Aquatics: Healthy Swimming Pools & the Importance of Chlorine


There are more than 10 million pools in the United States today. Pools have been a part of human culture going back to the third millennium B.C., in modern-day Pakistan with a pool made using a brick lining and tar sealant.

Chlorine was discovered in 1774 and first used in 1850 in an attempt to disinfect London's water supply during the cholera epidemic. It became widely employed as a disinfectant in the early 1900s, and its use has become known in the western world as a major public health achievement of the 20th century.

Recreational water illnesses (RWI) are maladies caused by germs and chemicals found in the water we swim in. Recreational water venues include swimming pools, waterparks, splash pads, hot tubs, lakes, rivers and swimming beaches.

Diarrheal illness is the most common type of RWI. Diarrheal illnesses can be caused by organisms like Cryptosporidium, Giardia, Shigella, norovirus, and E. coli., including the dreaded serotype O157:H7 (O157). There have been several outbreaks of O157 related to improperly chlorinated pool water, and complications of infection can lead to death. Proper chlorine levels in pool water will kill O157 in less than 1 minute.

Cryptosporidium forms cysts that can survive long periods of time outside of a host. The illness it causes, cryptosporidiosis (crypto), usually resolves spontaneously, but people with weakened immune systems may suffer complications, ranging from dehydration to death. Symptoms take an average of seven days to develop after exposure and can come and go for 30 days, while shedding of spores can continue for weeks after apparent recovery. This means the usual recommendation of avoiding pools for two weeks after having diarrhea is inadequate. Crypto cysts are relatively resistant to chlorine and can survive up to 10 days even in properly treated water, meaning that advanced disinfection techniques such as UV and ozone filtration may be necessary to treat affected pools. Pool closure along with hyperchlorination at a free chlorine level of 10ppm for 26 hours or 20ppm for 13 hours is necessary to eliminate the parasite.


Giardiasis (beaver fever) is a water-borne illness caused by a parasite (Giardia) that usually spreads person-to-person in humans, although pets and wetland mammals can also be a source of infection. The illness begins one to three weeks after exposure. Symptoms usually include diarrhea and cramping and can last up to six weeks. This organism is able to survive for weeks in room-temperature water and for months in water below 50 degrees. With proper chlorination, however, Giardia can be eliminated in about 45 minutes.

Shigella bacteria cause diarrheal illness that is bloody 25 to 50 percent of the time. It takes a relatively small number of bacteria to cause infection. Like many bacteria, it is rapidly killed in chlorinated water within the recommended range.

Legionella pneumophila is the bacterium that causes Legionnaire's disease. Infection occurs after exposure to aerosolized airborne bacteria or after aspiration of contaminated water. Legionella pneumonia carries a death rate of 10 percent. The organism thrives in warm-water locations, such as large air-conditioning condensers, water-cooling towers, whirlpool spas and even tap water from colonized distribution systems. Outbreaks have even occurred on cruise ships due to water distribution and spa filtration systems that became colonized with the bacteria. Chlorination is the most effective form of control for Legionella, but those same warm conditions that help the organism thrive also makes it harder to maintain adequate chlorine sanitization due to variables such as a faster rate of evaporation. Therefore, it's especially important to maintain water quality through routine testing to identify changes in water chemistry, as well as follow cleansing recommendations to avoid colonization of the equipment.

Pseudomonas aeruginosa is a type of free-living bacteria whose growth is favored by warm temperatures and aeration. It's reportedly present in 94 to 100 percent of hot tubs. Pseudomonas is killed by chlorine, but can sometimes survive in whirlpools with free chlorine concentrations of 3 ppm because it can protect itself by producing slime and hiding in biofilms. Spas and hot tubs are more prone to contamination with a higher concentration of human bodily contributions such as skin, sweat and urine due to the relatively small volume of water compared to swimming pools. Chlorine becomes less effective as it becomes combined with other molecules present in the water, while bromine still remains active after reacting with contaminants, so many people recommend using bromine to combat the issue of bacterial resistance to chlorine. Folliculitis, wound infections, ear infections and eye infections are caused by pseudomonas. Warm water opens the skin pores allowing introduction of pseudomonas into the soft tissues.


RWIs can also be caused by chemicals known collectively as disinfection byproducts, many of which are suspected carcinogens. Exposure to disinfectant byproducts (DBPs) occurs via skin contact, inhalation and consumption of water that has been disinfected with chlorine.

The disinfection byproducts are formed from the reaction of chlorine with organic material. There are two main sources of organic material in pool water: natural organic matter present in the original filling water, and bodily fluids and other human contributions.

Chloramines are a form of combined chlorine that results from chlorine reacting with organic materials in the water such as sweat and urine. This is the smell that people associate with chlorination. It causes irritation to skin, eyes and respiratory tract. Chloramines do get released from the water into the air, but much more slowly than chlorine does. Superchlorination can be used to destroy chloramines and is recommended when levels are greater than 0.3 ppm. This is accomplished by introducing more chlorine to get levels 10 to 20 times higher than the chloramine level and keep it there at least four hours. This means that if a pool has too much of a "chlorine" smell, additional chlorine can get rid of it.

Anything to decrease the demand for chlorine is helpful in this situation, such as supplementing disinfection with UV sterilization, rinsing off before entering the water and maintaining proper pH levels. The most important controllable variables when it comes to disinfection byproduct formation are the free chlorine concentration and the amount of organic matter coming from bathers.

The Model Aquatic Health Code (MAHC), developed in conjunction with the Centers for Disease Control, provides guidance for state and local authorities to make recreational water use safer. It's based on science and best practices and updated periodically.


From those guidelines, the target chlorine level is a minimum of 1 ppm for swimming pools not using cyanuric acid. For those using cyanuric acid, the minimum recommended chlorine level is 2 ppm, and 3 ppm for venues that pose more of a sanitization challenge, such as hot tubs. For any location the level shouldn't exceed 10 ppm when swimmers are present. Guidelines for bromine are a little different, with 3 ppm in aquatic venues other than hot tubs and spas, where 4 ppm is the minimum.

Chlorine's sanitizing power actually peaks at a pH of 5.5, but the MAHC recommends a pH between 7.2 and 7.8 to avoid skin and mucous membrane irritation. A calcium hardness level of 150 to 400 ppm is recommended. Cyanuric acid as a stabilizer or conditioner should be kept under the MAHC max of 90 ppm, and most experts recommend keeping it between 30 and 50 ppm. Higher levels may cause too much chlorine binding and result in "chlorine lock," rendering the chlorine ineffective.

It's crucial to have accurate testing equipment to verify the proper water chemistry and disinfectant levels. In facilities that rely on controllers to maintain the desired water parameters, the use of photometers/colorimeters is the most accurate method to verify system performance. As part of the MAHC, Water Quality Testing Devices (WQTD) must meet NSF/ANSI 50 standards. The current standards for WQTDs specify accuracy and precision requirements for chlorine, pH and other critical water parameters. Certified products are given an accuracy rating to one of three levels: L1, L2 or L3, with L1 (Level 1) being the most accurate. Whenever possible select a test kit with Level 1 accuracy to best ensure bather safety.



Dr. Chad Howard, M.D., is the Principal Medical Writer with Industrial Test Systems, Inc.


Chad Howard | M.D.