess moisture if its relative humidity is 50 percent or higher. This could mean that condensation puddles on windows, black mildew speckles walls and ceilings, or mold looms large in a closet.

Sometimes moisture damage is not as noticeable. For instance, moisture buildup in the walls can dampen insulation and rot wood framing. If left too long, excess dampness can produce mold-stained wallboard, water-soaked insulation, or rotten wood framing that needs replacing.

Here are several steps you can take to reduce moisture:

• Install a whole-house air-to-air heat exchanger, which brings fresh air inside as it exhausts stale, moist air outside. This appliance is ideal for a newer, airtight home.
• Outfit your furnace with a fresh air return to bring air inside when moisture levels get too high.
• Make sure ground moisture does not enter your home's foundation.
• Cover basement sumps so moisture is not produced as water evaporates. If your laundry room is in the basement, make sure your clothes dryer is vented to the outside to get rid of moisture, and don't let water stand in tubs.
• See that your attic is equipped with adequate vents. A moist attic means moist insulation, and moist insulation is less effective. Moisture will also rot wood, cause mildew, and trigger formation of water stains on the ceiling.
• To promote air circulation, you may want to replace solid doors with vented ones throughout your home. The door to the basement may especially benefit, allowing moist air to escape upstairs and outside.

Three Basic Strategies to Improve Indoor Air Quality

Source Control

Usually the most effective way to improve indoor air quality is to eliminate individual sources of pollution or to reduce their emissions. Some sources, like those that contain asbestos, can be sealed or enclosed; others, like gas stoves, can be adjusted to decrease the amount of emissions. In many cases, source control is also a more cost-efficient approach to protecting indoor air quality than increasing ventilation because increasing ventilation can increase energy costs.

Ventilation Improvements

Another approach to lowering the concentrations of indoor air pollutants in your home is to increase the amount of outdoor air coming indoors. Most home heating and cooling systems, including forced air heating systems, do not mechanically bring fresh air into the house. Opening windows and doors, operating window or attic fans, when the weather permits, or running a window air conditioner with the vent control open increases the outdoor ventilation rate. Local bathroom or kitchen fans that exhaust outdoors remove contaminants directly from the room where the fan is located and also increase the outdoor air ventilation rate.

New home designs are starting to feature mechanical systems that bring outdoor air into the home. Some of these designs include energy-efficient heat recovery ventilators (also known as air-to-air heat exchangers).

Air Cleaners

There are many types and sizes of air cleaners on the market, ranging from relatively inexpensive tabletop models to sophisticated and expensive whole-house systems. Some air cleaners are highly effective at particle removal, while others, including most tabletop models, are much less so. Air cleaners are generally not designed to remove gaseous pollutants.

The effectiveness of an air cleaner depends on how well it collects pollutants from indoor air (expressed as a percentage efficiency rate) and how much air it draws through the cleaning or filtering element (expressed in cubic feet per minute). A very efficient collector with a low air-circulation rate will not be effective, nor will a cleaner with a high air-circulation rate but a less efficient collector. The long-term performance of any air cleaner depends on maintaining it according to the manufacturer's directions.

Another important factor in determining the effectiveness of an air cleaner is the strength of the pollutant source. Tabletop air cleaners, in particular, may not remove satisfactory amounts of pollutants from strong nearby sources.

Building a New Home

When you are building a new home, you are probably thinking about floor plans and finish materials. Of course you want your new home to look good. But creating a comfortable and healthful environment for your family is even more important.

Building a new home provides the opportunity for preventing indoor air problems. However, it can result in exposure to higher levels of indoor air contaminants if careful attention is not given to potential pollution sources and the air exchange rate.

Express your concerns about indoor air quality to your builder and enlist his or her cooperation in taking measures to provide good indoor air quality. Talk both about purchasing building materials and furnishings that are low emitting and about providing an adequate amount of ventilation.

The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) recommends a ventilation rate of 0.35 ach (air changes per hour) for new homes, and some new homes are built to even tighter specifications. Particular care should be given in such homes to prevent the build-up of indoor air pollutants to high levels.

Here are a few important building techniques that can make a difference:

• Provide proper drainage and seal foundations in new construction.
• Become familiar with mechanical ventilation systems and consider installing one.
• Advanced designs of new homes are starting to feature mechanical systems that bring outdoor air into the home. Some of these designs include energy-efficient heat recovery ventilators (also known as air-to-air heat exchangers).
• Ensure that combustion appliances, including furnaces, fireplaces, woodstoves, and heaters, are properly vented and receive enough supply air.
• Combustion gases, including carbon monoxide, and particles can be back-drafted from the chimney or flue into the living space if the combustion appliance is not properly vented or does not receive enough supply air. Back drafting can be a particular problem in weatherized or tightly constructed homes. Installing a dedicated outdoor air supply for the combustion appliance can help prevent back drafting.

There are many actions a homeowner can take to select products that will prevent indoor air problems from occurring - a couple of them are mentioned here. First, use exterior-grade pressed wood products made with phenol-formaldehyde resin in floors, cabinetry, and wall surfaces. Or, as an alternative, consider using solid wood products. Secondly, if you plan to install wall-to-wall carpet on concrete in contact with the ground, especially concrete in basements, make sure that an effective moisture barrier is installed prior to installing the carpet. Do not permanently adhere carpet to concrete with adhesives so that the carpet can be removed if it becomes wet.

Building a healthy home might cost slightly more (3 to 5 percent) than a home built with traditional construction techniques. However, using these building techniques can end up saving you money in the long run by reducing utility bills 30 to 40 percent. Even more important, you and your family members will enjoy improved air quality and less exposure to toxic materials.