Tap Water or Bottled Water: Minerals, Microplastics and What to Actually Choose

Tap water is water treated by a waterworks from a surface or underground source and delivered by pipes to the home. Its composition depends on place: in some regions it is hard (rich in calcium and magnesium), in others soft. Quality depends on the water supplier and also on the plumbing in the building, which can affect the final composition. Local water suppliers usually publish information about water parameters, so it is worth checking for your address.

Bottled water is not one product. Waters of very different mineralisation are sold under that heading, from almost mineral-free to highly mineralised, in plastic (usually PET) or glass containers. A trade name such as ‘mineral’ or ‘spring’ does not by itself tell you how much calcium, magnesium or sodium it contains; the composition table on the label does. A review by Garzon and Eisenberg compared the composition of commercial bottled waters from North America and Europe: magnesium ranged from 0 to 126 mg/l, sodium from 0 to 1200 mg/l and calcium from 0 to 546 mg/l.

The practical conclusion is that comparing ‘tap’ and ‘bottled’ as two uniform categories makes no sense. What does make sense is comparing the specific tap water where you live with the specific bottled water you are considering, using the same composition table.

For most people water is a small but real source of minerals, especially calcium and magnesium, which occur in water in ionic form. It does not replace the diet, but in people who drink a lot of water (for example 1.5-2 litres a day), water high in magnesium or calcium can supply a noticeable part of the daily requirement. Conversely, waters high in sodium can supply more of it than ordinary water; in the Garzon and Eisenberg review there were waters with from several hundred to over a thousand mg of sodium per litre.

Mineralisation affects who should choose carefully. People with hypertension or heart failure should pay attention to sodium. People prone to kidney stones should discuss their choice of water with a doctor, because water composition (calcium, bicarbonate, magnesium) affects the composition of urine and hydration is key; we write about this in the article on hydration and kidney stones. People with kidney disease often have fluid and electrolyte recommendations that must be agreed with a nephrologist. We cover the basics of electrolytes in the entry on sodium, potassium and chlorides.

One of the most studied questions is the link between water hardness (calcium and magnesium content) and heart disease. A 2016 meta-analysis of ten studies and over 77,000 deaths from coronary heart disease found an inverse relationship between magnesium level in drinking water and coronary mortality (RR 0.89; 95% CI 0.79-0.99), with high heterogeneity of results; in European studies the effect was somewhat stronger (RR 0.83; 95% CI 0.69-0.98).

A similar picture comes from a 2023 systematic review by Polish authors from Poznań: of 25 studies analysed qualitatively, 17 pointed to a significant relationship between water hardness and protection from cardiovascular mortality, and the quantitative analysis indicated a significant reduction in mortality with high hardness, but with high heterogeneity and many confounders. The authors noted that further research is needed.

The most-cited analysis is the 2019 paper by Cox et al., which pooled 402 data points from 26 studies (over 3600 processed samples) and estimated how many microplastic particles enter Americans’ bodies through food, water and air. One of its calculations concerns water: a person meeting the recommended water intake solely from bottles may swallow about 90,000 additional microplastic particles a year, compared with about 4,000 for a person drinking only tap water.

These numbers are estimates aggregated from different studies, not measurements in particular people, and methods of detecting microplastics differ between laboratories (size of the smallest particle detected, technique). It is also worth knowing a second study that complicates the picture. Oßmann et al. examined 32 samples of bottled mineral water by micro-Raman spectroscopy (smallest analysed particle 1 μm) and found microplastics in waters from all types of packaging, including glass bottles.

What follows? First, a glass bottle does not guarantee water free of microplastics. Second, the spread of results within a single study is huge, so ‘one bottle’ can have many times more or fewer particles than the average. Third, such measurements concern the number of particles, not their toxicity. We write more broadly about what is known on microplastics in the body in the article on whether we should worry about them.

The question of harm is open. In 2019 the World Health Organization published a report on microplastics in drinking-water that critically examines the evidence on the occurrence of microplastics in the water cycle (including tap and bottled water), the potential health impacts of exposure, and the removal of microplastics during treatment. The report makes recommendations on monitoring and managing microplastics and plastics in the environment and on better assessing risks to human health, and it identifies key knowledge gaps; the accompanying WHO release called for more research.

For a healthy adult living where tap water has good quality and taste, tap water is a reasonable, cheap default; on microplastics the estimates are more favourable to it, and mineral needs are met by the diet. Bottled water makes sense when, for some reason, you do not trust the local water, when you cannot use it (travel, poor network quality), or when you deliberately choose a water of a specific mineral composition.

This article does not assess water filters, which change composition and need a separate analysis (type of filter, maintenance, effect on minerals). If you are considering a filter, check exactly what it removes and what it leaves, rather than assuming that filtered water is by definition better or worse.

The ‘tap versus bottled’ dispute is usually badly framed. There is no universal winner, because both tap and bottled water are broad sets of products of differing composition. For a healthy person with good local water, tap is a cheap and sensible choice, and the estimates of microplastic exposure favour it if anything. For people with hypertension, kidney stones or kidney disease, composition counts and is worth checking on the label and in the water supplier’s report.