Vapor Intrusion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 416

can have significant influences on vapor intrusion. Where buildings are heated, con-
vection cells develop with hot air rising and leaking through roofs and upper-floor
windows. This phenomenon is referred to as the “stack effect”, and is commonly
assumed to cause de-pressurization in the bottom floor of a building by a few
Pascals. The “stack-effect” is a function of the height of a building and the tempera-
ture difference between inside and outside, and can result in under-pressurization
of hundreds of Pascals in very tall buildings in cold climates. The escaping air
will be replaced to some extent by soil gas entry at lower levels/basements. Even
where buildings are not mechanically heated, solar radiation on rooftops can heat
air in the attic significantly and cause the same effect. Wind-load on the side of a
building can also cause a pressure differential across the building, and wind flow
over the building can create a vacuum within the building via Bernoulli's principle.
Bathroom exhaust fans, central vacuum cleaners, clothes dryers and kitchen exhaust
fans all remove air from a building. Elevator shafts can also cause localized pressure
gradients. Window-mounted air conditioners blow air into a building. In commer-
cial/industrial buildings, heating, ventilation and air conditioning (HVAC) units are
usually mounted on the roof, and blow air into the building, heating or cooling it as
appropriate for the season or climate. These units may also be designed to exhaust
a portion of the indoor air to provide a certain amount of outdoor air into the build-
ing as fresh air or “make-up air”. Operation of HVAC units can generate sufficient
pressure or vacuum to significantly influence vapor intrusion, and may be worth
considering as part of the conceptual model. Verification of these pressure gradi-
ents typically requires a digital micro-manometer, which are in common use in the
HVAC industry. In many cases, valuable information like air exchange rate, build-
ing pressure, seasonal changes, et cetera, can be obtained from the HVAC engineer,
with minimal effort.
10.2.4 Vapor Intrusion Assessment Approach
The approach for assessing vapor intrusion will vary from site-to-site, but there
are certain elements that are appropriate in most cases (see Fig. 10.6 ). It is usually
advisable to begin a vapor intrusion assessment with a thorough planning step. This
step typically includes gathering readily available existing information, formulating
an initial conceptual model, establishing a scope for an initial phase of investiga-
tion, and developing a logical plan for future directions in response to the probable
range of outcomes. Having a clear and logical plan will facilitate communication
with building owners, occupants, regulators and other stakeholders. The plan should
include a rationale or logic for how the data will be interpreted, including the basis
for the indoor air quality target, any threshold above which an interim action will be
required, whether confirmatory sampling will be required if concentrations are all
below target levels, and similar considerations.
In some cases it may be appropriate to conduct a focused assessment of one par-
ticular aspect of the pathway, for example at sites where there is reason to believe
that a particular process is effectively preventing subsurface vapor intrusion, and a