Berkeley Lab Model Tracks Indoor Anthrax Dispersal
Three fumigations spanning three months were needed 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 EETD's Indoor Environment Department. "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, begun 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 submitted 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 that almost 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 might transport anthrax spores between rooms.
Combined, the two models paint a rough picture of what happens when an anthrax-laden letter is opened. For example, because anthrax is a relatively large aerosol, (between two and four microns in size), 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 would be true for the same amount of a smaller, combustion-produced aerosol, which is more likely to adhere to walls and ceilings. The models also predict how much aerosol leaks through a building's shell and accumulates in air ducts.
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 areas-surfaces on which people can walk and unwittingly disturb deposited anthrax-and inaccessible areas composed of hard-to-get-to surfaces like corners and areas behind desks; once anthrax settles in such places, it typically isn't tracked or resuspended. These additional variables enable the model to map in detail the chain of events that affects 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 his or her 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.
For each scenario, anthrax exposures of each individual are predicted as well as the concentrations of anthrax spores that end up on various indoor surfaces. 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 and—ultimately— to avoid high exposures, the model can help focus decontamination efforts by determining where anthrax accumulates."
For more information, contact:
- Richard Sextro
- (510) 486-62957; fax (510) 486-6658
Dan Krotz is a writer in Berkeley Lab's Public Information Department.