This article was first published on 22 December 2009 and updated on 27 November 2024

• Waters available on the market are classified according to their fixed residue.
• The WHO sets no reference health values for fixed residue.
• Based on the content of individual mineral salts, you can choose the water most suited to your needs.

The fixed residue is a classification parameter for water intended for human consumption: it indicates the content of solids (mineral salts) dissolved in water after evaporation at 180°. The measurement is expressed in milligrams per litre and is shown on the label. Water with a high residue is richer in mineral salts than water with a low residue.

How commercially available water is classified according to fixed residue

Based on their fixed residue, commercially available waters are classified as minimally mineralised, oligomineral, medium-mineral or salt-rich. Here are the reference values:

• Minimally mineralised, with the lowest quantity of dissolved salts, where the fixed residue does not exceed 50 mg/litre;
• Oligomineral, with a fixed residue below 500 mg/litre;
• Medium-mineral, where the fixed residue ranges between 500 and 1,500 mg/litre;
• Salt-rich, with a fixed residue above 1,500 mg/litre, preferably consumed under medical supervision as it may have therapeutic properties.

Among the elements most commonly making up the residue are calcium, phosphates, nitrates, sodium, potassium, magnesium and chloride. Depending on the quantities in which these are present, water can also be classified, for example, as sulphated, calcic, sodic (with more than 200 mg/l of sodium), or "suitable for low-sodium diets" (with less than 20 mg/l of sodium).

Is it better to have water with a high or low fixed residue?

The fixed residue value does not indicate that one water is healthier than another, as water marketing can sometimes imply. Just a few weeks ago, the monitoring committee of the Institute of Advertising Self-Regulation (Iap) banned the use of a slogan by a well-known water brand, deeming it misleading: the slogan associated the "lightness" of the water — due to its low mineral salt content — with wellbeing, whereas the committee stressed that this "lightness" does not represent a real benefit for everyday wellbeing for all consumers.

What the WHO says about fixed residue

The WHO states that no reliable data are available on the possible health effects associated with fixed residue in water, and no guideline value is proposed for consumption. In any case, prolonged consumption of minimally mineralised or salt-rich water should be monitored by a doctor.

A water for every age

Depending on one's stage of life, state of health or physical condition, there are waters with recommended characteristics. According to the Fondazione Umberto Veronesi, children are advised to drink medium-mineral water, rich in calcium, magnesium and fluoride. For adolescents, medium-mineral bicarbonate-calcic and magnesian water is recommended; for adults, oligomineral and medium-mineral water depending on lifestyle; for pregnant women, calcic water; the same applies to menopausal women; calcic and magnesium-sulphate water is recommended for the elderly. Those who suffer from hypertension should take particular care to avoid waters high in sodium.

What to look for on a mineral water label

When choosing water, it is worth paying attention to the sulphate content as well (400 mg/l is the limit for drinking water), as high concentrations could cause laxative effects and gastrointestinal irritation. An excessive content of nitrates (more than 10 mg/l) can also cause problems, particularly for infants or pregnant women, and can indirectly increase the risk of stomach and intestinal cancer.