Is low TDS actually good for you?

Walk into any water purifier showroom in India and someone will hold up a TDS meter. Tap water reads 300. Purified water reads 20. The lower number wins, and the sale closes.

It is a compelling demonstration. It is also measuring the wrong thing.

What TDS actually measures

TDS stands for Total Dissolved Solids. It is a single number for everything dissolved in your water, expressed in milligrams per litre. Everything. Calcium is dissolved solids. So is magnesium, potassium and bicarbonate. So is lead, arsenic and nitrate.

A TDS meter cannot tell you which is which. It passes a small current through the water and infers the total quantity of dissolved material from how easily that current travels. It is a headcount, not a guest list.

The distilled water problem

The cleanest illustration is distilled water. It has a TDS reading close to zero — the best possible score by the showroom logic. It is used in laboratories, in steam irons and in car batteries.

It is not sold for drinking. Water that pure is an aggressive solvent and offers the body nothing at all. Nobody markets it as the healthiest water available, because a score of zero is not an achievement. It is an absence.

So what should you ask instead?

The useful question is not how low the number is. It is what is left behind it.

• Have the contaminants been removed — heavy metals, nitrates, microbes?
• Have the beneficial minerals survived — calcium, magnesium, bicarbonates?
• Is the pH balanced, or has the water been left slightly acidic?

A purifier that answers yes to the first and no to the other two has done half a job well and the other half backwards. It has removed everything because it could not tell the difference.

The short version

Stop asking how low your TDS is. Start asking what is left in your water. Those are different questions, and only one of them has anything to do with your health.