|
 |
 |
 |
|
|
 |
An anthrax-contaminated letter closed
the Hart Senate Office Building for several months. Berkeley Lab researchers
have developed a model for studying the dispersal of anthrax spores
that may eventually be used to guide decontamination efforts in such
situations. |
|
|
It took three fumigations spanning three months to rid the Hart Senate
Office Building of anthrax after a single contaminated letter was sent
to Senator Tom Daschle last October.
Although the epicenter of the $14 million cleanup was Daschle's office,
the nine-story building was sealed after traces of anthrax were found
in other rooms. No one knows precisely how the aerosolized spores drifted
from the envelope to the far corners of the building, but Berkeley Lab
researchers are zeroing in on an understanding.
"We've always included aerosol behavior in our modeling and experimental
work, but the seed crystal was what happened in the Hart Building,"
says Richard Sextro, of the Indoor Environment Department in the Environmental
Energy Technologies Division. "It became very clear that one of the
big unknowns is what happens after you open the envelope. Where does the
anthrax go?"
The indoor anthrax model developed by Sextro and colleagues David Lorenzetti,
Tracy Thatcher, and Mike Sohn had its origins in the Department of Energy's
Chemical and Biological National Security Program, initiated in 1997.
The program initially included only Lawrence Livermore and Los Alamos
laboratories' work on outdoor modeling of biological and chemical attacks.
However, because Berkeley Lab's Indoor Environment Department has one
of the nation's most comprehensive indoor air programs, Joan Daisey (the
late head of the department) successfully pitched a proposal to DOE in
1998 for funding to explore chemical and biological agent dispersion in
buildings. A fourth DOE lab, Argonne, rounds out the program by modeling
subway contamination.
Sextro and his colleagues have developed a model with a singular purpose:
to track the fate of airborne anthrax spores and use these simulations
to estimate exposures. Their rationale is based on the unnerving fact
that one gram of anthrax contains 100 billion spores, and only 10,000
spores are needed to spur a lethal case of inhalation anthrax. This also
means nearly every spore counts, so the model has to be robust enough
to depict anthrax dispersal in considerable detail.
To start, the team used information obtained from Indoor Environment
Department experiments that studied aerosol transport and deposition in
both rooms and ducts. In addition, a multizone building airflow model
developed in part by Berkeley Lab scientists was used to simulate the
room-to-room airflows that can potentially transport anthrax spores between
rooms.
 |
|
Aerosol research conducted at this
dispersion testing facility was used to help develop the indoor anthrax
model. (Photo by Robert Couto) |
|
Combined, the two models paint a rough picture of what happens when an anthrax-laden
letter is opened. For example, at between two and four microns in size,
anthrax is a relatively large aerosol, so the models reflect that it is
more susceptible to gravitational settling than smaller particles. In other
words, more of a given amount of anthrax settles on tabletops and carpets
than the same amount of a smaller, combustion-produced aerosol, which is
more likely to adhere to walls and ceilings. The models also predict the
amount of aerosol that leaks through a building's shell and accumulates
in air ducts.
However, most airflow models do not account for the activities of people.
What happens when someone steps in anthrax that has settled on the floor
and tracks it from room to room? Or resuspends it into the air by simply
walking on the floor? To explore this poorly understood component of anthrax
dispersion, the modeling team incorporated terms that describe foot traffic's
influence on deposition and suspension. Delving deeper, they subdivided
surfaces into two types: accessible areassurfaces on which people
can walk and unwittingly disturb deposited anthraxand inaccessible
areas comprised of hard-to-get-to surfaces like corners and behind desks;
once anthrax settles in such places, it typically isn't tracked or resuspended.
These additional variables enable the model to more fully map the chain
of events that affect anthrax dispersal.
"This pushes us, conceptually, into a new area of knowing what happens
to particles on accessible surfaces where they can be resuspended or tracked,"
Sextro says. "This is important, because by examining anthrax dispersal
in as complete a picture as possible, we determine where we need to focus
our research."
So far, the model has been unleashed in a hypothetical, computer-generated,
190-square-meter office floor, subdivided into a main hallway surrounded
by six offices, each occupied by one person. A letter carrying one gram
of anthrax is opened in one room. Some anthrax remains in the envelope,
some settles on the floor, and some disperses into the air. Several scenarios
are played out. In one, everyone remains in their office and the HVAC
system is the sole means of dispersal. In more complex scenarios, people
move from room to room and track, resuspend, and redeposit anthrax throughout
the office floor.
Disturbingly, in even the simple scenario, everyone exceeds or comes
close to receiving a lethal dose.
Although the model is still under development and is primarily a research
tool, Sextro believes it can eventually be used to map real-world exposure
cases. "It's very important to know how much anthrax is in the HVAC
system, on the floor, and on the backside of ceiling tiles," Sextro
says. "In addition to the important task of estimating potential
exposures, andultimatelyhow to avoid high exposures, the model
can help focus decontamination efforts by determining where anthrax accumulates."
Additional information:
|