A total of 15,000 dogs are being used by police departments throughout the United States. Most of the dogs are used for routine patrol and investigations on the street. An increasing number are being used for bomb and drug detection and subway patrol. Dogs are well-suited to police work because they are loyal to their handlers, able to get into spaces too small for humans, have extraordinary powers of smell, are fearless and can intimidate people. Philadelphia is one of the few cities in which dogs are still used for crowd control, but most departments forbid their use. Each police dog works with a single officer and will defend that officer to the death. Although dogs have occasionally attacked their handlers or their handlers' families, such incidents are rare. The dogs' most valuable function is bomb detection. They are trained to detect dynamite, plastic explosives, smokeless black powder, detonation cord bombs, and incendiary mixtures. Training procedures for these and other functions are fairly uniform across the country. In the District of Columbia, dogs begin training when they are between 1 and 3 years of age. The cost of putting one dog on the street is $7,000. Dogs receive agility training, training which uses simulated street situations, and training in systematic search. Dogs must also be able to track a human scent 1 hour old for a minimum of 300 yards. Training is a game to the dog and praise their only reward.