hazards present in this situation. The first is the rocket-propelled grenades and IEDs themselves. The warheads on the rockets can be extremely powerful. High explosive warheads detonate on impact and produce a relatively high degree of fragmentation. Both the initial impact and the impact of the fragments can do a significant amount of damage to a person as well as to any chemical containers. If the rockets are armed with high-explosive anti-tank rounds, the rounds will have enough force to break through an armored vehicle.

IEDs too can do a significant amount of damage to people as well as any chemical containers. (The number of casualties in Iraq and Afghanistan due to IED attacks is a testament to how lethal IEDs can be. A person who is injured by an RPG round or an IED would be doubly vulnerable to gas released in the attack because he or she would be less able to get into a shelter away from toxic chemicals.

Chlorine gas is a highly toxic chemical. It was used as a chemical weapon in World War I. In the lungs, chlorine gas mixes with water in the pulmonary tissue to form hydrochloric acid, which can be lethal. Combining sodium hypochlorite and sodium chloride can produce additional chloride gas.

The fires themselves present a number of hazards. Smoke inhalation is deadly even when toxins like chlorine gas are not present. Heating chlorine gas makes it more dangerous and a person whose body temperature has risen is also more vulnerable to the effects of toxins since his or metabolism, heart-rate, and blood pressure will all be elevated, thus increasing the effects of the poison. Of course, the fire itself poses the potential to burn people. It might also do structural damage to any buildings, making them unsafe.

At the second site, the release of MDI can cause irritation to the nose, throat, and eyes along with coughing, chest pain, asthmatic attacks, and other problems breathing (NIOSH pocket guide to hazardous chemicals.) According to the CDC, Acrylonitrile is both toxic and highly flammable. It is explosive (which has its own potential hazards in terms of injury). When it burns, it produces hydrogen cyanide among other by-products: This is a highly poisonous gas. The by-products of burning acrylonitrile are classified as a Class 2B carcinogen, which means that it is possibly carcinogenic.

The heavy metals in the smoke plume also present a number of potential hazards. Nickel can produce skin irritation, allergic asthma, and lung irritation. Molybdenum, in the form of smoke, can be toxic when inhaled or ingested. It cause relatively low-level harm (including skin and eye irritation) although inhalation can lead to relatively serious lung irritation. Longer-term exposure (which might occur if the smoke were to stay in the air for some period of time).

NIOSH lists a number of serious consequences for exposure to cadmium and its compounds. Exposure to cadmium fumes tends to produce the range of symptoms that the flu does -- fever, chills, and aches. More severe reactions -- to longer exposure or exposure to cadmium in higher concentrations -- can include pulmonary edema. Exposure to cadmium in smoke can lead to kidney damage and even fatal kidney failure. Gout is also a possible effect of exposure to smoke with cadmium in it. A number of cadmium compounds are also carcinogenic.

NIOSH classifies mercury is a highly toxic chemical that can affect a range of organic systems. When it is vaporized, mercury can permeate the central nervous system. In general, acute exposure to inhaled mercury can produce in flu-like symptoms, difficulty in breathing, chest pain, fatigue, confusion, nausea and vomiting. It can also produce potentially lethal pulmonary emboli.

Scenario Two

In this scenario, we are asked to consider the relative effects of two different biological agents: bubonic plague spread by infected fleas and aerosolized anthrax.

According to the Natural Institute of Health, bubonic plague has an incubation period of between two to ten days. Initial symptoms include malaise, high fevers, and tenderness around each lymph node. (Pneumonic plague is caused by the same pathogen; however, bubonic plague would be more likely in the case of an outbreak caused through a release of infected fleas. The bacteria is easily killed by direct sunlight; however, while it is inside a living organism (whether flea or human) it will remain alive. It can live for an extended time outside of a host in cold weather and in a moist environment it can survive for several days. While it survives it remains infectious.

Almost all human cases of bubonic plague result from being bitten by an infected flea: There is very little human-to-human transmission. This is not true with pneumonic plague. As noted above, pneumonic plague is caused by the same bacterium that causes bubonic plague. The only difference is where the bacteria takes up residence in the body. If the bacteria lodges in the lungs, it manifests itself as pneumonic plague. An individual with pneumonic plague spreads the bacteria in droplets of saliva when she or he coughs or sneezes, thus passing the bacteria on to others.

Because the population in question is a commuter one, the risk of transmission is lower than it would be if the population were tightly concentrated. The great plagues of the Middle Ages were so lethal because so many people were living in very crowded neighborhoods that had many rats carrying infected fleas living close by them. Individuals had numerous chances each day to come into contact with infected fleas. In this scenario the only way that the plague could be widely spread would be if the commuting individuals carried the fleas with them back to their homes.

How many fleas would be carried by the commuters would depend on a number of different factors, including what mode of transportation was used (car vs. train, for example) and the weather. (In warmer weather, people would be wearing less clothing, which would afford fewer places for fleas to hide, for example, which would cut down on the rate of transmission. In general, the potential for contagion is lower in this scenario is lower than it would be in a more densely populated, stationery population.

There is an available vaccine for bubonic plague. If the population in question had been vaccinated, the incidence of the disease would be substantially reduced. It is also important to note that the disease can be successfully treated with antibiotics.

Overall, an attack that used bubonic plague spread by infectious fleas would probably be more psychologically traumatic than lethal. The disease could certainly be spread in this fashion, but this is not a highly efficient method for infecting a large population -- and even less efficient if the goal of the attack is to cause a high number of fatalities.

Using anthrax in an aerosolized form is also an ineffective potential weapon, according to the Centers for Disease Control. The major problem in using anthrax as a weapon is that it has to be aerosolized, which is something that has never been accomplished by a terrorist organization. The scenario asks us to assume that a non-governmental organization has been able to aerosolize it, so we will assume that somehow this has become possible.

However, even if terrorists succeeded in producing an aerosolized form, it would not be a very effective weapon. Spores of anthrax would quickly fall to the ground. In the natural world, the primary means of transmission of anthrax is by animals inhaling it and ingesting it as they graze. Because of this, anthrax spores are designed to fall to the ground and stick there. There would not be enough of the spores kicked back up into the air to infect humans. Thus while many spores would remain viable, they would not infect humans.

Anthrax is often fatal, which is no doubt why it is so frightening as a potential weapon. When people hear the words "plague" or "anthrax" they become extremely frightened because they know the terrible toll that these diseases have taken on humanity in the past. However, that was before the era of antibiotics (anthrax can also be treated with antibiotics and it too has a vaccine) and occurred in the context of natural infection and transmission routes.

However, as bio-terrorism weapons, neither of these bacteria would be significantly harmful in these particular scenarios. The plague would be a greater threat of the two.

Scenario Three

Like biological weapons, dirty bombs are one of the potential threats that darken the prospects of citizens in the twenty-first century. This scenario about a dirty bomb demonstrates how easy it would be for a terrorist organization to come up with the parts and materials needed to make such a bomb. According to the Nuclear Regulatory Commission, a dirty bomb is a weapon (as of yet none has actually been produced) that combines radioactive material with conventional explosive material. The goal of such a weapon would be to distribute radioactive material over a large area.