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Appendix A

Glossary of Definitions


Flow of water through filter element(s) or media in a reverse direction to dislodge and remove accumulated dirt, debris, or filter aid from the filter tank.

bather load

The number of bathers using a swimming pool or spa at any one time. The maximum bather load is usually determined by a state or local pool code and is based on surface area and depth of the pool or spa.


Microbial cells that adhere to a moist or water-covered surface through a matrix of primarily polysaccharide materials in which they are encapsulated. Biofilms can grow on piping and surfaces of aquatic venues and can be very difficult to remove. They protect microbes from disinfectants (e.g., chlorine) in the water.

cercarial dermatitis

Dermatitis caused by contact with or direct invasion through the skin or a break in the skin by the cercariae (larval stage) of certain species of schistosomes. The normal hosts of these species are birds and non-human mammals. Dermatitis is an allergic response to contact with cercariae and does not lead to parasitic infestation in humans and produces no long-term disease.


Waterborne disease outbreaks are classified according to the strength of the epidemiologic and clinical laboratory data, and environmental data implicating recreational water as the source of the outbreak (see Table 1).


A group of disinfection by-products or weak disinfectants formed when free chlorine combines with nitrogen-containing compounds (e.g., urine or perspiration) in the water. Tri- and di-chloramine can cause eye and respiratory (e.g., lung and throat) irritation and can accumulate in the water and air surrounding treated recreational water venues. In drinking water treatment, monochloramine is used for disinfection to reduce formation of disinfection by-products created when using chlorine as a disinfectant (see combined chlorine level).

combined chlorine

Chlorine that has combined with organic or inorganic compounds in the water and is no longer an effective disinfectant for recreational water. The combined chlorine level is derived by subtracting the water's free chlorine test level from its total chlorine test level. This level is likely to include combined compounds in addition to chloramines (see chloramines).

communitywide outbreak

This outbreak typically starts as a focal outbreak associated with one recreational water venue and evolves into an outbreak associated with multiple recreational water venues or other settings, such as child care centers in a community.

contact time

The length of time that recreational water and any pathogens in the water are exposed to a disinfectant, usually measured in minutes (e.g., chlorine contact time).


A treatment that kills microorganisms (e.g., bacteria, viruses, and parasites); in water treatment, a chemical (commonly chlorine, chloramine, or ozone) or physical process (e.g., ultraviolet radiation) can be used.

disinfection by-products

Chemicals formed in water through reactions between organic or inorganic matter and disinfectants. Examples include chloramines, also known as combined chlorines. These chemicals might have acute or chronic health effects.


The pathogen, chemical, or toxin causing a waterborne disease outbreak or other health event. Infectious etiologies include bacteria, parasites, and viruses.

fill-and-drain pools

Small pools or slides that often are constructed of plastic and that might be inflatable. These pools and slides are filled with tap water without any ongoing chemical disinfection or filtration.


In water treatment, the process of passing water through one or more permeable membranes or media of small diameter (e.g., sand, anthracite, or diatomaceous earth) to remove suspended particles from the water. Filters might be effective in removing pathogens, depending on the type and operation.

free chlorine

Chlorine in water (found as an aqueous mixture of hypochlorous acid and hypochlorite anion) that has not combined with other constituents; therefore, it is able to serve as an effective disinfectant (also referred to as free available chlorine or residual chlorine). Measuring the free chlorine level is a common water quality test.


Untreated, nonmarine surface water (e.g., water from lakes, rivers, or ponds).

interactive fountain

A fountain or water spray device that is either intended for or accessible for recreational use and typically does not have standing water aboveground; recirculated water typically is stored in an underground holding tank. These fountains are sometimes called spray pads, splash pads, wet decks, or spray grounds. In contrast, a noninteractive (ornamental or decorative) fountain intended for public display rather than recreational use is often located in front of buildings and monuments, and the water is not easily accessible for public use.


An inclusive term for the U.S. states, District of Columbia, territories, and Freely Associated States (FAS) (e.g., the Republic of the Marshall Islands, the Federated States of Micronesia, and the Republic of Palau) that report waterborne disease outbreaks to the Waterborne Disease and Outbreak Surveillance System (WBDOSS). Waterborne disease outbreaks investigated by local public health agencies are reported to the state, territorial, or FAS public health agency. Jurisdictions can also report select recreational water--associated health events to WBDOSS.

marine water

Untreated recreational water at an ocean or estuarine setting.

pool chemical--associated health events

Injuries or illnesses resulting from exposure to pool chemicals (e.g., halogens or disinfection by-products) used to maintain quality of treated recreational water. These events might not meet the criteria for a waterborne disease outbreak depending on whether they involve exposure to recreational water or two or more persons. These events must be associated with treated recreational water venues.

recreational water venue

A body of water used for recreation (e.g., swimming, soaking, or athletics), including any structure that encloses this water. It can include a lake, pond, river, spring, ocean, or a man-made venue (e.g., swimming pool and spa); some recreational water venues do not include standing water (e.g., interactive fountains).

reservoir, impoundment

An artificially maintained lake or other body of water used for the collection and storage of water. It can be available as a source of raw water for drinking purposes or recreational use.


Location where exposure to contaminated water occurred.


Any structure, basin, chamber, or tank (located either indoors or outdoors) containing a body of water that is intended to be used for recreational or therapeutic purposes and that usually contains a waterjet or aeration system. It is operated at high temperatures and usually is not drained, cleaned, or refilled after each use. It also is referred to as a hot tub or whirlpool.

spray park

A recreational water venue consisting of multiple interactive fountains.

total chlorine

A common water quality test that measures the chlorine in water that is free for disinfection (see free chlorine) plus that combined with organic or inorganic materials (see combined chlorine level). The water's combined chlorine level is derived by subtracting the free chlorine test level from the total chlorine test level.

treated water

Water that has undergone a systematic disinfection process (e.g., chlorination and filtration) to maintain good microbiologic quality for recreation. Typically, this refers to any recreational water in an enclosed, manufactured structure. This includes water in swimming or wading pools, fountains, and spas but might also include water in fill-and-drain pools filled with treated tap water or untreated water (e.g., mineral spring water) that receives no further treatment.

untreated water

Water that has not undergone a disinfection or treatment process to maintain good microbiological quality for recreation (e.g., lakes, rivers, oceans, and reservoirs).


Any person or bather entering recreational water. Might also be referred to as a patron at some membership clubs or recreational water venues.

water quality indicator

A microbial, chemical, or physical parameter that indicates the potential risk for infectious diseases associated with using the water for drinking, bathing, or recreational purposes. Standards might vary based on type and degree of water exposure associated with different water uses. Ideally, density or concentration correlates with health effects. Examples include turbidity, coliform counts, fecal coliforms, Escherichia coli, enterococci, and free chlorine level.

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