Handbook of air conditioning and refrigeration / Shan K

Basic Strategies to Improve Indoor Air Quality
INDOOR AND OUTDOOR DESIGN CONDITIONS 4.29
There are three basic strategies to improve indoor air quality: control the contaminated source,
remove air contaminants from the indoor air by air cleaner, and use outdoor ventilation air to dilute
the concentrations of indoor air contaminants.
To eliminate or to reduce the emissions of air contaminants from the contaminated source is often
the most effective way to improve the IAQ as well as periodically cleaning the duct’s interior
surface and coil’s condensate pan and using building materials and carpets that do not release or release
only negligible volatile organic compounds and dust. However, emissions and odor released
from occupants are difficult to eliminate or to reduce, and smoking is now prohibited in many public
places and limited to specified areas in many commercial buildings.
The volatile organic compounds and combustion products contain numerous minute particles of
size between 0.003 and 1 �m. Only high-efficiency air filters and activated carbon filters can remove
these minute particles and odors from the airstream effectively. High efficiency air filters and
carbon filters are expensive to install, operate, and maintain.
Adequate outdoor ventilation air to dilute the air contaminations in practice has been proved an
essential, practical, and cost-effective means to improve the indoor air quality. ASHRAE Standard
62-1999, Ventilation for Acceptable Indoor Air Quality, specifies two alternative procedures to obtain
acceptable IAQ: the ventilation rate procedure and indoor air quality procedure.
● In the ventilation rate procedure, acceptable indoor air quality is achieved by providing ventilation
air of specified quality and quantity to the space.
● In the IAQ procedure, acceptable air quality is achieved within the space by controlling known
and specifiable contaminants.
The ventilation rate and IAQ procedurs are discused again in Sec. 23.2.
ASHRAE Standard 62-1999 defined acceptable indoor air quality as air in which there are no
known contaminants at harmful concentrations as determined by cognizant authorities and with
which a substantial majority (80 percent or more) of the people exposed do not express dissatisfaction.
ASHRAE Standard 62-1999 defines ventilation air as that portion of supply air that is outdoor
air plus any recirculated air that has been treated for the purpose of maintaining acceptable indoor
air quality. Ventilation is the process of supplying and removing ventilation by natural and mechanical
means, and the ventilation rate means the rate of ventilation air supplied to the conditioned
space through the air system.
If outdoor air is used to dilute the concentration of indoor contaminants, its quality must meet
the National Primary Ambient-Air Quality Standard provided by the EPA. Part of the time average
concentrations are shown below:
Long-term concentration Short-term concentration
Average period Average period
Pollutants �g/m3 ppm of exposure �g/m3 ppm of exposure
Particulate matter 50 1 year 150 24 hours
Sulfur oxides 80 0.03 1 year 365 0.14 24 hours
Carbon monoxide 40,000 35 1 hour
10,000 9 8 hours
Nitrogen dioxide 100 0.055 1 year
Oxidants (ozone) 235 0.12 1 hour
Lead 1.5 3 months
Only particulate matter is expressed in annual geometric means; the other two are annual arithmetic
means. For carbon monoxide and ozone, both values are not to be exceeded more than once a year.