Page 555 - Mechanical Engineers' Handbook (Volume 4)

544 Indoor Environmental Control

Table 3 Human Irritation Responses to TVOC Mixtures
Concentration
3
(mg/m ) Health Effect Response Exposure Definition
0.20 No irritation Comfort
0.20–3.0 Irritation & discomfort Multifactorial exposure range
3.0–25 Exposure effect and headache Discomfort
25 Additional neurotoxic effects Toxic

• Inadequate ventilation, 52%
• Contaminant from inside the building, 16%
• Contaminant from outside the building, 10%
• Microbial contamination, 5%
• Contamination from building fabric, 4%
• Unknown sources, 13%
Appropriate design and maintenance of HVAC system can eliminate the majority of
indoor air-quality problems. ANSI/ASHRAE Standard 62-2001 has a prescriptive ventilation
rate procedure that defines adequate ventilation. The standard uses CO as an indicator of
2
adequate IAQ because CO is a ‘‘marker’’ for human contaminants. The standard assumes
2
that if CO is kept at a required level, then other contaminants are also successfully diluted.
2
Typically required fresh air quantities are 8–10 L/(s person). The rest of the air may be
recirculated in order to save energy used by the HVAC system.
To solve IAQ problems, building owners or managers could use one of the following
three strategies: (1) eliminate or modify the contaminant source, (2) dilute the contaminant
with outdoor air and air distribution, or (3) use air cleaning (filtering). Even though these
strategies sound straightforward, they may not be easy to implement. For example, for mold
and fungi prevention, it is important to properly design HVAC systems for humidity control
and it is just as crucial to have a properly constructed building envelope. Effective use of
air distribution requires proper maintenance and balancing of the building air-distribution
systems. Finally, the use of different types of filters and regular replacement of filters can
significantly reduce IAQ problems. Table 4 indicates the type of contaminants that certain
filters are capable removing; typically, two or more types of filters are combined to achieve
proper removal of contaminants.
Recently, in addition to health concerns, HVAC systems have been charged with pro-
tecting buildings and occupants from internal or external releases of chemical, biological, or
radiological contaminants. These new requirements for the removal of extremely harmful,

Table 4 Use of Different Filters for Different Types of Contaminants
Media HEPA Activated Carbon UV Photocatalytic
Filtration Filters Adsorption Oxidation
Odors X X
VOCs X X
Bio-aerosols X X
Dust X X