Order Code RS21920
Updated August 9, 2006
CRS Report for Congress
Received through the CRS Web
Detection of Explosives on Airline
Passengers: Recommendation of the 9/11
Commission and Related Issues
Dana A. Shea and Daniel Morgan
Analysts in Science and Technology Policy
Resources, Science, and Industry Division
Summary
The National Commission on Terrorist Attacks Upon the United States, known as
the 9/11 Commission, recommended that Congress and the Transportation Security
Administration give priority attention to screening airline passengers for explosives.
The key issue for Congress is balancing the costs of mandating passenger explosives
trace detection against other aviation security needs. Passenger explosives screening
technologies have been under development for several years and are now being deployed
in selected airports. Their technical capabilities have not been fully established, and
operational and policy issues have not yet been resolved. Critical factors for
implementation in airports include reliability, passenger throughput, and passenger
privacy concerns. Presuming the successful development and deployment of this
technology, certification standards, operational policy, and screening procedures for
federal use will need to be established. This topic, which was addressed by Intelligence
Reform and Terrorism Prevention Act of 2004 (P.L. 108-458), continues to be of
congressional interest in the 109th Congress. This report will not be updated.
Introduction
In its discussion of strategies for aviation security, the 9/11 Commission
recommended that:
The TSA [Transportation Security Administration] and the Congress must give
priority attention to improving the ability of screening checkpoints to detect
explosives on passengers. As a start, each individual selected for special screening
should be screened for explosives.1
1 Final Report of the National Commission on Terrorist Attacks Upon the United States, July
2004, p. 393.
Congressional Research Service ˜ The Library of Congress

---

CRS-2
The Intelligence Reform and Terrorism Prevention Act of 2004 (P.L. 108-458)
directs the Department of Homeland Security to place high priority on developing and
deploying equipment for passenger explosives screening; requires TSA to submit a
strategic plan for deploying such equipment; and authorizes additional research funding.
It also requires that passengers who are selected for additional screening be screened for
explosives, as an interim measure until all passengers can be screened for explosives.
Congressional interest in this topic continues in the 109th Congress. This report discusses
the current state of passenger explosives trace detection, ongoing federal R&D efforts and
pilot equipment deployments, and related policy issues.
Current State of Trace Explosives Detection Technology
Explosives detection for aviation security has been an area of federal concern for
many years. Much effort has been focused on direct detection of explosive materials in
carry-on and checked luggage, but techniques have also been developed to detect and
identify residual traces that may indicate a passenger’s recent contact with explosive
materials. The trace detection techniques use separation and detection technologies, such
as mass spectrometry, gas chromatography, chemical luminescence, or ion mobility
spectrometry, to measure the chemical properties of vapor or particulate matter collected
from passengers or their carry-on luggage. Parallel efforts in explosives vapor detection
have employed specially trained animals, usually dogs, as detectors.
The effectiveness of chemical trace analysis is highly dependent on three distinct
steps: (1) sample collection, (2) sample analysis, and (3) comparison of results with
known standards.2 If any of these steps is suboptimal, the test may fail to detect
explosives that are present. When trace analysis is used for passenger screening,
additional goals may include nonintrusive or minimally intrusive sample collection, fast
sample analysis and identification, and low cost. While no universal solution has yet been
achieved, ion mobility spectrometry is most often used in currently deployed equipment.
Several technologies have been developed and deployed on a test or prototype basis.
One approach is to direct passengers through a portal, similar to a large doorframe, that
contains detectors able to collect, analyze, and identify explosive residues on the person’s
body or clothing. The portal may rely on the passenger’s own body heat to volatilize
traces of explosive material for detection as a vapor, or it may use puffs of air that can
dislodge small particles as an aerosol. Alternatively, a handheld vacuum “wand” may be
used to collect a sample. In both cases, the collected samples are analyzed chemically.
A different approach is to test an object handled by the passenger, such as a boarding pass,
for residues transferred from the passenger’s hands. In this case, the secondary object is
used as the carrier between the passenger and the analyzing equipment.
The olfactory ability of dogs is sensitive enough to detect trace amounts of many
compounds, but several factors have inhibited the regular use of canines as passenger
explosives trace detectors. Dogs trained in explosives detection can generally only work
for brief periods, have significant upkeep costs, are unable to communicate the identity
of the detected explosives residue, and require a human handler when performing their
2 National Research Council, Configuration Management and Performance Verification of
Explosives-Detection Systems, 1998.

