Three Strategies
Three basic strategies to reduce pollutant concentrations in indoor air are source control, ventilation, and air cleaning.
The use of air cleaners alone cannot ensure adequate air quality.
Source control eliminates individual sources of pollutants or reduces their emission. It is usually the most effective strategy for reducing pollutants. There are many sources of pollutants in the home that can be controlled or removed. For example, solid wood or alternative materials can be used in place of pressed wood products that are likely to be significant sources of formaldehyde. Smokers can smoke outdoors. Combustion appliances can be adjusted to decrease their emissions.
Ventilation is also a strategy for decreasing indoor air pollutant concentrations. It exchanges air between the inside and outside of a building. The introduction of outdoor air is important for good air quality. 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. Natural ventilation describes air movement through open windows and doors. Most residential forced air-heating systems and air-conditioning systems do not bring outdoor air into the house mechanically. Two primary ventilation methods can be used in most homes: general ventilation and local ventilation.
• General ventilation of the living space, by way of infiltration, natural ventilation, or mechanical ventilation, brings outdoor air indoors, circulates air throughout the home, and exhausts polluted air outdoors. Although limited by weather conditions, this method removes or dilutes indoor airborne pollutants, thereby reducing the level of contaminants and improving indoor air quality (IAQ). Special consideration should be given to the outdoor air used for ventilation. It should be of acceptable quality and should not contain pollutants in quantities that would be considered objectionable or harmful if introduced indoors. The use of ventilation to reduce indoor air pollutants should be evaluated carefully where there may be outdoor sources of pollutants.
• Localized ventilation by means of exhaust fans in bathrooms and kitchens, and in some cases by open windows and doors, removes excess moisture and strong, local pollutants and keeps them from spreading to other areas. Using exhaust fans increases the amount of outdoor air that enters a house.
Advanced designs for new homes are starting to add a mechanical feature that brings outdoor air into the home through the HVAC system. Some of these designs include energy efficient heat recovery ventilators to mitigate the cost of cooling and heating this air during the summer and winter.
Air cleaning may be useful when used along with source control and ventilation, but it is not a substitute for either method. The use of air cleaners alone cannot ensure adequate air quality, particularly where significant sources are present and ventilation is insufficient. While air cleaning may help control the levels of airborne particles including those associated with allergens and, in some cases, gaseous pollutants in a home, air cleaning may not decrease adverse health effects from indoor air pollutants.
Types of Air Cleaners
Various technologies can be used in air-cleaning devices. Filtration and electrostatic attraction are effective in removing airborne particles. Adsorption or chemisorption captures some gaseous and vaporous contaminants. Some air cleaners use ultraviolet light (UV) technology. Ultraviolet germicidal irradiation (UVGI) has been used to kill some microorganisms growing on surfaces. Photocatalytic oxidation (PCO), another UV light technology under development, has the potential to destroy gaseous contaminants. Ozone-generating devices sold as air cleaners use UV light or corona discharge and are meant to control indoor air pollutants.
Table 1 provides a brief summary of air-cleaning technologies and the pollutants they are designed to control. Some air-cleaning devices are designed to be installed in the ductwork of HVAC systems or to be used in portable, stand-alone units.
In-duct or whole-house air cleaning devices typically are installed in the return ducts of HVAC systems, as shown in Figure 1. The typical furnace air filter is a simple air cleaner that captures particles in the airstream to protect fan motors, heat exchangers, and ducts from soiling. Such filters are not designed to improve indoor air quality, but the HVAC system can be upgraded by using more efficient air filters to trap additional particles. Other air-cleaning devices such as electrostatic precipitators, UV lamps, and gas-phase filters use sorption and chemical reaction and are sometimes used in the ductwork of home HVAC systems.
The fans in residential HVAC systems may operate intermittently or continuously. Continuous operation improves air circulation and air cleaning, but this operation mode also increases electrical energy consumption and costs.
Portable air cleaners are available as small tabletop units and larger console units. They are used to clean the air in a single room, but not in an entire house. The units can be moved to wherever continuous and localized air cleaning is needed. Larger console units may be useful in houses that are not equipped with forced air-heating systems and air-conditioning systems. Portable air cleaners generally have a fan to circulate the air and a cleaning device such as a mechanical air filter, electrostatic precipitator, ion generator, or UV lamp.
Some units marketed as having the quietest operation may have no fan; however, units that do not have a fan typically are much less effective than units that have one. Air cleaners may also have a panel filter with bonded fine particles of activated carbon, or an activated carbon filter encased in a frame, to remove gases and odours. Some portable air cleaners referred to as hybrid air cleaners use a combination of two or more of the devices discussed above.
Courtesy: Environmental Protection Agency
