Environmental Engineering Reference
In-Depth Information
ies in which ventilation rates are increased from the low to the high end of
the range of 0 to 20 CFM (0 to 9.54 L/s)/person have shown significant
reductions in symptom reporting rates, whereas studies at ventilation rates
>20 CFM (9.54 L/s)/person have not. This indicates that ventilation rates <20
CFM/person are a risk factor for sick building-type symptoms.
The mixed results observed for studies designed to evaluate relation-
ships between ventilation and building-related health complaints may be
due to a number of confounding factors. These include: (1) differences in
study designs, (2) problems encountered in controlling actual ventilation
rates by mechanical system adjustments, (3) differences in ventilation effi-
ciencies, (4) presence of sources with high emission strengths, (5) dynamic
interactions between source emissions and ventilation, (6) relationships
between contaminant concentrations and health effects, (7) ventilation sys-
tems serving as contaminant sources, and (8) multifactorial causes of build-
ing-related symptoms.
Only a few of these potential confounders are described here to suggest
why few research studies have been able to establish a strong relationship
between illness symptoms and building ventilation conditions. Most notable
are relationships between contaminant concentrations, ventilation and
health effects, ventilation systems as sources of contaminants, and the mul-
tifactorial nature of building-related health complaints.
It is often assumed that increasing ventilation by a factor of 2 will result
in a reduction of contaminant levels by 50% and a concomitant 50% decrease
in symptom prevalence. For many contaminant sources, emissions are cou-
pled to ventilation rates as a consequence of vapor pressure phenomena.
Increasing the ventilation rate increases the emission rate of emitting mate-
rials. The net result of this coupling effect is to reduce the effectiveness of
ventilation in reducing contaminant concentrations in indoor air.
In addition to this nonlinear relationship between contaminant levels
and ventilation, the effectiveness of ventilation is further diminished by the
nonlinear relationship between irritation symptoms (typical SBS symptoms)
and contaminant exposure. The relationship between symptoms and expo-
sure concentrations is typically log-linear as evidenced by mouse bioassay
studies of irritant responses (
Figure 11.20
)
. Given the log-linear response,
reductions in contaminant levels of 90% or so would be required to achieve
a 50% reduction in irritation/symptom response. Such reductions cannot be
easily achieved by mechanical ventilation systems in buildings except when
the initial ventilation condition is near zero. This is consistent with study
results that show reductions in symptom reporting rates and occupant dis-
satisfaction with increasing ventilation rates up to 20 CFM/person. In
Figure
11.13
it can be seen that increases in ventilation rates above 10 L/s/olf (or
10 L/s/person) apparently do not result in any significant improvement in
occupant satisfaction with air quality. Thus, above a maximum ventilation
rate (circa 20 to 30 CFM [10 to 15 L/s]/person), ventilation is relatively
ineffective in reducing symptom reporting rates and increasing occupant
satisfaction with air quality.
