Environmental Engineering Reference
In-Depth Information
where C
= initial tracer gas concentration (ppmv)
o
C
= tracer gas concentration at time, t (ppmv)
t
T
= hours
n
= air exchange rate/hr (ACH)
The ventilation rate per person can be calculated by multiplying the air
exchange rate by the volume of the building space and dividing by the
number of individuals present.
Use of tracer gases such as SF
to determine building ventilation rates
is technically more demanding than using CO
6
and thermal balance tech-
niques. It requires systems for tracer gas injection, sampling, and analysis.
It cannot be used to measure air exchange rates in large buildings. An
alternative to the SF
2
levels
during and after building occupancy. There are two limitations to the use of
CO
method for large buildings is to monitor CO
6
2
as a tracer gas: (1) CO
measurements must be made after working
2
2
hours and (2) CO
is often not uniformly distributed in building spaces.
2
Environmental measurements are
commonly conducted in problem building investigations. Protocols devel-
oped by NIOSH for health hazard evaluations, and the USEPA/NIOSH
protocol for in-house personnel, de-emphasize contaminant measurements,
reasoning that in most cases no contaminant will have a sufficiently high
concentration to explain reported symptoms. Carbon dioxide is the excep-
tion. It is measured in most building investigations to determine the ade-
quacy of ventilation.
Most investigators will conduct environmental measurements. In the
early stages these will typically include CO
d.
Environmental measurements.
, temperature, and relative
humidity. Significant quantitative contaminant measurements are usually
reserved for advanced stages of an investigation, when the walk-through
inspection indicates what potential causal agents may be, or when initial
efforts fail to identify the problem. In theory, it would be desirable to conduct
measurements of only those contaminants that have a reasonable chance of
being a potential cause of complaints based on symptom types, patterns,
and potential sources present. These could include CO, HCHO, TVOCs (total
volatile organic compounds), airborne mold, respirable particles, and surface
dust. Surface dust would be analyzed for common allergens associated with
cats, dogs, etc., or a variety of semivolatile organic compounds.
2
3.
Interpretation of investigation results
Once a problem building investigation has been completed, the investigator
has the task of evaluating the data collected, assessing the problem in both
technical and political terms, and formulating hypotheses that may be tested
by comparison to IAQ/comfort guidelines and by the implementation of
mitigation measures.
