Any chemical that is either mishandled or improperly used is dangerous. Gas chlorine was briefly tried as a war weapon during World War I, but was quickly replaced as such because it was ineffective for the use. It dissipated too fast, was neither toxic nor persistent enough to do the desired damage to troops, and its effects were too easily remedied. It was therefore quickly replaced by more toxic and effective compounds. Please bear with us, because this one gets a little more technical.
Total chlorine is a combination of free chlorine and combined chlorine.
Free active chlorine is the effective sanitizing agent in the disinfection process, and the passive form is a reserve for when the active form gets used. When chlorine is added to the pool on the service day, the initial addition creates a superchlorination, or shock effect in the pool. This is a beneficial way of removing any combined chlorine or impurities in the water. Any remaining chlorine then becomes free chlorine waiting to be used and is referred to as the chlorine residual. The active is ready to kill bacteria and algae, and the passive replenishes the active.