tent.
Evaluation of Bacillus anthracis Contamination Inside the Brentwood Mail Processing and Distribution Center December 21, 2001 / Vol. 50 / No. 50
U.S. DEPARTMENT OF HEALTH & HUMAN SERVICES
Evaluation of
Bacillus anthracis Contamination Inside
the Brentwood Mail Processing and Distribution Center
District of Columbia, October 2001
During October 1921, 2001, four postal workers at the Brentwood Mail Processing
and Distribution Center in the District of Columbia were hospitalized with inhalational
anthrax; two of the workers died. The building, which was closed on October 21, was
believed to have been contaminated by a letter containing
Bacillus anthracis spores sent
to the Hart Senate Office Building (HSOB) that had passed through the postal facility on
October 12. A second contaminated letter addressed to another U.S. senator that was
processed through the same mail sorter and sort run as the first letter was discovered on
November 17. This report describes the results of CDCs evaluation of
B. anthracis in the
facility, which showed widespread contamination of the facility and suggest that wipe
samples and high efficiency particulate air (HEPA) vacuum samples complement each
other in assessing contamination.
A U.S. Postal Service investigation indicated that, on late October 11 or early October
12, the letter sent to one U.S. senator entered the building in a mailbag through a loading
dock near the Postal Vehicle Transportation Office (Figure 1). The bag was opened and
the contents separated into bar-coded trays and moved by all-purpose carrier (APC) to a
large tray-sorting machine. The APC tray then went to delivery bar-code sorter (DBCS)*
17, where the letter was manually fed into the machine at 7:10 a.m. The letter was then
transported by APC to the government mail section of the facility and was transported to
HSOB at approximately noon on October 12. Sometime during 8 a.m.9:40 a.m., the
DBCS machine that processed the letter was opened, and compressed air at 70 lbs. per
square inch was used to clean debris and dust from conveyor belts and optical reading
heads.
On October 18, before recognition of inhalational anthrax cases, a Postal Service
contractor collected 29 swab samples from the mail sorting area of the Brentwood facil-
ity. On October 20, CDC initiated an investigation of the Brentwood facility. As part of this
investigation, CDC extended the evaluation of
B. anthracis contamination in the
Brentwood facility.
On October 23, CDC investigators and Postal Service contractors selected and marked
sampling locations. Sampling for
B. anthracis spores began on October 24 using three
*The DBCS machines move mail along internal conveyor belts and rollers through a series of
turns and compressions at 32 miles per hour until the mail lands in the appropriate collection
bin for distribution.
1129 Evaluation of
Bacillus anthracis
Contamination Inside the Brentwood
Mail Processing and Distribution
Center District of Columbia,
October 2001
1133 Progress Toward Interrupting
Indigenous Measles Transmission
Region of the Americas, January
November 2001
1137 Rubella Outbreak Arkansas, 1999
1140 Notices to Readers 1130
MMWR
December 21, 2001
Brentwood Facility Continued
Delivery Dock
DBCS 5
DBCS 14
DBCS 17
Positive Wipe Locations
Positive Vacuum Locations
A Postal Vehicle Transportation Office
B Express Mail Room
C Customer Service Area
D Government Mail
A
D
C
B
25
50
100 Ft
FIGURE 1. Diagram of Brentwood Mail Processing and Distribution Center and location
of positive identification of
Bacillus anthracis spores District of Columbia, October
2001
techniques: surface wipe sampling, surface vacuum sampling, and air sampling (
1 ). The
evaluation focused on the path of the HSOB letter through the facility and the work
locations of the known anthrax patients. To evaluate the extent of
B. anthracis contami-
nation, additional samples were collected throughout the facility, including the adminis-
trative areas on the second level and the customer service area at the front of the
building. Wipe samples were submitted to CDC for culture and analysis. Vacuum and air
samples were analyzed by a contract laboratory. Suspect culture colonies were screened
using standardized Laboratory Response Network (LRN) Level A testing procedures for
identification of
B. anthracis (2 ) and were confirmed by direct fluorescent antibody stain-
ing and gamma phage lysis (
3 ).