---

CRS-3
detection role.3 In addition, direct contact between dogs and airline passengers raises
liability concerns.
Detection of Bulk Explosives. Direct detection of explosives concealed on
passengers in bulk quantities has been another area of federal interest. Federal and
industry efforts in this area include the development of portal systems utilizing techniques
such as x-ray backscatter imaging, millimeter wave energy analysis, or terahertz imaging.
As such systems detect only bulk quantities of explosives, they would not raise “nuisance
alarms” on passengers who have recently handled explosives for innocuous reasons.
Some versions could simultaneously detect other threats, such as nonmetallic weapons.
On the other hand, trace detection techniques would likely also detect bulk quantities of
explosives, and may alert screening personnel to security concerns about a passenger who
has had contact with explosives but is not actually carrying an explosive device when
screened. Current research efforts are focused primarily on trace detection, and the
remainder of this report does not discuss bulk detection. However, many of the policy
issues discussed below would also apply to bulk detection equipment.
Federal R&D Activities and Deployed Equipment
Multiple federal agencies, including the TSA in the Department of Homeland
Security (DHS), its predecessor organization formerly in the Federal Aviation
Administration, the Department of Energy, the Department of Justice, the National
Institute for Standards and Technology, and the interagency Technical Support Working
Group, have funded research related to trace explosives detection. Explosives trace
detection portals have been considered as potential solutions to other homeland security
needs, such as securing federal buildings and monuments. Much of this research has been
dedicated to overcoming technical challenges, such as increasing sensitivity and reducing
the time required for sample analysis.
The DHS is the main funder of research on explosives detection for airport use.
However, most of this research has focused on detecting explosives in baggage rather than
testing passengers. Some of the underlying technology for passenger explosives trace
detection was developed at Department of Energy national laboratories. Commercial
equipment vendors using such technology under license have incorporated research
performed at Los Alamos National Laboratory and Oak Ridge National Laboratory into
equipment currently being procured by TSA for airport use.
TSA deployed portal equipment for operational testing and evaluation through pilot
projects beginning in 2004. Since then, TSA has reportedly deployed 93 detection portals
in 36 airports.4 Deployments in additional airports are ongoing. The TSA has also
implemented pilot projects for operational testing and evaluation of document scanners
that detect traces of explosives on boarding passes.5
3 National Institute of Justice, Guide for the Selection of Commercial Explosives Detection
Systems for Law Enforcement Applications, NIJ Guide 100-99, September, 1999, p. 36.
4 Jon Hilkevitch, “Midway Gets an Extra Blast of Security,” Chicago Tribune, August 7, 2006.
5 Asa Hutchinson, Under Secretary for Border and Transportation Security, Department of
(continued...)

---

CRS-4
Policy Issues
Operational issues for trace detection include the impact of increased screening time,
the consequences of erroneous and innocuous detections, the ability to detect novel
explosives, and the potential for intentional disruption of the screening process. The
needs for technical and regulatory standards and future cost implications are also of
congressional interest. For security reasons, many technical details of equipment
performance are not publicly available, which makes independent analysis of technical
performance challenging.
Impact on Screening Time. When multiplied by the large number of airline
passengers each day, even small increases in screening times may become logistically
prohibitive. The TSA goal for passenger wait time at airports is less than 10 minutes, and
screening systems reportedly operate at a rate between 7 to 10 passengers per minute;6
additional screening that slows passenger throughput and increases passenger wait time
may add to airport congestion and have a detrimental economic impact. A 1996 General
Accounting Office (GAO) study of explosives detection equipment for airline security
states that throughput goals for portal technologies at that time were equivalent to 6
passengers per minute.7 According to the same study, non-portal technologies, such as
secondary object analysis, had slightly higher throughput goals.
The TSA’s pilot deployment of passenger explosives trace detection equipment will
likely shed light on the question of passenger throughput. If no appreciable increase in
screening times occurs, then passenger explosives screening would seemingly involve few
additional direct economic costs beyond the costs of procuring, deploying, operating, and
maintaining the equipment. If passenger throughput is drastically decreased, then
alternatives for passenger screening may need to be considered. In between these
extremes, it may be possible to moderate the economic impact by adding screening lanes
or by using explosives detection equipment only on those passengers who are selected for
secondary screening (as recommended by the 9/11 Commission as a possible initial step).
Erroneous and Innocuous Detection. A potential complication of explosives
trace detection is the accuracy of detector performance. False positives, false negatives,
and innocuous true positives are all challenges. If the detection system often detects the
presence of an explosive when there actually is none (a false positive) then there will be
a high burden in verifying results through additional procedures. Because of the large
volume of air passengers, even small false positive rates may be unacceptable.
5 (...continued)
Homeland Security, testimony before the Senate Committee on Commerce, Science, and
Transportation, August 16, 2004; “Knoxville Airport to Test Oak Ridge Explosives Sniffer,” USA
Today, July 21, 2004.
6 General Accounting Office, National Preparedness: Technologies to Secure Federal Buildings,
GAO-02-687T, April 25, 2002 and John E. Parmeter, David W. Hannum, Kevin L. Linker, and
Charles L. Rhykerd, Jr., Overview of Explosives Detection Research and Development in
Department 5848 at Sandia National Laboratories, 16th Annual NDIA Security Technology
Symposium & Exhibition, June 26-29, 2000.
7 General Accounting Office, Terrorism and Drug Trafficking: Technologies for Detecting
Explosives and Narcotics, GAO/NSIAD/RCED-96-252, September 1996.

