ACH and health effects





One of the most critical issues now a days in  Occupational Health Safety (OHS) is bad indoor air quality (IAQ) in densely  populated offices.  IAQ greatly depends on air changes rate per hour in work offices. Here is details of it.

Air changes per hour, or　air change rate, abbreviated as ACH or ac/h, is a measure of the air volume added to or removed from a space (normally a room or house) divided by the volume of the space. If the air in the space is either uniform or perfectly mixed, air changes per hour is a measure of how many times the air within a defined space is replaced.

In many air distribution arrangements, air is neither uniform or perfectly mixed. The actual percentage of an enclosure's air which is exchanged in a period depends on the airflow efficiency of the enclosure and the methods used to ventilate it. The actual amount of air changed in a well mixed ventilation scenario will be 63.2% after 1 hour and 1 ACH. In order to achieve equilibrium pressure, the amount of air leaving the space and entering the space must be the same.

ACH equation in　Imperial unit
             60XQ
N = -------------------
             Vol.
Where:
N　= number of air changes per hour
Q　= Volumetric flow rate of air in cubic feet per minute (cfm)
Vol　= Space volume　L　×　W　×　H, in cubic feet
Ventilation rates are often expressed as a volume rate per person (CFM per person, L/s per person). The conversion between air changes per hour and ventilation rate per person is as follows:
              ACPH X D X h
Rp =        -------------------
                      60

Where:

Rp　= ventilation rate per person (CFM per person, L/s per person)
ACPH　= Air changes per hour
D　= Occupant density (occupants per square foot, occupants per square meter)
h　= Ceiling height (ft, meters)

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