Pollutant Removal and Decrease in Indoor Air Pollutant Concentration by Air Cleaners
The extent to which an air cleaner reduces the indoor pollutant concentration depends on the rate of pollutant removal by the air cleaner and on other factors, such as the building's outdoor air ventilation rate. The performance of some air cleaners may also depend on the rate at which pollutants are generated indoors. For many air cleaners:
Pollutant Removal Rate of Air Cleaner = (Air Flow Rate) x (Fraction of Time Air Cleaner Operates) x (Pollutant Removal Efficiency) x (Indoor Air Pollutant Concentration)
Some air cleaners sold for residential use have a very low air flow rates and, consequently, cannot substantially improve indoor air quality. The pollutant removal efficiency varies from 0 to 1, or 0% to 100%. If the pollutant removal efficiency is 0.5 (or 50%) for a particular pollutant, the concentration of the pollutant in air exiting the air cleaner is 50% of the concentration of the pollutant in the air entering the air cleaner. For many particle air cleaners, the pollutant removal efficiency varies with particle size, and the efficiency is lowest for particles with a diameter of approximately 0.2 micrometers. Some particle filters have MERV ratings to indicate their particle removal efficiency . A higher MERV rating indicates a higher efficiency in removing the small particles most important to health.
To substantially reduce the indoor pollutant concentration, the rate of pollutant removal by the air cleaner must be equal to or larger the rate of pollutant removal by ventilation, assuming clean ventilation air. A typical home has a ventilation airflow rate, due to air leakage, of approximately 0.5 indoor air volumes per hour. In a 1500 square foot (139 square meter) home with 8 feet (2.4 m) ceilings, this ventilation air flow rate equals 100 cubic feet per minute (47 liters per second). Thus, if the air cleaning is applied to the entire house, the air flow rate of the air cleaner multiplied by the fraction of time the air cleaner operates and also multiplied by the pollutant removal efficiency should be 100 cubic feet per minute (47 liters per second) or preferably substantially larger.
The Clean Air Delivery Rate (CADR) is a parameter used by the Association of Home Appliance Manufacturers (AHAM) to indicate the rates of particle removal by portable air cleaners. AHAM recommends different minimum CADRs depending on the size of the room.
Avoiding Air Cleaners That Produce Significant Ozone
California prohibits sale of air cleaners that produce more than a specified amount of ozone. For a list of air cleaners that meet California's standard go to the Air Resources Board web site.
For more information, also refer to the U.S. EPA's web site "Ozone Generators that are Sold as Air Cleaners".
Air Cleaning Technologies
For air cleaning, particle filters and/or media for removing gaseous pollutants may be installed in the ductwork of the heating and cooling system as illustrated in Figure 1. Alternately, one or more standalone air cleaning devices, often called portable air cleaners or air purifiers, may be deployed in commonly used rooms as depicted in Figure 2.
|Figure 1. Filter installed in a forced-air heating system.||Figure 2. Portable air cleaner installed in a bedroom.|
Table 1 and Table 2 provide an introductory description of the most commonly used air cleaning technologies, with information on their advantages and disadvantages, modes of application, and implications for health summarized briefly. There are a very large number of technologies and variants on technologies. The table is not comprehensive and the entries in the table do not apply to every variant or application of a technology. Many air cleaners employ multiple of the listed technologies. For additional information on the basic designs for gas phase air cleaners, see the U.S EPA’s web site or reference .
This web site has focused on air cleaners that use fans to move air through a system that removes pollutants. Limited research has been performed on passive air cleaning in which natural indoor air movement is used together with materials that remove air pollutants. These systems have no fans, thus they are quiet and energy efficient. For example a cloth containing activated carbon that removes ozone can be mounted on a wall, or the normal gypsum wall board can be replaced with a wall board that removes pollutants. No data were identified indicating how these systems affect health or perceived air quality.