---

CRS-5
Conversely, if the system fails to detect the presence of an explosive (a false negative)
then the potential consequences may be serious. Assuming the system has adequate
sensitivity to detect explosives traces in an operational environment, the detection
threshold or criteria required for an alarm can generally be adjusted, enabling a tradeoff
between false positives and false negatives, but neither can be eliminated entirely; the
appropriate balance may be a matter of debate.
Innocuous true positives occur when a passenger has been in contact with explosives,
but for legitimate reasons. Examples include individuals who take nitroglycerin for
medical purposes or individuals in the mining or construction industry who use explosives
in their work. Such passengers would be regularly subject to additional security scrutiny.8
On the other hand, similar issues arise from the current use of trace detection equipment
on some airline passenger carry-on baggage, and innocuous true positives in such cases
are generally handled without incident. The impact of innocuous true positives will likely
depend on their frequency and on the proportion of passengers who are subject to
explosives trace detection.
Passenger Acceptance. Some passengers may have personal concerns about the
addition of passenger explosives trace detection to the screening process. Issues of
privacy may be raised by the connection between innocuous true positives and passenger
medical status or field of employment. Also, equipment that uses a vacuum “wand” or
puffs of air for sample collection may offend some passengers’ sense of propriety or
modesty. Passenger reluctance could then increase screening times. Allowing alternative
forms of screening, such as within privacy enclosures or through different imaging
technology, might mitigate passenger concerns in some cases.
Detection of Novel Explosives. Explosives detectors are generally designed to
detect specific explosives, both to limit the number of false positives and so that a
determination can be made regarding the identity of the explosive detected. As a result,
novel explosive materials will probably not be detected by these systems. Only after
identifying characteristics and reference standards are developed for the novel explosives
will detection and identification become practical. Unlike detection of bulk quantities of
explosives, where human intervention may identify a suspicious device, trace analysis
provides no comparable detection through operator experience or intuition. While too
narrow a detection window may exclude novel or binary explosives and therefore increase
risk,9 too wide a detection window, in an attempt to detect all possible explosives, will
likely increase the rate of false positives and innocuous true positives. Such results would
limit passenger throughput and increasing the potential for negative economic impact.
Potential for Intentional Disruption. Another concern is the possibility that a
passenger screening regimen that includes explosives trace detection could be exploited
8 An analogous incident occurred in the New York City subway system. A passenger who was
being treated with radioisotopes set off radiation detectors in the subway. The passenger was
repeatedly stopped and held by law enforcement until his medical status could be verified.
Christoph Buettner and Martin I. Surks, “Police Detainment of a Patient Following Treatment
With Radioactive Iodine.” Journal of the American Medical Association, Vol. 288, 2002, p. 2687.
9 Binary explosives are mixtures of two substances that are not explosive separately but become
so when mixed.

---

CRS-6
intentionally to disrupt the operation of an airport. The dissemination of trace quantities
of an explosive material on commonly touched objects within the airport might lead to
many positive detections on passengers. This would make trace detection less effective
or ineffective for security screening, and might disrupt airport operations generally until
alternative screening procedures, such as enhanced baggage screening by TSA personnel,
could be put in place or the contamination source could be identified and eliminated.
Standards, Certification, Regulation, and the Establishment of
Screening Procedures. Standards for the performance of passenger explosives trace
detection equipment, procedures for evaluation and certification of the equipment, and
regulations for its use are all areas yet to be established. Regulations and screening
procedures have been established for explosives trace detection on luggage.10 Detection
on passengers is a more complicated venture, involving possible privacy concerns, greater
difficulty in sampling, and potentially different sensitivity requirements. Nevertheless,
the current luggage regulations could be a model for future certification criteria for
passenger screening. Procedures will also need to be established for the use of the
equipment, such as how an operator should resolve detector alarms to distinguish genuine
security threats from false positives and innocuous true positives.
Cost of Operation. The total cost of deploying explosives detection equipment
for passenger screening is unknown. According to TSA, the portal systems currently being
deployed in U.S. airports cost more than $160,000 each.11 Document scanning systems
are somewhat less expensive; according to the 2002 GAO study, similar tabletop systems
used for screening carry-on baggage can cost from $20,000 to $65,000. It is possible that
technology improvements or bulk purchasing could lower costs. The number of devices
required would depend on throughput rates, device reliability and lifetime, and
deployment strategy. Alternative deployment strategies might screen all passengers or
just a select subgroup, and might conduct screening at all airports or just those where the
perceived threat is greatest. There are over 400 commercial passenger airports in the
United States; possibly several thousand devices could be required, corresponding to a
total capital cost for equipment of up to hundreds of millions of dollars. Installation and
maintenance costs would be additional. Operating the equipment would require
additional screening procedures and might lead to costs for additional screening
personnel, or else create indirect costs by increasing passenger wait times. It is unknown
whether the personnel limit for TSA screeners, currently set at 45,000 full time equivalent
screeners nationwide (P.L. 108-90), could accommodate the potential additional staffing
requirements.
10 67 Fed. Reg. 48506-48509.
11 Transportation Security Administration press release, “Trace Portal Machines Deployed to
Pittsburgh Airport,” October 13, 2005.

---