Environmental Engineering Reference
In-Depth Information
The Berge equation is a relatively good predictor of HCHO concentration
at standard conditions when measured under a variety of environmental
conditions. It has been reported to have a standard error of ±12% within a
95% confidence level.
Formaldehyde levels
decrease significantly with time. A generalized relationship between HCHO
levels and product or home age with time can be seen in
Figure 4.1
.
Rapid
reductions of HCHO levels can be seen to occur in the early life of formal-
dehyde-contaminated residences or emitting products. After an initial rapid
decline, HCHO levels decrease at a much slower rate, with relatively ele-
vated levels continuing for years.
Several investigators have attempted to model changes in HCHO levels
with time, using exponential model equations as well as statistical analyses.
Exponential models that describe the decay of radioactive isotopes as well
as first-order chemical reactions predict a constant half-life. Studies of field
data indicate that HCHO decreases with time are exponential only in part,
with half-lives that increase in time. Statistical and graphical evaluations of
Wisconsin mobile homes tested for HCHO in the early 1980s indicated half-
life values of 3, 5, 12, and 72 months.
Because HCHO levels depend on a variety of source and environmental
variables, it is unlikely that a model equation could be developed that would
reliably predict the decay of HCHO levels under the many source and envi-
ronmental conditions that have and continue to exist in North American
residences. However, double exponential models have been shown to be
relatively good predictors of changing HCHO levels with time (see Chapter 9).
The decay rate is dependent on emission rates that are affected by tem-
perature, relative humidity, and interaction effects between formaldehyde-
emitting materials, as well as ventilation rates (increasing temperature,
humidity, and ventilation rates increase emission rates and, as a conse-
quence, decay rate). Therefore, half-lives would be expected to be shortened
c.
Decrease in formaldehyde levels with time.
Figure 4.1
Generalized decrease of formaldehyde with time.
