Civil Defense was an effort designed to help in case of emergencies and disasters.
In the 1960s and '70s, the two lowest levels of the basements under the Crystal Palace theatre were used as air raid shelters by Civil Defense, and were stocked with water cans and radiation detectors. (Trojan Horse)
During Day 5, Civil Defense gave CTU's Lynn McGill new estimates of casualties for the release of Sentox nerve gas in Los Angeles. The new estimates said that each canister could generate a kill zone of 1 to 3 square miles. Since the terrorists had 20 canisters in their possession, the casualties could've mounted to approximately a million. ("Day 5: 1:00pm-2:00pm")