Over the years a number of “truths” have been reasonably
well established relative to air pollution with chemicals. One of the strongest of these is that our
exposures to air pollution are typically dominated by what we breathe while
indoors. There are a number of reasons
for this:
- We spend the vast majority of our time indoors
- Most outdoor pollution can penetrate to the indoor environment
- Mixing air change rates of fresh air are invariably lower indoors than they are outside
- Indoor sources of chemical pollutants often dominate over outdoors sources
One way of looking at our world from the perspective of inhalation exposure is to divide the pollution we experience into two primary sources; namely, near-field and far-field. The cartoon below is designed to provide
some insight into this dichotomy.
We all know and many of us are concerned about large
far-field sources of air pollution such as industrial emissions from smoke
stakes or from other large sources like automotive emissions, large spills or releases or forest
files. SMOG or ozone alerts fit into
this category. Indeed, the majority of
regulatory effort such as laws like the Clean Air Act have been directed at these sources. They are clearly important; however, it has
been shown that indoor sources of pollution can be just as and, in some cases,
more important that the large sources.
Over 30 years ago Lance Wallace from the EPA conducted
pioneering studies under the acronym:
TEAM or Total Exposure Assessment
Methodology http://nepis.epa.gov/Adobe/PDF/2000UC5T.PDF.
He concluded that indoor airborne
concentrations resulting from essentially all eleven target chemicals that he looked at were greater
than the outdoor concentrations at various locations in the
US. He and his team were the first to find that in most case, even in locations with concentrated industrial sources, the
indoor sources significantly outweighed the impact of traditional far-field sources such
as chemical plants, petroleum refineries, local dry cleaners or service
stations.
It is worth noting that this study was done by the US EPA whose primary charge is to focus on non-occupational sources and exposures. Presumably, the vast majority of significant
exposures are even more intense at work and are from near-field sources.
How does one measure near-field inhalation exposure? The direct method is to monitor the breathing
zone of persons while the source is present or while they are engaged in
activity that will generate a near-field source. For example, applying hair-spray might be
considered a fairly intense acute near-field exposure event to the chemicals present within the
sprayed product. Other relatively
intense near-field sources include any activity that releases a toxicant and that is done at “arm’s length”
or at “tool’s length” such as digging, mopping or sweeping. Measuring and timing these acute exposures
will allow one to reasonably characterize them.
Another form of
near-field exposure in this context are the relatively constant emissions and
exposures that may be coming from indoor sources like building products. Here “capturing” the time or spatial element of the resulting concentrations may
not as critical especially if the sources are “spread-out” like off-gassing
paint or carpeting.
Near-field exposures indoors can also be modeled but here the
terminology can get a bit confusing. If
one considers the entire volume of the residence as the near-field then one does not have to
use a “near-field” exposure model which
sizes the near-field volume as the conceptual geometric space (often a sphere
hemisphere or a cylinder) which includes the source and the nose and mouth of
the exposed person. If that volume
includes the entire house or entire room within a house and the source is “spread-out” as
mention above, then a well-mixed box model could be a good choice. If, however, the source and the person are
only in a fraction of the room's volume, then one would use the 2 zone model with the
near-field including the volume containing the source and the breathing zone. The far-field would include the rest of the room’s volume.
It is important to remember what Lance Wallace taught us;
namely, that most human inhalation exposures occur in the near-field. Of course, dermal exposure, almost by
definition, occurs within the near-field.
As usual I would love to hear of your thoughts about and experience with the near-field exposure assessment especially compared to what might be happening or not happening in the far-field.
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