Shalasai Huangprasert                                  PHEH 414    Rural Sanitation & Appropriate Technology

 

1. Types of pools and spas

Pools may be private (domestic), semi-public (e.g., hotel, school, health club, condominium, cruise ship) or public (municipal or governmental). Pools may be located indoors, outdoors or both. In terms of structure, the conventional pool is often referred to as the "main" pool or public or municipal pool. It is by tradition rectangular, with no extra water "features," and it is used by people of all ages and abilities. However, there are many "specialist" pools for a particular user type — for example, paddling pools, diving pools, pools with special features such as "flumes" or water slides, and spas. A "spa" for the purposes of this lecture is defined to include hot tubs (domestic), whirlpools (commercial/facility) and natural mineral baths. There are also pools containing thermal or medicinal waters, such as physical therapy pools, in which treatments for a variety of physical symptoms are performed by professionals on persons with neurological, orthopaedic, cardiac or other diseases, in warm water.

In addition, the type, design and use of the pool may predispose the user to certain hazards. Bubble pools or whirlpools, leisure pools and spas, for example, may be subject to high bather loads relative to the volume of water. Where there are high water temperatures and rapid agitation of water, it may become difficult to maintain satisfactory pH, microbiological quality and disinfectant residuals. Pools with moveable floors will have variable depths, and, as such, the greatest bathing load is likely to be in the shallower areas. In any pool with concentrated bather loads, pollution can be high. In addition, some special provisions of pools, such as forced recirculation and aeration, may contribute to bacterial overgrowth.

Pools without water treatment and recirculation are permitted in some countries, as their construction does not impose a major economic burden. These pools may be associated with a higher risk of transmission of waterborne diseases among users and thus pose a special set of problems to operators.

Spas with thermal and medicinal waters are also a special case, as they are generally impossible to treat in the usual way — they cannot be recycled or disinfected, because the therapeutic agent, such as sulfides, would be eliminated or impaired. As well, chemical substances of geological origin in some sorts of deep thermal springs and artesian wells, such as humic substances and ammonium, may hamper the effect of disinfectants when these waters are used to fill pools without any pretreatment. These spas therefore require non-oxidative methods of water treatment to keep the water effective and microbiologically safe at the same time. A very high rate of water exchange is necessary — even if not effective enough — if there is no other way of preventing microbial contamination.

Pools and spas on ships are also a special case, as the source water may be either seawater or from the potable water supply for the ship. The pool configurations should be of a safe design, as with land pools. However, the hydraulic and circulation system of the pool will necessitate a unique design, depending upon ship size and pool location. The filtration and disinfection systems will also require adaptation to the water quality.

2.Types of users

The type of pool reflects the users, which may include:

• the general public;
• children/babies in small teaching groups;
• hotel/motel guests;
• tourists on board cruise ships;
• health club members;
• medical patients in therapy pools;
• competitive swimmers;
• clients of outdoor camping parks;
• leisure bathers, including clients of theme parks; and
• specialist sporting users, including scuba divers and water polo participants.

Certain groups of users may be more predisposed to hazards than others. Children, for example, particularly when unattended, may cause an elevated risk of accidents for themselves and others because of their uncontrolled physical competitive spirit and desire for attention. They are also generally reluctant to observe formal rules of safety and hygiene. In addition, they generally play longer in recreational waters and are more likely to intentionally or accidentally swallow water

3. Hazard and risk

Popularly, the terms hazard and risk are used interchangeably. Correctly, a hazard is a set of circumstances that could lead to harm — harm being injury, illness or loss of life. The risk of such an event is defined as the probability that it will occur as a result of exposure to a defined quantum of hazard. In simpler terms, hazard is the potential for harm, while risk is the chance that harm will actually occur. The rate of incidence or attack rate is the expected number of events that occur for this defined quantum of hazard. Strictly speaking, probabilities and rates obey different laws; however, if the probabilities are small and the events are independent, the two values will be approximately equal.

 Types of hazard encountered

The most prominent adverse health outcomes arising from the use of swimming pools, spas and similar recreational-water environments are:

• drowning and near-drowning;
• major impact injuries (e.g., spinal injury);
• slip, trip and fall accidents;
• cuts, lesions and punctures;
• infection arising from inhalation of, ingestion of or contact with pathogenic bacteria, viruses, protozoa and fungi that may be present in water as a result of faecal contamination, carried by participants using the water or naturally present; and
• adverse effects relating to toxic chemicals, with exposure arising from inhalation, ingestion and dermal exposure.

 

Data on risk related to the use of swimming pools, spas and similar recreational-water environments take four main forms:

• national and regional statistics of illness and deaths;
• clinical surveillance of the incidence of illness and outbreaks;
• epidemiological studies and surveys; and
• accident and injury records held by swimming pool owners/managers and local authorities.

Although "incident records" held by local pools and authoritative bodies are often comprehensive, published statistics are seldom sufficiently detailed for risk assessment. Surveillance is the process of continuous and vigilant assessment of the state of public health and of safety and acceptability. Processes for surveillance of drinking-water supplies have been recommended by WHO (1976, 1997) and involve the dual responsibility of a national, governmental regulator and the supplier or provider of the service. Systems for surveillance of public health operate in most countries.

