Chlorine is a greenish-yellow gas that dissolves easily in water.

Chlorine is a greenish-yellow gas that dissolves easily in water. It has a pungent, noxious odor that some people can smell at concentrations above 0.3 parts per million. Because chlorine is an excellent disinfectant, it is commonly added to most drinking water supplies in the US. In parts of the world where chlorine is not added to drinking water, thousands of people die each day from waterborne diseases like typhoid and cholera.Chlorine is also used as a disinfectant in wastewater treatment plants and swimming pools. It is widely used as a bleaching agent in textile factories and paper mills, and it’s an important ingredient in many laundry bleaches.
Free chlorine (chlorine gas dissolved in water) is toxic to fish and aquatic organisms, even in very small amounts. (See table.) However, its dangers are relatively short-lived compared to the dangers of most other highly poisonous substances. That is because chlorine reacts quickly with other substances in water (and forms combined chlorine) or dissipates as a gas into the atmosphere. The free chlorine test measures only the amount of free or dissolved chlorine in water. The total chlorine test measures both free and combined forms of chlorine.
If water contains a lot of decaying materials, free chlorine can combine with them to form compounds called trihalomethanes or THMs. Some THMs in high concentrations are carcinogenic to people. Unlike free chlorine, THMs are persistent and can pose a health threat to living things for a long time.
People who are adding chlorine to water for disinfection must be careful for two reasons: 1) Chlorine gas even at low concentrations can irritate eyes, nasal passages and lungs; it can even kill in a few breaths; and 2) The formation of THM compounds must be minimized because of the long-term health effects.
Less than one-half (0.5) mg/L of free chlorine is needed to kill bacteria without causing water to smell or taste unpleasant. Most people can’t detect the presence of chlorine in water at double (1.0 mg/L) that amount. Although 1.0 mg/L chlorine is not harmful to people, it does cause problems for fish if they are exposed to it over a long period of time.
Effects of chlorine on industrial processes
Chlorine may cause canned or frozen food to taste "funny". It also may effect the smoothness or brightness of plated metals. Chlorine levels as low as 0.3 mg/L can spoil the quality of high-grade paper during the manufacturing process.
Effects of chlorine in water used for irrigation
The concentration of chlorine in city water or treated wastewater rarely reaches 1.0 mg/L (ppm). So chlorine usually is not a problem to farmers and gardeners using either city water or wastewater to irrigate their crops.
Effects of chlorine on fish and aquatic life
The table shows how chlorine affects fish and aquatic organisms. It is important to realize chlorine becomes more toxic as the pH level of the water drops. And it becomes even more toxic when it is combined with other toxic substances such as cyanides, phenols and ammonia.
Phenols are organic chemicals produced when coal and wood are distilled and when oil is refined. Phenols are found in a number of products—from organic wastes to sheep dip. Although phenols are very toxic, dilute solutions of a phenol (carbolic acid) are used as a disinfectant.

Table 3. Effects of chlorine on fish and aquatic organisms
Total chlorine (in mg/L)	Effect
0.006	Kills trout fry in two days.
0.01	Recommended maximum for all fish and aquatic life.
0.01	Kills Chinook Salmon and Coho Salmon.
0.01-0.05	Oysters have difficulty pumping water through their bodies.
0.02	Maximum Brook and Brown Trout can withstand.
0.05	Maximum amount that can be tolerated by young Pacific Salmon in the ocean.
0.1	Kills most marine plankton.
0.25	Only the hardiest fish can survive.
0.37	Maximum fish can tolerate.
1.0	Kills oysters.
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