Surface Wipe Sampling
Selected surfaces (e.g., table or desk tops, sorting machines, sorting bins, control
consoles of sorting machines, and ventilation ducts) were sampled using moistened
sterile cotton gauze pads. Cultures from samples were reported as either positive or
negative for colonies of
B. anthracis.
Twelve days after the contaminated letter sent to HSOB passed through the facility,
eight (7%) of 114 surface wipe samples were positive for isolates of
B. anthracis. Four of
the positive samples were collected on and around DBCS machine 17, which processed Vol. 50 / No. 50
MMWR
1131
Brentwood Facility Continued
the contaminated letters, and one was from an air supply duct approximately 12 feet
above the machine. The remaining three positive samples were from areas on distant
DBCS machines. None of the wipe samples collected in the administration area or in the
customer service area was positive for isolates of
B. anthracis. All wipe samples col-
lected in the Postal Vehicle Transportation office, express mail room, and the govern-
ment mail area were negative.
Surface Vacuum Sampling
Surface vacuum samples were collected by inserting a cone-shaped filtering sock
(dust collection trap) into the nozzle of a HEPA vacuum cleaner with a high-efficiency (0.1
µm pore size) filter. The vacuum nozzle was mechanically cleaned with an alcohol wipe
between samples to dislodge spores and prevent cross-contamination. Several grams of
dust were collected inside each vacuum sock (
1 ) and were submitted to a contract
laboratory for culture and analysis. Results were reported as number of colony forming
units per gram of material collected (CFU/g); a CFU can represent a single
B. anthracis
spore or an aggregate of several spores and may not correlate directly to the number of
spores present.
Of 39 vacuum dust samples,
B. anthracis was isolated in 27 (69%). Reported
B. anthracis concentrations in positive samples ranged from 3 CFU/g to 9.7 million CFU/g.
All eight samples collected in the government mail area were positive. No wipe samples
collected in this area were positive. All samples from the high-speed sorting machines
and from areas near DBCS sorting machines were positive (8,700 CFU/g to 2 million
CFU/g). A relatively high concentration of spores was found in the sample collected on
the overnight hot mail sorting bin (13,000 CFU/g), which was near the end of DBCS
machine 5 that had a positive wipe sample collected inside it but had not processed the
contaminated letters addressed to the U.S. senators. Concentrations on the loading dock
and in the express mail room were relatively low. Although the concentrations tended to
decrease with distance from the DBCS machine that processed both letters, spores also
were found in areas far from DBCS machines. The three samples collected in the second
floor administration area and two samples collected in the customer service area were
negative. The vacuum samples indicated wide distribution of
B. anthracis spores, with
the greatest concentrations associated with work areas along the path of the HSOB
letter.
Air Sampling
Air samples were collected on open-faced 37 mm mixed cellulose ester filters (0.8
µm
pore size) in polystyrene cassettes attached to sampling pumps operated at 2.0 liters per
minute. The sampling pumps were placed in fixed locations throughout the facility for
approximately 30 hours. Results were reported as positive or negative for isolates of
B. anthracis.
Twelve air samples for airborne
B. anthracis spores were collected 12 days after the
contaminated letters were processed, which was 4 days after the building was closed
and the ventilation system was turned off. The ventilation system was not operating
during the sampling period. All air samples were negative for
B. anthracis, indicating that
no airborne spores were detectable during the sampling period.
Reported by: D Small, IT Corporation, Virginia Beach, Virginia. B Klusaritz, IT Corporation,
Somerset, New Jersey. P Muller, Muller Architects, Cincinnati, Ohio. National Institute for
Occupational Safety and Health; National Center for Infectious Diseases; and EIS officers,
CDC. 1132
MMWR
December 21, 2001
Brentwood Facility Continued
Editorial Note: The four inhalational anthrax cases among Brentwood facility employees
indicate that aerosolization of
B. anthracis occurred at the facility. The extent to which
environmental sampling can detect potential aerosol dispersion and widespread
contamination is uncertain. In the absence of positive air samples, contamination detected
by wipe or vacuum sampling away from the path of the known source of contamination
(i.e., the letters addressed to the two U.S. senators) could indicate either airborne
dispersion from that source or contamination from a different, unrecognized source
(e.g., another contaminated letter). However, even without positive air samples, two
patterns of sampling results are particularly useful as evidence of possible aerosolization.
Either contamination of surfaces such as air