Q:
If chlorine is so effective, why do I need to remove it?

A:
Studies have shown that the same reactive nature of chlorine that kills hamful bacteria also allows it to react with humic acid from decaying organic matter in the water to form cancer-causing trihalomethanes.

For more information on trihalomethanes, please see the article What Are Trihalomethanes.

Also, the presence of chlorine in tap water causes taste and odour problems that are distasteful to most people.