Ventilation as a Control Mechanism for VOC Concentrations
Effect of Outside Air Ventilation Rate on VOC Concentrations in Office Buildings
The importance of adequate outdoor air ventilation in office buildings
is supported by a number of studies that have investigated the association
between ventilation rates and human responses (Seppanen
et al., 1999). Ventilation increases above 10 liters per second
(L/s) per person have been found in some studies to decrease symptom
prevalence or to improve perceptions. These improvements were presumably
due to reductions in the concentrations of airborne contaminants
such as VOCs. Simple mass-balance models directly relate ventilation
and concentrations of VOCs generated indoors. For example, at steady-state
ventilation conditions, a two-fold increase in ventilation rate
should reduce airborne concentrations of VOC produced by steady
sources by half. However, this simple relationship is affected by
a number of factors potentially affecting air mixing, VOC emission
and removal rates and chemical reactions. Thus, the effectiveness
of ventilation rate for controlling indoor VOC concentrations may
vary widely within and among buildings.
Design of Study to Measure Effect of Ventilation Rate on VOC Concentrations in a Call Center
A study was conducted during 2000 in a call center located in the
San Francisco Bay Area to investigate the effect of ventilation
rate on indoor VOC concentrations and emissions (Hodgson
et al., 2002). This was part of a larger investigation of ventilation
rate and worker productivity (Fisk et al., 2002).
The building, constructed in 1998, had two floors, an area of 4,600
m2, and about 300 workers. The building was ventilated
and conditioned by four air handling units (AHUs 1-1, 2-1, 2-2 and
2-3) located on the rooftop. Each AHU served a defined interior
zone although there likely was considerable air mixing among zones.
Equipment was added to enable manipulation and measurement of outside
air ventilation rates. For the three AHUs serving the study population,
three fixed damper positions were selected to provide periods of
low, medium, and high ventilation rates. The positions for the low
period were fixed to provide the code-minimum outside air supply
rate of 76 L/s per 100 m2. The positions for the medium
and high periods were selected to provide about 2- and 4-times the
code minimum. The dampers in AHU 2-2 were fixed in one position.
Periods of ventilation were scheduled over 13 weeks in one of four
modes: low, medium, high, and economizer mode. Carbon dioxide concentrations
were measured in all supply and return air systems and outside air
on a regular cycle. Samples for VOCs and aldehydes were collected
from the return air ducts and outside air over a Tuesday workday
in weeks 2, 3, 6-8, 11 and 13. VOC samples were collected on sorbent
tubes and analyzed for 49 compounds using thermal desorption GC/MS.
Samples for formaldehyde and acetaldehyde samples were collected
on treated cartridges and analyzed by HPLC.