Details of how the fire contributed to the collapse of the WTC towers are still being studied. Some estimates suggest that the jet fuel probably burned out within a few minutes of impact, but not before igniting building materials and contents on multiple floors simultaneously. This would mean that the fires were fed primarily by materials that are equivalent to those in most other high-rise office buildings. The 1988 First Interstate Bank fire in Los Angeles and the 1991 One Meridian Plaza fire in Philadelphia burned out multiple floors in very intense fires fed only by the ordinary combustible furnishings and finishes within these office buildings (Nelson, 1989; Routley et al., 1991), but they did not collapse. An important issue, then, is whether a similar fire in the WTC and or similarly constructed megastructures could cause the building to fall even without airliner impacts.

Single, localized ignition is assumed in building design (Ingberg, 1928). However, a low-grade explosive incendiary device or other method of starting multiple small fires could potentially cause enough damage and spread fire over a large enough area to overwhelm the building’s sprinkler system and lead to an uncontrolled fire. Redundant water supply for fire protection and/or redundant sprinkler systems might provide additional protection for these situations and for some types of attacks.

In addition to damage to the building itself, the hazards of impact, explosion, or fire also include flying glass shards (there may literally be millions of them) and airborne toxins.

Heating, ventilation, and air conditioning (HVAC) systems could disperse airborne materials. While most HVAC systems in new buildings are partitioned, serving groups of several hundred people or fewer, older HVAC systems may serve much larger areas. In some high-rise buildings, openings for elevators and plenums run the entire height of the building, creating a chimney effect. Outdoor air enters the building at the lowest levels and rises to the top as it is warmed. These paths provide a ready mechanism for distribution of toxins throughout the entire building.

One way to prevent HVAC units from becoming the entry point for toxic agents is to restrict access to the outdoor air intakes and fan rooms. Outdoor air intakes are commonly located in the walls of buildings, accessible to the street level. In existing urban high-rise buildings, relocating them would be quite expensive and therefore unattractive to building owners. Rooftop HVAC systems are less vulnerable. In new buildings, outdoor air intakes can more easily be protected, and fan rooms can be secured. Such changes are achievable through building codes. While most HVAC systems use air filters, they are not capable of removing many types of toxins. Filters that could remove both biotoxins and chemical toxins are available, but they are costly to install and operate. Few building owners would find them worthwhile in today’s real estate markets. However, filters to remove just biotoxins (e.g., anthrax) can be installed and operated; these might be a reasonable compromise. Meanwhile, no technology is



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