Indoor Air Quality
Indoor air contaminant materials are particulates, gases, and vapors that may be generated due to the nature of the indoor space, by occupants and their activities in a space, or brought in from the outdoors. Airborne materials may:
- occur from emissions and/or shedding of building materials and systems,
- originate in outside air, and/or
- be from building operating and maintenance programs and procedures
These materials include bioaerosols, particles, volatile organic compounds, and inorganic and organic gases. Bioaerosols are airborne microbiological particulate matter derived from viruses, bacteria, protozoa, algae, mites, pollen, and their cellular or cell mass components. Bioaerosols are present in both indoor and outdoor environments. Further discussion can be found in the ASHRAE Fundamentals Handbook Chapter Indoor Environmental Health.
Outdoor air requirements for acceptable indoor air quality (IAQ) have long been debated, and different opinions have produced different ventilation standards. Historically, the major considerations have included the amount of outdoor air required to control moisture, carbon dioxide (CO2), odors, and tobacco smoke generated by occupants. These considerations have led to prescriptions of a minimum rate of outdoor air supply per occupant. More recently, the maintenance of acceptable indoor concentrations of a variety of additional pollutants that are not generated primarily by occupants has been a major concern. The most common pollutants of concern and their sources are presented in Table 1 "Indoor Air Pollutants" in the Ventilation and Infiltration Chapter of the ASHRAE Fundamentals Handbook.
Various factors make odor control a primary consideration in ventilation engineering:
- modern buildings permit less air infiltration through walls and have more indoor sources of odors associated with building materials, furnishings, and office equipment
- outdoor air is often polluted; and
- energy costs have encouraged ventilation rate reductions at a time when requirements for a relatively odor-free environment are greater than ever.
Dilution of Odors by Ventilation
Comparisons of occupancy odor with smoke odor have been made by relating perceived odor intensity and odor acceptability during smoking and nonsmoking under controlled chamber conditions. When smoking took place, they found that the odor intensity was nearly five times as high as the odor intensity perceived under severe occupancy conditions (hot, humid, and low ventilation) but with no smoking. Determinations of the ventilation rates required to control occupancy odor and tobacco smoke odor to acceptable levels have been incorporated in the recently revised ASHRAE Standard 62 on Ventilation for Acceptable Indoor Air Quality.
Indoor Air Quality Sensors
Indoor air quality control can be categorized into two divisions-ventilation control and contamination protection.
- Ventilation control measures levels of carbon dioxide or other contaminants in a space and controls the amount of outdoor air introduced into the occupied space. The demand control of ventilation helps to maintain proper ventilation rates under all levels of occupancy. Typical control setpoint levels of carbon dioxide are 800 to 1000 ppm.
- Contamination protection sensors monitor levels of hazardous or toxic substances and either issue warning signals and/or initiate corrective actions through the building automation system. An example of this type of sensor is the carbon monoxide sensor used to control and alarm CO levels in parking garages. Another is the commonly used smoke detector.
Oxygen depletion sensors used to measure, alarm and initiate ventilation purging in enclosed spaces which house refrigeration equipment to prevent suffocation of occupants upon a refrigeration leak. The application of these sensors determines the type selected, the substances monitored, and the action taken.
Additional information can be found in the Sick Building Syndrome and Ventilation segments.
ASHRAE Standard 62 provides further information on ventilation for acceptable indoor air quality. Additional data is available from the EPA, some of which is shown below.
Most people are aware that outdoor air pollution can damage their health but may not know that indoor air pollution can also have significant effects. EPA studies of human exposure to air pollutants indicate that indoor air levels of many pollutants may be 2-5 times, and occasionally, more than 100 times higher than outdoor levels. These levels of indoor air pollutants are of particular concern because it is estimated that most people spend as much as 90% of their time indoors.
Over the past several decades, our exposure to indoor air pollutants is believed to have increased due to a variety of factors, including the construction of more tightly sealed buildings, reduced ventilation rates to save energy, the use of synthetic building materials and furnishings, and the use of chemically formulated personal care products, pesticides, and cleaners.
In recent years, comparative risk studies performed by EPA and its Science Advisory Board (SAB) have consistently ranked indoor air pollution among the top five environmental risks to public health. EPA, in close cooperation with other Federal agencies and the private sector, is actively involved in a concerted effort to better understand indoor air pollution and to reduce people's exposure to air pollutants in offices, schools and other indoor environments where people live, work, and play.
What Causes 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, 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; cleaning and maintenance; central heating and cooling systems and humidification devices; and outdoor sources, such as radon, of 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 flame 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 unvented 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. High pollutant concentrations can remain in the air for long periods after some of these activities.
Indoor Air Quality in Large Buildings
Indoor air quality (IAQ) problems are found many office buildings which have significant air pollution sources. Some of these buildings may be inadequately ventilated. For example, mechanical ventilation systems may not be designed or operated to provide adequate amounts of outdoor air. Finally, people generally have less control over the indoor environment in their offices than they do in their homes. As a result, there has been an increase in the incidence of reported health problems.
The Building Air Quality Guide, developed by the EPA and the National Institute for Occupational Safety and Health, provides practical suggestions on preventing, identifying, and resolving indoor air quality (IAQ) problems in public and commercial buildings. This guidance provides information on factors affecting indoor air quality; describes how to develop an IAQ profile of building conditions and create an IAQ management plan; describes investigative strategies to identify causes of IAQ problems; and provides criteria for assessing alternative mitigation strategies, determining whether a problem has been resolved, and deciding whether to consult outside technical specialists. Other topics included in the guide are key problem causing factors; air quality sampling; heating, ventilation, and air conditioning systems; moisture problems; and additional sources of information.
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