As a water disinfectant, chlorine is almost without comparison. Relatively low in price, chlorine is used by many swimming pool owners precisely for disinfection purposes. Chlorine quickly works to eliminate potentially harmful bacteria in a pool's water, for one. However, chlorine is a very strong chemical, and its presence in water should only be at recommended levels. For example, if swimming pool chlorine levels are too high, swimmers shouldn't use the pool until levels decline.
Safe levels of chlorine should be maintained in a swimming pool at all times; there should never be an excessively low amount of chlorine nor should there be an excessively high amount. For swimming, the recommended level of chlorine is 2 parts per million (ppm), with 1 ppm to 3 ppm as the recommended range. When pool chlorine levels increase to 10 ppm or more, definite issues will occur.
Chlorine is a very strong chemical disinfectant, and the first thing pool owners notice when its levels are too high is a chlorine smell. And that smell often becomes almost unbearable at 10 ppm of chlorine. Also, at 10 ppm of chlorine, swimmers will experience strong levels of skin, eye and mucous membrane irritation. You shouldn't swim in any pool with chlorine levels exceeding about 6 ppm.
After swimming has been prohibited in a pool with excess chlorine levels, chlorine can be neutralised by adding sodium thiosulfate. When used in swimming pools, sodium thiosulfate can be added once chlorine levels exceed 5.5 ppm. Add 170gr. of sodium thiosulfate per 10,000 gallons of water to reduce chlorine by 1 ppm. You should also ensure that your pool's cyanuric acid (CA, a chlorine stabiliser), levels range from 30 to 95 ppm.
One of the best ways to lower chlorine levels in a swimming pool is to stop adding chlorine until those levels have declined to 1 to 3 ppm. You don't need to consistently employ sodium thiosulfate to lower pool chlorine levels. You can test for chlorine, pH (which is an important adjunct to chlorine) and CA levels by using a good all-around pool testing kit. Such pool test kits are available at any swimming pool supply store.