Why good drinking water is important

Drinking water is essential for human living. Drinking water needs to be safe, wholesome and clean and acceptable to consumers. Existing regulation aims to protect the public against unsafe drinking water. Considerable effort is being made by government, the water industry, WHO, consumer groups etc to safeguard and, where possible, improve drinking water quality.

In Europe the new Council Directive 98/83/EC on the quality of water intended for human consumption provides a sound basis for both the consumers throughout the EU and the suppliers of drinking water and provides guidelines, but many challenges lay ahead as a result of (expected) shortage of water supply in certain regions, increasing water use, new scientific evidence in relation to the adverse health effects of conventional water treatment and existing pollutants in the water, and the effect of climate change.

Water treatment

There are a number of sources for drinking water such as ground water, upland or lowland surface water, re-used water or (desalinated) sea water. The water generally needs to be made fit for human consumption. Drinking water treatment techniques include disinfection and mechanical and membrane filtration.

Disinfection of water to eliminate disease-causing organisms is the most important step in the drinking water treatment process.
The process may include rapid or slow filtration, coagulation, flocculation, GAC and disinfection methods such as chlorination, ozonation, chloramination or use of chlorine dioxide. The process is generally highly effective, but disinfection methods may produce some disinfection by-products that have caused some concerns. However, the benefits of its use generally outweigh the risks.

Water ingestion and filtration

Ingestion of tap water and use of household filters differs between countries and affects the uptake of any contaminants in drinking water. Whereas in Northern European countries such the Netherlands and United Kingdom 60-70% of the people drink water straight from the tap, in the France, Italy or Spain very few people do. However, most people use tap water to make hot beverages such as tea and coffee. Household filter use varies within and between countries but generally the use of household filters is below around 20%.

Ingestion is not the only route of uptake of contaminants in water since uptake of for example trihalomethanes, a main group of disinfection by-products, may occur through skin absorption and inhalation during showering, bathing and swimming.

European regulations

In Europe, Council Directive 98/83/EC, the Drinking Water Directive (DWD), concerns the quality of water intended for human consumption. The objective of the Drinking Water Directive is to protect the health of the consumers in the European Union and to make sure the water is wholesome and clean (free of unacceptable taste, odour, colour) and that it has a pleasant appearance.

To make sure drinking water everywhere in the EU is indeed healthy, clean and tasty, the Drinking Water Directive sets standards for the most common substances (so-called parameters) that can be found in drinking water. In the DWD a total of 48 microbiological and chemical parameters must be monitored and tested regularly. In principle WHO guidelines for drinking water are used as a basis for the standards in the Drinking Water Directive.
While translating the Drinking Water Directive into their own national legislation (transposition of the DWD), the Member States of the European Union can include additional requirements e.g. regulate additional substances that are relevant within their territory or set higher standards. But Member States are not allowed to set lower standards as the level of protection of human health should be the same within the whole EU.

Member States have to monitor the quality of the drinking water supplied to their citizens and this has to be done mainly at the tap inside private and public premises. Also the quality of drinking water used in the food production industry has to be monitored to make sure it complies with the EU standards. Member States report at three yearly intervals the monitoring results to the European Commission.
The Commission assesses the results of water quality monitoring against the standards in the Drinking Water Directive. After each reporting cycle the Commission produces a synthesis report, which summarises the quality of drinking water and its improvement at a European level.

Reproductive and cancer effects of Disinfection by-products in epidemiological studies
Chlorination disinfection by-products (DBPs) are formed when water is chlorinated and the organic matter in the water reacts with chlorine to form these by-products. The formation and occurrence depends on many factors, including the chlorine dose, type of treatment, pH, temperature, residence time, bromine levels (Nieuwenhuijsen et al 2000a, IPCS 2000). Up to 500 different by-products have been identified (Richardson 1998). Different mixtures of by-product may exist in different locations depending on the various factors mentioned above, making it more difficult to assess any health effects of DBPs, particularly in epidemiological studies.

The health effects of DBPs in drinking water have been a concern since DBPs were first reported in the seventies (Rook, 1974). Early studies focused on cancer outcomes, while the more recent studies have focused on reproductive outcomes (IPCS 2000). According to the recent review by IPCS (2000): “more studies have considered bladder cancer than any other cancer. The authors of the most recently reported results for bladder cancer risks caution against a simple interpretation of the observed associations. The epidemiological evidence for an increased relative risk for bladder cancer is not consistent – different risks are reported for smokers and non-smokers, for men and women, and for low and high water consumption. Risk may differ among various geographic areas because the DBP mix may be different or because other water contaminants are also present. More comprehensive water quality data must be collected or simulated to improve exposure assessments for epidemiological studies.” A recent pooled analysis by Villanueva et al (2004), that provided quantitative information, confirmed this. For men there was an exposure response related relationship between DBP intake and bladder cancer, but there was no relationship in women.

