Environmental Engineering Reference
In-Depth Information
reported. Initial statistical analysis often shows strong correlations between
increasing temperatures and increasing symptom prevalence. On more
intensive analysis, temperature usually is shown to vary colinearly with
other factors which investigators determine to be more directly related to
SBS-type complaints. However, increasing symptom prevalence with
increasing temperature, independent of other factors, has been reported.
Several studies have shown significant increases in symptom preva-
lence/reporting rates at temperatures >73.5°F (22°C).
Factors which affect the thermal balance (thermal comfort) of the human
body include air temperature, mean radiant temperature, air velocity, and
R.H. Relative humidity affects thermal comfort because it affects evaporative
heat loss from the body. With increasing R.H., water vapor pressure gradi-
ents between the skin and surrounding air decrease, resulting in reduced
evaporation of perspiration and therefore reduced heat loss. Air velocity
affects heat loss by convection; higher velocities result in greater convective
heat loss.
Local thermal discomfort occurs when parts of the body are subjected
to different thermal heat gain or loss conditions. An example of such a
phenomenon is when significant vertical differences in air temperature exist
between head and feet, e.g., when floor temperature is warmer or cooler
than air temperature.
Local thermal discomfort also occurs as a result of excessive air velocity
(drafts) or radiant temperature asymmetries. Radiant field asymmetries are
a common cause of thermal comfort complaints. When an exterior wall is
warm, it radiates heat toward an individual who absorbs it. When it is cool,
heat is readily lost from that portion of the body facing it; as a consequence,
an individual may experience a localized chilling effect. Radiant asymme-
tries are noticeable at different seasons on north and south faces of buildings.
Seasonal air temperatures, air velocities, and radiant asymmetries associated
with optimal thermal comfort are summarized in Tables 7.7 and 7.8 .
2.
Humidity
In addition to influencing thermal comfort, relative humidity can affect
mucous membranes. In low-humidity, winter-time conditions, low R.H.
(<30%) appears to be a risk factor for SBS-type symptoms. Humidification
to attain a range of 30 to 40% R.H. has been reported to reduce symptom
prevalence/reporting rates.
3.
Air flow and air movement
Air movement (or lack thereof) in building spaces may directly affect human
comfort, perceived air quality, and even symptoms. When air movement is
perceived as excessively cool, it is described as a draft and may be thermally
uncomfortable. Uncomfortable drafts may result from air leakage through
the building envelope during cold weather conditions or during the cooling
season when chilled air flows from diffusers at high velocity onto individuals
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