Remember one of the simplest of models? Its C
= G/Q where, at equilibrium with a constant rate of
G and Q, C is the steady-state concentration (mg/m

^{3}), G is the source or emission rate (mg/min) and Q is the ventilation rate (m^{3}/min) for the volume (V) into which G is flowing. In the last few blogs we have been concentrating Q and how to estimate it with tracer gas. We also went over the first order decay model for use with a tracer gas. This week I am going to talk about source (G) characterization.
But first and briefly, I just want to go over a much more
simple way of estimating Q in a room that has obvious input or output streams
of ventilation. We know that every inhabitable
room receives as much air as it exhausts or else the room would explode or the
folks within it would die from lack of oxygen.
Thus, if you can measure the amount of air coming into or leaving the
room you can determine its ventilation rate (Q). You simply take the area of the incoming or
outgoing stream and multiply it by the velocity of the air going in or coming
out. Q = air velocity x area. If you
have both output and input streams measure them both, the one with the highest
calculated Q is the most accurate estimator.

Back to sources (G).
In general we assume that sources are constant or that they undergo first order decay of an initial mass. These are all generalized descriptions of a
source and like all models they attempt to portray reality but they are not strictly
speaking, reality. IF they describe reality reasonably well then
they are useful. If they do not then we
need a better model.

A constant source means that G does not vary with time; it
is constant. Consider someone spraying
an aerosol around their head in the application of hairspray. The can of hairspray is used for 1 minute
and then the spraying is stopped.
Measure the weight of the can at the beginning and at the end of this
period and the difference is the rate of application G in units of mg/min. You
can determine the source rate of a certain ingredients within the can by
knowing its concentration. For example,
if 10% of the stuff in the can is ethanol then 10% of the weight loss was this
alcohol.

Imagine a leaking valve that would also put out a relatively
constant mass of vapor per min as a constant source. Another constant source would be an open and
relatively deep container of solvent.
The evaporation would be constant given a constant vaporizing surface
area in which the temperature remains relatively constant. If the temperature remains virtually the
same and the resulting airborne concentration does not approach a reasonable
fraction of the saturation concentration of that vapor, then the source can be
considered constant. I will go over the
details estimating evaporation rates and relevance of saturation concentration
and “backpressure effect” in next week’s blog.

A first-order decaying source is completely analogous to the
first order decaying concentration of a tracer gas. Consider a spill of a volatile solvent on
the floor. If you plot the natural log
of the weight of the remaining spill versus time you will, in most cases with thin spills, get a straight-line; that is, a good fit to
the first order decay model. Go back two blogs to see how this is done with Excel.

Another way of measuring the parameters (initial mass and half-life) of a first-order decaying source is to measure the time course of concentration decay in a volume with well mixed ventilation. A series of C,t points should follow a first-order decay model and one can then back calculate the generation rate parameters that caused them. If there is interest in more of the details of this technique I can go over them in a future blog.

There is a significantly more complicated source which is a
combination of an increasing AND a first-order decaying source. This may seem daunting but it is doable. Consider someone cleaning a surface with a
solvent. The solvent goes on the first
little section of the surface and starts to evaporate until there is no more of
it left. This is classical first order
decay of G. But that is not the whole story;
indeed, solvent is applied continuously and somewhat constantly until the
surface cleaning is complete. This is a growing
rate of generation (G) as the area gets larger. This type of source has fortunately been
described mathematically within the documentation for the EPA Model
E-FAST2. Reference: http://www.epa.gov/opptintr/exposure/pubs/efast2man.pdf

Both constant and first order sources are presented and
coded individually in many of the important models presented in IH MOD which you can get at:

http://www.aiha.org/get-involved/VolunteerGroups/Pages/Exposure-Assessment-Strategies-Committee.aspx. Unfortunately, the combination of an increasing
and first order decaying source types, as described in the above paragraph, has
not been coded into IH MOD but it may be in future versions of this very
valuable tool.

I will go over exactly what the freeware IH MOD offers in a future blog.

I will go over exactly what the freeware IH MOD offers in a future blog.

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