Home water treatment systems are often promoted on the basis of the purported health (rather than aesthetic) benefits of using them. This is particularly in relation to urban drinking water given the full treatment — coagulation, sedimentation, filtration and disinfection — where such claims usually constitute misleading advertising. In this review I will focus on a number of misconceptions about the health benefits of water treatment, examining each assertion in its wider context. The ensuing discussion applies less to rural water supplies, where valid reasons often exist for use of treatments — eg removing nitrate or protecting against giardia.

Recently it has been emphasised that water purification systems require a degree of periodic maintenance which the modern householder is not very good at delivering. Over time, poorly maintained systems can become health hazards in their own right, as bacteria start to flourish, and contaminants previously retained may leach back into the water. This issue has already been exposed to the full glare of the New Zealand media’s 40-watt bulb, and I will side-step it by assuming (in what follows) that the treatment systems involved always function according to ideal specifications.

Fallacy No. 1: Filtered water is Pure

There is a range of home water-purification systems available on the market, each good at a particular job (eg, filters based on activated carbon are good at removing small organic compounds but poor at removing heavy metals; by contrast, cation exchange resins will remove metals, but allow most organic compounds to pass through). In addition, no system could be regarded as completely efficient, even at jobs it does well. Therefore, the assertion that use of a given system will result in water being transformed from “dirty” to “clean” is just another of those oversimplifications which dwell in ad-world.

In a cosmos without hyperbole, a typical advertising slogan would read more like this:

“Buy the ZX500 water-filter. It makes dirty water somewhat less dirty.”

Cleaner water is required at a number of institutions around New Zealand, for applications as diverse as culturing bacteria and undertaking trace-level chemical analyses. The capital outlay required for a single system which delivers reasonably “clean” water (ie. for research) is sufficient to buy 30-40 typical home water treatment systems.

Another point is even the “cleanest” natural systems can be found to contain most of the elements in the Periodic Table, given sufficiently sensitive analytical instrumentation.

Fallacy No. 2: Drinking Water Should be Pure

I was amused to find an advert for home water filters which stated:

“Our bodies are made up of some 65 per cent water, so the water we drink needs to be pure and free of chemicals.”

Apart from the obvious point that water is a chemical, critical readers will notice the assertion made in the second half of the sentence doesn’t actually follow from the opening statement. For instance, using similar abstruse reasoning techniques, it could be argued because our bodies are about 18% carbon, all the food we eat should be cooked until thoroughly black. Toast should only be eaten after flames have been seen to emerge from the toaster, and steaks grilled until they resemble small lumps of coal.

In reality, even the “major” nutrient constituents of normal drinking water (sodium, calcium, potassium, and magnesium) are present at concentrations many times lower than levels required by our bodies. Swallowing normal drinking water already acts in the direction of diluting what’s already there, and your body adjusts the amounts of the major ions excreted to maintain a pre-set homeostatic balance.

However, if a person drinks enough water (8-9 litres) within a short-enough period, homeostatic mechanisms can be overcome, and dilution can occur to such an extent that cells all over the body simultaneously experience osmotic shock, resulting in death. The toxic dose of pure water is actually lower than the toxic dose of normal drinking water, because pure water leads to more effective dilution.

Fallacy No. 3: Aluminium in Drinking Water will give you Alzheimer’s Disease

Aluminium does show a correlation with Alzheimer’s disease, just as the reduction in the number of storks sighted in Copenhagen over the years correlates with the declining birth-rate; yet neither are enough evidence to ascribe cause. However, I can leave aside the question of whether or not aluminium is implicated in the pathogenesis of Alzheimer’s disease, because it’s not actually relevant to the issue of water treatment.

Assume for the sake of argument that aluminium is involved.

If you drink 2.5 litres of water a day, your daily intake of aluminium from drinking water (city supply) will be about 68 millionths of a gram. This corresponds to one gram swallowed in 40 years of hard drinking, and represents only 0.07-0.7% of your daily intake from food sources (typically 10-100 thousandths of a gram per day). Thus, even if you have a perfect filter which removes all the aluminium from your drinking water, you are still only affecting the last 1% of your daily intake. It is stretching things a little to suggest it just happens to be this final 1% which is critical in causing a person to develop Alzheimer’s disease.

Fallacy No. 4: Filtering heavy metals from drinking water will significantly reduce daily intakes

The error in this idea once again becomes obvious when exposure via drinking water is set in its proper context, which is total exposure from all sources (food, water, air, cosmetics, etc). For most people, the main exposure route is eating, and the main source is food. This low-level background intake of heavy metals — and nutrient metals also — is a simple consequence of their natural presence in soil and the wider environment. “Clean” soil contains about 0.2 parts per million (ppm) cadmium and mercury, 15 ppm lead, 30 ppm copper, 60 ppm zinc, and over 50,000 ppm iron. Plants take a portion of each of these up, and they enter the food chain, eventually making it to your breakfast.

Your average daily intake of the heavy metal lead from all sources (mainly food) is about 220 millionths of a gram. Your daily intake from water is responsible for no more than 1% of this. It seems unreasonable to assert that anyone on a city supply will derive significant health benefits by reducing the amount of lead in their drinking water. Figures for other heavy metals are similar: drinking water is responsible for only 3% and 0.7% of your daily mercury and cadmium intakes, respectively. It accounts for less than 1% of daily intake of copper, manganese, iron, zinc, bismuth, and selenium (five of which are essential elements), and less than 10% of daily intake of silver, molybdenum, nickel, gallium, chromium, beryllium, antimony and thallium (four of which are essential anyway).

Fallacy No. 5: Filtering organic compounds from drinking water will significantly reduce daily pesticide intakes

Some 49 natural pesticides have been identified in cabbage, and many of these are indistinguishable in their action from synthetic (“man-made”) pesticides. In some cases, natural and synthetic are the same thing — the compound known as 1080 naturally occurs in some Australian plants. It’s estimated we eat about 1.5 grams of natural pesticides daily, a figure 10,000 times higher than synthetic pesticide residue intake. Levels of synthetic pesticide residues in city water are extremely low, if present at all. Even if drinking water accounted for a generous 10% of synthetic pesticide intake, this represents 0.001% of total pesticide exposure.

Fallacy No. 6: “Chlorine” in drinking water is both unnecessary and bad for health

With a pH value of 2, the hydrochloric acid in your stomach already has a concentration of chlorine about 1025 times greater than average levels in treated water.

This comparison may represent an over-simplification, because the active ingredient in chlorinated water is really something called the hypochlorite ion; however, very few toxic (rather than aesthetic) effects have been associated with water containing high chlorine concentrations, despite it having been used in disinfection since 1896, where chloride-of-lime stopped a typhoid epidemic. Indeed, the general modern lack of typhoid epidemics and other deadly water-borne diseases in developed countries is testimony to the effectiveness of ongoing chlorine disinfection.

The only health issues currently considered to be realistic with regard to chlorination are (a) reports of asthma being triggered in susceptible individuals on exposure to highly chlorinated waters, and (b) generation of chlorinated organic compounds during treatment itself, which is a facet that is therefore routinely monitored in city supplies (also, see point 5).


Home-water treatment systems can be used to effectuate pleasing aesthetic improvements — removal of the chlorine’ taste — and it’s reasonable to promote such systems on this basis. However, overall, for municipal water supplies, it’s not valid to argue genuine health benefits will ensue as a result of removing selected chemical constituents from drinking water. Removal of worries and stress caused by the advertising itself could be regarded as a health benefit, but it is clearly one which is generated artificially. One wonders about the ethics of creating consumer need like this.

Recommended Posts