Unconventional-weapons threat

UNCONVENTIONAL-WEAPONS THREAT

When a harmful agent is found in a building, experts say, managers should shut down the ventilation system, release stored, purified air, and evacuate if necessary. But detection tech is still not advanced enough to make a skyscraper completely immune to attacks by chemical and biological weapons or a dirty bomb. Here, some of the most cutting-edge current solutions:

RADIATION: Detectors made of zinc sulfide and silver send an alert when levels of alpha radiation (the most dangerous kind) register at more than 10 times the background level.

CHEMICAL WEAPONS: Ion mobility spectroscopy sniffs out sarin, mustard gas and other chemical agents within 15 seconds by giving the air samples an electrical charge; suspect contaminants are then identified based on the rate at which they travel through an electromagnetic field. The units are costly ($35,000 or more) and not yet capable of spotting all potentially harmful chemicals.

GERM WEAPONS: Ultraviolet lamps are used in hospitals to irradiate and kill microorganisms, but they aren´t strong enough to eliminate a large quantity of germs introduced all at once, so skyscraper engineers rely on detection. One state-of-the-art technology is a fluorescent particle counter. A laser shines on air samples; if particles fluoresce, that indicates that living organisms may be present. The organisms are quickly filtered out, but determining whether they are harmful takes 30 minutes or more-too long for those at risk.

The Freedom Tower´s designers say they will put biological and chemical filters in the ventilation units on each floor but have not been specific about which variety they plan to use.