Reproductive health outcomes should be easier to study from an exposure point of view, because of the shorter relevant exposure period. Amongst others, birth weight, prematurity, spontaneous abortion, congenital anomalies and still birth have been the focus of these studies. Various thorough reviews have been conducted and concluded that there are still many problems to overcome and that the results are inconsistent and inconclusive (Reif et al 1996, Nieuwenhuijsen et al 2000b, Gevecker Graves et al 2001, Bove et al 2002, IPCS 2000).

A number of studies found statistically significant positive associations between THMs and neural tube defects, one of the most studied group of congenital anomalies (Bove et al 1995, Klotz and Pyrch 1999, Dodds et al 2001), while others did not (Magnus et al 1999, Kallen et al 2000, Hwang et al 2002, Dodds et al 1999, Shaw et al 2003). Klotz and Pyrch (1999) found a statistically significant association between TTHM levels in the water and neural tube defects, but not with haloacetonitriles and haloacetates. Also, the effects were most pronounced in offspring from women that did not take supplementary vitamins, but these findings were not confirmed by the Shaw et al (2003) study. Inclusion of information on ingestion, showering, bathing and swimming made little difference to the risk estimates.

Hwang et al (2002) and Cedergren et al (2002) found significant associations between chlorinated water and levels of TTHM above 10 mg/l respectively and respiratory defects, but other studies did not find such an association (Magnus et al 1999, Kallen et al 2000, Bove et al 1995, Dodds et al 1999, Dodds et al 2001, Shaw et al 2003). Studies on chlorinated water and respiratory effects have been rare, but two studies found a significant positive association (Aschengrau et al 1993 and Hwang et 2002). Similarly, for urinary tract defects, although only three studies have been conducted they all reported statistically significant associations (Aschengrau et al 1993, Magnus et al 1999, Hwang et al 2002). Studies on oral cleft or cleft palate have largely been negative, except for the study by Bove et al (1995). In a meta-analysis Hwang et al (2003) reported evidence for an effect of exposure chlorination by-products on the risk of neural tube and urinary system defects, but results for respiratory system, major cardiac and oral cleft defects were heterogeneous and inconclusive.

Only a few studies have assessed the relationship between DBPs and spontaneous abortion. The California study has attracted the most attention since they found a statistically significant association between TTHM and BDCM and spontaneous abortion (Waller et al 1998). The effects were even stronger after re-analysis (Waller et al 2001)

A number of Canadian studies and one English found statistically positive associations between DBPs and stillbirth (Dodds et al 1999, King et al 2000, Dodds et al 2004, Toledano et al 2005). However the case control study by Dodds et al 2004 did not show a monotonic relationship between THM levels and stillbirth, and they did not find an association between HAAs and still birth (King et al 2005).

Studies on pre-term delivery have generally shown no association with DBPs, with the exception of the study by Yang et al (2000). Study results on low birth weight have been more mixed, with some studies reporting statistically significant associations (Kallen et al 2000, Bove et al 1995, Gallagher et al 1998) while others did not find any statistically significant associations (Kanitz et al 1996, Jaakkola et al 2001, Kramer et al 1992, Savitz et al 1995, Dodds et al 1999, Wright et al 2003, Toledano et al 2005). Studies on small for gestational age and/or intrauterine growth retardation showed some more consistent results, and a good proportion of them have found statistically significant associations (Kramer et al 1992, Bove et al 1995, Gallagher et al 1998, Wright et al 2003, Wright et al 2004). Wright et al (2004) found statistically significant associations with THMs and a measure of mutagenicity, but not with HAAs or MX. Infante-Rivard (2004) found that the association between THMs and intrauterine growth retardation was modified by a metabolic polymorphism, with newborns without the CYP2E1 (G1259C) variant at high risk.

In summary

Epidemiological studies on neural tube defects, urinary tract defects and small for gestation age/intra growth retardation have shown the most consistent statistically significant associations with an index of DBPs, but generally the risk estimates are small. The interpretation of the studies is not straight forward because they may not be directly comparable because of differences in DBP mixtures, exposure categories and actually uptake of DBPs due to differences in e.g. ingestion rates, showering, bathing, and swimming. Only few specific DBPs have been studied and THMs have often been used as a marker for other DBPs, since they are often routinely available. However they may not be well correlated with other DBPs and therefore not be a good marker. Sample sizes, and therefore power, have at times been low, particularly when the population was split into exposure categories. Although most studies considered some confounders, (residual) confounding by other water contaminants or other factors related to water intake, cannot always be excluded. Case ascertainment, for outcomes such as spontaneous abortion and certain congenital anomalies is far from straight forward, and for the latter at times anomalies are lumped together with different aetiology, which may be inappropriate. Furthermore, as with many reproductive epidemiological, if the putative agent affects both early pregnancy loss and later birth outcomes such as congenital anomalies, interpretation of later birth outcomes may be more difficult.