He used pop culture to describe "Dispersion Characteristics of Aerosol Contaminants in Underground Facilities."

Sanchez showed a clip from the movie Ben Hur that depicted a chariot being driven on arid land. Dust billows from behind the chariot as Charlton Heston drives it; when he pulls up to a stop, the dust keeps moving until it obscures the shot so that only faint outlines of the actors can be seen. That, he explained, is how contaminants would move in tunnels.

"The wake of dust expands and keeps moving after the chariot stops," Sanchez said. "Likewise, it keeps moving behind a subway train." A train moving in an opposite direction disperses aerosols throughout the entire space.

Aerosols are a suspension of particles in gas. Those particles range from 0.01 to 100 µm, he explained. Particles greater than 5 to 10 µm are handled by the upper respiratory tract. Those less than 5 µm can penetrate the lungs. They are carried in a medium - in this discussion, air - which controls how the particles are dispersed. Viruses, bacteria, and fungal spores are in hazardous size range, he pointed out.

There is "almost never air velocity low enough for these particles to settle," he said. Subway systems also have ventilation shafts, exits, and stairways. The "piston effect" dumps aerosols back up to sea level if those areas are not sealed.

When contaminants finally come to settle, that's when cleaning can occur. "Venting is not the answer," Sanchez stated. "Containment is more critical. Once an aerosol is released in the tunnel, it's there."

The best way to avoid dispersion, he said, is to prevent a substance's release. Filtration can work, but the number of units required and the degree of filtration they would need to provide make this an undesirable solution from the owners' perspective.

Detection is another possible strategy, one that the U.S. Department of Homeland Security is trying to encourage. However, once a chemical, biological, or radiological (CBR) agent is detected in an underground facility, "the damage is already done," Sanchez said.

In answer to his own question - "Can we limit the dispersion of immediately harmful gases in underground areas, such as subways, tunnels, and parking areas?" - Sanchez said the answer is probably no. The best that can be done is to prevent the spread of CBRs to ground level and stations.

Smoke Models Don't Apply

Igor Maevski, Ph.D., P.E., of Jacobs Engineering in New York, described the Nov. 11, 2000, disaster at Kitzsteinhorn Tunnel near Kaprun, Austria; 155 people died, including three people in the upper terminal, due to smoke inhalation from a fire that started on the funicular train (a hybrid cable and rail transport system).

There is a great need to protect people upstream of the smoke, he pointed out. "Can we use smoke ventilation strategies for chemical or biological attacks? No."

The paper he presented, "Protection of Washington Metro Stations from Tunnel Fire Smoke with Jet Fans Ventilation System," was co-authored with Kanu Desai, P.E., of the Washington Metropolitan Area Transit Authority (WMATA), Washington.

WMATA's Largo line and systems should have its station open by the end of this year, Maevski said. It will have 34 jet fans and two tunnel fans in the upper station. Typical station protection from tunnel smoke, he said, is to have shafts or jet fans at crossovers (areas where secondary tunnels are cut between the main tunnels).

In cases of terrorist attacks near an enclosed vehicle facility, "We need to know if an attack is external or internal," he said. "If it is external, people might use the station for protection."

Applying air filters for tunnels is "problematic," he said. Can stations and stairways be protected with pressurization and filtration? "Do we need additional requirements for tunnel dampers, for isolation? What about protecting the station from the tunnels with physical barriers?"

It is unpleasant and unsettling, but "We have to think about this," Maesvski said. "It is very difficult to stop with our mechanical systems" when it comes to isolating areas.

Collateral Damage

"The isolation of the area of attack might mean the sacrifice of the people inside to protect the people outside," said Kirk McDaniel, P.E., of Earth Tech Inc., Oakland, Calif. He has a background in nuclear radiation, so he is familiar with this strategy.

While researching for his paper ("Can We Limit the Access of CB Substances in Enclosed Transit Facilities?"), "It didn't take me much effort to find information" on releasing substances in enclosed, underground facilities, he said. "Anybody could find this information. Transportation systems are ripe targets."

On March 20, 1995, terrorists released a nerve gas at several points in the Tokyo subway system, McDaniel recounted. The attack was initiated by a cult, which used sarin gas, released by means of a crude delivery method while the terrorists were on trains. The attack was partially botched and therefore resulted in minimal losses - 12 dead and more than 5,500 hospitalized. "With more effective delivery, those numbers could have been reversed," McDaniel said.

Chemical exposure is fast-acting, he explained. There are two classes of chemical weapons: nerve-acting or vesicating (blistering). Proper identification of the gases can be difficult and time consuming. It is also difficult to detect in vapor form, he said. "Monitors can be very expensive" and are still mostly used to gauge post-dispersion contamination.

Biological contaminants, on the other hand, are classified as slow acting due to their incubation periods. Victims can be widely dispersed, and proper diagnosis requires cell culturing before mutation occurs, he said.

Mass transit facilities, he pointed out, have been designed for easy access and mobility, with fairly anonymous entrance and exit portals and multiple access points along the line. There is minimal monitoring of persons and packages; security staff tends to watch for overt criminal behavior.

Nondirect sampling is being developed, he said, but it's likely going to be very expensive. Preventing the release of CB weapons could include the installation of monitoring equipment at all entrances and/or limitation of entrances, and monitoring the identity of passengers. These would result in higher fares, longer queues, decreased popularity of the systems, and would be a hard sell to system owners.

"Can we limit the threat?" McDaniel asked. No, he answered; at least, "not without fundamentally changing the nature of mass transit systems."

Isolation

Paul C. Miclea, P.E., of Earth Tech Inc., Oakland, Calif., discussed "Practical Means to Isolate the Station from the Tunnels in Case of a CBR Event."

"At a minimum, separate the system from the tunnels," he said. "Stations are confined spaces." Limitations must be put in place to control the environment and evacuations.

"The problem is accommodating a large number of patrons at once," he said. Potential damage to structures and systems due to an attack also needs to be factored in, Miclea said.

"Approximately 20 percent of all terrorism is against transportation," he said, adding, "No station can evacuate at full capacity."

Tunnel evacuation is still more difficult, he said. An attack could result in poor visibility, loss of track power, traffic jams, and blocked evacuation routes. In addition, "The contaminated cloud will move faster than the speed of the evacuees."

He described stations as being "wide-open enclosed spaces" with unrestricted airflow between levels. "As designers, we are very proud of these things!" Platforms are connected to tunnels, and it all has a sophisticated ventilation system.

Isolating tunnels from platforms could involve platform screen doors, roll-up/roll-down doors, tunnel gates/doors at the ends of platforms (as used in the former Soviet Union, where stations were meant to be used as shelters in case of nuclear attack), and inflatable barriers. There could still be some leakage, because some barriers are up to 20-percent porous, Miclea said.

He posed several questions, including:

Can we have more smoke/fume extraction points to prevent contaminants from moving to the station?

Can there be dedicated evacuation routes for contaminated people?

"Maybe we cannot design for all terrorist events," he said. However, actions can be taken for site-specific prevention and detection, including continuous monitoring of sensitive areas.

Publication date: 08/02/2004