Protecting Subway Riders from a Chemical Attack

In 2010, New York City had the fourth highest annual subway ridership in the world – more than 1.6 billion people, according to the Metropolitan Transportation Authority. In that same year, a much smaller number of passengers – 713 million – boarded airplanes across the United States, according to FAA (Federal Aviation Administration) statistics. Despite this significant difference, the nation’s subway systems have not imposed strict passenger or baggage screening requirements similar to those used in civil aviation.

In other words, there is no passenger vetting, similar to what is used to compile the aviation industry’s “no-fly” lists, to prevent someoneentified as a potential terrorist from boarding a subway car – or, for that matter, any of the nation’s trains and buses.

The Vulnerabilities of Subway Systems 

Literally millions of Americans (and foreign visitors) ride subways every day. One of the most important challenges facing those responsible for the safety of these transit systems, therefore, is to protect them against chemical terrorism – i.e., the use of chemical agents by persons seeking to kill or injure others, intentionally harm the environment, and/or adversely affect the nation’s economy. The two principal categories of chemical weapons or devices typically used by terrorists are chemical warfare agents (CWAs) and toxic industrial chemicals (TICs). Most CWAs, which are designed primarily to disrupt enemy assaults on the battlefield, are produced in mass quantities. Most TICs are manufactured by private-sector companies to create a broad spectrum of commercial products – including plastics, fuels, fiberglass, and household cleaners – that are readily available for purchase in supermarkets and many other stores throughout the country.

Enclosed spaces such as subway systems are particularly susceptible to attacks using TICs because most chemical agents can be released simply by attaching an explosive device to a canister of some type that is being used to contain the agent. The heat and pressure produced by the explosion may not substantially degrade the toxic characteristics of the agent, which means that it may still cause significant harm. In addition, the “push/pull” airflow created by incoming and outgoing railcars that are traveling, usually at high speed, through subway tunnels can rapidly disperse chemicals from their source of release toward unsuspecting passengers – those already aboard the train as well as those waiting in the station. Further complicating the problem, and exacerbating the danger, is that the typically limited egress from many stations hampers a rapid evacuation, and that secondary problem also could cause crushing injuries, and additional deaths, as people stampede to the nearest exit.

To detect and warn patrons and authorities – and potentially save lives – transportation planners must design a system for integrating, into current and future subway systems, chemical detection systems and devices that can work in any of the stations in any given subway system. What follows are six key steps – partially developed and strongly recommended by planners in the DHS Office of Health Affairs’ Chemical Defense Program – to help protect subway riders from a chemical attack:

  1. Develop the risk assessment methodology needed to: (a) characterize specific chemical threats; and (b) carry out a vulnerability assessment of a subway system that can be used to protect against those same threats. Chemical detection systems are designed specifically to save lives – but will be able to do so only if the detectors procured and installed detect and warn against the presence of the agents used in an attack.
  2. Establish detection performance specifications, based on the vulnerability assessment, to determine the detection technology requirements. In addition to developing these specifications, the target agents should be tested, to validate the performance claims made by the vendors of detection equipment, by using a set of Acute Exposure Guideline Levels (AEGLs) – a task managed by the U.S. Environmental Protection Agency – and/or a NIOSH (National Institute for Occupational Safety and Health) list of “Immediately Dangerous to Life and Health” (IDLH) values. The AEGL list defines the threshold exposure limits of specific hazardous chemicals, under emergency conditions, whereby – at or above those levels – harmful health effects are most likely to occur. AEGL values apply to first responders as well as to the general public. TheLH values also define exposure limits, and are generally higher than the AEGL values. AtLH levels, which apply to first responders only, escape from the immediate area within 30 minutes is critical, or permanent disability or death may result. Moreover, performance specifications must include a list of chemicals commonly present and/or used in a subway system for cleaning or maintenance, and – to minimize false positive alarm rates – should require that such chemicals not be able to trigger an alarm by the chemical detectors. Rigorous and effective performance specifications are essential to determine the appropriate detection technology that should be used.
  3. Evaluate the information available about the various types of detection technologies that are being considered for use as stationary, autonomous detection systems. Single detection technologies involving one type of process typically have a fast detection response and are relatively small in size. Orthogonal detection technologies involving two or more types of in-series detection processes usually have a longer detection response time than single detection technology detectors but, because of their ability to separate the chemical constituents present in an air sample mixture, possess greater sensitivity. In addition, orthogonal detectors often have lower false positive alarm rates than those that are characteristic of detectors with only a single detection technology. Evaluating different types of technology is key to success in this area, because detection technology systems are not “one size fits all” products, and the use of more than one type of technology may in many situations not only be advisable but mandatory.
  4. Use a detector placement method, primarily through the use of dispersion modeling and field studies to determine the optimal number and placement of detectors needed to provide the full range of detection response capabilities needed. There are a number of different ways to use both methods to predict the downwind spread of chemical vapors and gas through a subway system – while also taking into account the need, if and when possible, to continue routine subway system operations. To determine the number of detectors needed for a particular system, computational modeling canentify the appropriate detector placement locations, and thus the number of units required to best detect a chemical agent immediately after its release – from either a single source or multiple sources. A methodological approach to detector placement is particularly important to ensure adequate and effective system-wide coverage.
  5. Develop a concept of operations (CONOPS) to coordinate all elements of the system’s detect-to-warn-to-response capabilities. Integration of the system’s new chemical detection system should, in fact, be the principal factor used in developing a response plan specifically designed to protect against chemical agents.
  6. Create and implement a training and exercise program to help first responders familiarize themselves with the actions that they must take after a detector has signaled a release. A key component of this program should focus on “patron awareness” of the detect-to-warn capabilities of the new system. This step could also provide valuable feedback from users toentify gaps – in the detector systems or in the response plans – that may have been overlooked during the initial design and development processes. Continued training and a broad spectrum of exercises are needed to ensure that all personnel involved not only know their individual, and collective, roles and responsibilities but also are able to carry them out both fast and efficiently.

To briefly summarize, a properly planned and implemented chemical detection architecture can assist immensely in the design and implementation of the effective chemical-protection capabilities needed in the subway transit environment. The key steps described above – risk assessment methods, establishment of performance specifications, review of current detection technologies, detector placement, CONOPS development, and training and exercise programs – are the essential building blocks needed to help transit authorities, and subway system personnel, evaluate and install the detection products that best meet their needs.

A deliberate approach, as proposed here, will help transit authorities and homeland security professionals minimize casualties in the event of a chemical agent release in a subway system. The sooner such a framework is implemented, the better prepared they will be should such an attack occur at any time in the foreseeable future.

Joselito S. Ignacio

Captain Joselito S. Ignacio, M.A., M.P.H., is a U.S. Public Health Service Officer now serving as Acting Director of the Chemical Defense Branch in the U.S. Department of Homeland Security’s Office of Health Affairs. He previously served as Deputy Director of the Chemical Defense Branch, which he joined in 2010. He has, among other responsibilities, overseen a two-year demonstration project in Baltimore on how to protect the city’s subway system in the event of a chemical attack. He holds a master’s degree in public health from the University of California at Los Angeles as well as a master’s degree in homeland defense and security from the Naval Postgraduate School.



No tags to display


Translate »