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Because of their length, ubiquity, and remoteness, pipelines can be nearly impossible to defend.

Following the Sept. 11, 2001, attacks on the World Trade Center and the Pentagon, many experts have become more concerned that terrorists may maliciously spread biological agents such as anthrax or smallpox.

U.S. chemical plants are vulnerable to acts of terrorism.

While petrochemical plants get the most attention, statistics from the Chemical Safety Board suggest that media overlook three quite common and widespread hazards: chlorine, ammonia, and propane

Various nations have developed and produced chemical weapons -- substances whose main use is to harm people -- such as nerve gas or mustard gas.

There are places along the Mississippi River (e.g. New Orleans) where many people live in areas that would normally be flooded, were it not for levees or other flood control works.

The WTC victims were killed not just by terrorists, but by the buildings themselves.

Cropduster flights resumed Sept. 25, 2001. The FBI had ordered cropdusters grounded nationwide temporarily in late Sept. 2001.

The President's Commission on Critical Infrastructure Protection was set up in July 1996 under President Clinton's Executive Order 13010.

A likely upcoming shift in California's strategy to reduce smog from cars could affect new vehicles rolling onto roads nationwide over the next few years.