Because chlorine likes to combine with other elements in the water, it is effective at killing bacteria when they combine. However, when ammonia is present in the water, the two combine to form chloramines. When combined, the chlorine is in a weakened state and is not as effective against algae and bacteria.
Ammonia (Nitrogen) in the water can come from human sweat, urine, leaves and fertilizer. Because one active swimmer in the water can produce one quart of sweat in an hour, it is highly likely that ammonia will be present. When you smell chlorine in a pool or have irritated eyes, chloramines are the cause – not the presence of too much chlorine. Chlorine gets a bad rap in this case. Chlorine is by far the most efficient and effective way to control algae and bacteria in your pool. The only way to reduce the chloramines is to add more chlorine, shock, to neutralize the chloramines so that the chlorine will go back to oxidizing the bacteria and algae instead of combining with the ammonia in the water.
You can calculate combined chlorine in your water by subtracting the amount of free chlorine from total chlorine. This value should be as close to zero as possible. Whether your pool is used heavily or not, chloramines can form in your pool. Shocking on a weekly or biweekly basis is required.