There are other reasons why it is difficult to estimate risk directly, such as the following:

• In most active water sports, enjoyment arises from the use of skill to avoid and overcome perceived hazards. The degree of competence of participants and the use of properly designed equipment, accompanied by supervision and training, will considerably modify the risk.
• Risks of acquiring infectious disease will be considerably influenced by innate and acquired immunity (for examples, see Gerba et al., 1996). The former comprises a wide range of biological and environmental factors (age, sex, nutrition, socioeconomic and geographic), as well as body defences (impregnability of the skin, lysozyme secretion in tears, mucus and sweat, the digestive tract and phagocytosis). Previous challenge by pathogens often results in transient or long-lasting immunity. Immunocompromised individuals will be at greater risk of acquiring infectious diseases.
• Assessment of harm itself and the degree of harm suffered depends upon judgement at the time. Medical certification of injury and of physiological illness and infection, accompanied by clinical diagnosis, is the most reliable information. Information obtained by survey or questionnaire will contain a variable degree of uncertainty caused by the subjects' understanding of the questions, their memory of the events and any personal bias of the subject and interviewer. Survey information is only as good as the care that has gone into its design and conduct.
• The causes of harm must be ascertained as far as possible at the time. There are considerable difficulties in determining causes in the cases of low-level exposures to chemical and physical agents that have a cumulative or threshold effect and of infectious diseases caused by those pathogens that have more than one route of infection or have a long period of incubation. For example, gastroenteric infections at swimming pools may result from person-to-person contact or faulty food hygiene in catering, as well as from ingesting excreta-contaminated water.
• Where data are in the form of published regional or national statistics giving attack rates, the exact basis on which the data are collected and classified must be ascertained. For example, national statistics on deaths by drowning will usually include suicides, occupational accidents (e.g., lifeguards) and misadventure in recreation.
• It cannot be assumed that risk is directly proportional to exposure or that risks from multiple exposures or a combination of different factors will combine additively.

 The use of epidemiology in assessing risk

Epidemiology is a discipline designed to reduce sources of bias and errors in the interpretation of statistical and other data for assessing risks. However, epidemiological studies are usually limited to single or a few closely related hazards and carefully defined populations. Hence, epidemiological approaches do not always measure the full range of variation in population responses (Grassman, 1996). The results of epidemiological studies, usually presented in the form of relative risks or odds ratios (see Box 1.1), can therefore be applied only to activities and subjects similar to those studied. There is only a little epidemiological information available concerning hazards associated with swimming pools and spas.

Box 1.1: Definitions of relative risk and odds ratio Controlled epidemiological studies involve a comparison of attack rates between the experimental group exposed to the hazard and an unexposed control group, carefully selected to be otherwise as identical to the experimental group as possible. The objects of such studies take the form of statistical examination to disprove the null hypothesis that there is no significant difference in the outcome between the two groups. The results are therefore usually presented in the form of relative risk (risk of outcome in the exposed group / risk of outcome in the control group) or the odds ratio (odds of outcome in the exposed group / odds of outcome in the control group), together with a statement of the level of statistical significance (the probability that the stated result could have occurred by chance). By way of definition, if the baseline rate of illness unrelated to exposure is r (the fraction of the control group who become ill) and exposure to the hazard studied increases it by a factor b, the rate observed in the exposed group is obviously br, and the relative risk is br/b = r. The odds ratio is defined as [br/(1 - br)]/[r/(1 - r)], or the ratio of ill to well exposed subjects divided by the ratio of ill to well control subjects. The odds ratio is larger than the relative risk, but the differences are small when the direct risks are 1% or less. Odds ratios are readily calculated in the analytical procedure known as logistic regression analysis, which is commonly used to analyse the effects of different factors on illness in large, multivariate epidemiological studies. Relative risk has no real meaning in retrospective case–control studies of outbreaks, where the number of well, but exposed, subjects is an unknown fraction of the total population who were exposed to the hazard, and the odds ratio is therefore given instead.

 The use of quantitative microbiological risk assessment in assessing risk

Quantitative microbiological risk assessment (QMRA) can play a useful role in assessing the risk of infection from the use of recreational water. In its simplest form, it consists of four steps:

• hazard assessment;
• exposure assessment;
• dose–response analysis; and
• risk characterization.

In terms of recreational water, the hazard assessment relates to the microorganism of concern, Cryptosporidium, for example. The exposure assessment determines the likely dose (or range of doses) of microorganisms received by the water user and is a function of the level of microbiological contamination and amount of water ingested. Information on microbiological contamination is usually derived from the results of routine monitoring. Few data exist on the amount of water ingested during a "typical" recreational-water exposure, so a standard default value of 100 ml per day is generally used (Haas, 1983). The dose–response analysis step relates the dose of microorganisms received to the likely infection. These data are usually determined from limited healthy adult volunteer feeding experiments and then extrapolated, using a mathematical model, to the low doses typically experienced from recreational waters. Human dose–response data are available for a number of microorganisms, including Cryptosporidium parvum, which has documented swimming pool transmission (Lemmon et al., 1996; see also chapter 3). The risk characterization step combines the information on exposure and dose–response and results in an overall estimation of the likelihood of infection, which is often expressed as an annual risk of infection of, say, 1/10 000.

The results of QMRA can provide useful guidance, but a number of factors must be borne in mind. Monitoring data may present only a "snapshot" picture of the true situation and, depending upon how the monitoring is conducted, may not be representative. Data are lacking on the level of ingestion of recreational water, and levels may vary significantly between different groups of users. The dose–response relationships may be derived from single experiments involving a small number of healthy adult volunteers; the results of these experiments may not be appropriate for all user groups. In its simplest form, QMRA does not account for issues such as secondary infection, although more sophisticated techniques, employing epidemiologically based models, are being pioneered (Eisenberg et al., 1996).

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