Water is the building block of our very existence. The average human body consists of 60 to 70 per cent water, and without it, we die in five, possibly even three days.

'It plays a vital part in digestion, circulation, tissue building, maintaining temperature and flushing toxins and body wastes,' says dietitian Mimi Sham. 'Water is also an important lubricant, helping to cushion joints and vital organs and keeping eyes, lungs and air passages moist and healthy.'

Healthy living begins with drinking adequate amounts of good quality water. Health professionals recommend eight glasses containing 240ml each day, and more in hot weather, or during exercise.

'On a temperate day, without any physical exertion, the body loses about half a litre sweating,' says Walker. 'But on a hot day in Hong Kong you can lose three to five litres.' Some of this fluid loss is replaced by water in our food and by other drinks. Nonetheless, most of us are permanently parched, with the average person drinking just three glasses of water a day.

'Humans do not have a very good instinct with water, so we don't naturally know how much is enough,' says Sham.

Walker says: 'People who are used to less water lose their thirst impulse, so that by the time they feel thirsty, it usually means they are already dehydrated.'

Rather than relying on your body to tell you, keep an eye on the colour of your urine. 'When you are under-hydrated, water is reabsorbed from the urine in the kidney tubes, so it becomes darker,' says Walker. 'Ideally, it should be light straw-coloured.'

While mild dehydration isn't lethal, if someone loses two per cent of their lean mass of water, they'll suffer a measurable drop in mental and physical performance. And when people start drinking enough, they often see unexpected improvements in their overall health and well-being.

'Many lose weight, firstly because their metabolism improves, and secondly because they eat less with the stomach full of water,' says Walker. 'They sleep better and their skin improves.'

Unfortunately, the answer for dried-out Hongkongers may not be as simple as turning on the tap. Although any water is arguably better than none, what comes out may be laced with so many toxic chemicals that the benefits are diluted by the risks.

In independent tests, samples of Hong Kong tap water were found to be up to three times more contaminated than other developed countries. The total dissolved solids were around 90ppm (parts per million), compared with less than 20ppm for Australia and America, and readings of more than 300ppm from undeveloped countries.

Dozens of different contaminants are permitted in drinking water, including E.coli bacteria, disinfectants such as bleach, toxic industrial waste such as cadmium, cyanide, and mercury, cancer-causing herbicides and pesticides and products of the pipe system such as lead and copper.

The World Health Organisation sets permitted levels of these contaminants, below which they are not believed to pose a significant risk to health.

While the Water Supplies Department is careful to ensure our drinking water meets these standards when it leaves treatment plants, by the time it reaches our taps, this may not always be the case.

'Only 30 per cent of Hong Kong's water comes from natural local supplies, and the rest is imported from Guangdong province,' says Professor Ho Kin-chung, a water expert from the Open University of Hong Kong. 'We pump in water from the East River, which is lined with housing developments and contaminated by industry.'

Hi-tech water treatment systems help to make this unacceptable water acceptable. 'When the water leaves the treatment plants it is very high quality, much higher than in many other countries,' says Ho.

But further down the pipeline, it's a different story. 'There is 15 per cent leakage from the pipes, and cracked pipes means contaminants can leak in from the soil,' says Walker.

Ho says: 'In Hong Kong there is new construction work going on all the time, and pipes underground often get damaged, allowing more contaminants to get in.'

Old building infrastructures are also to blame. 'High rises mean more pipes and more potential for contamination,' the professor says. 'Many pipes are 30 to 50 years old, and some are even pre-war, so may leach metals such as lead, and be prone to cracking.

'Newer buildings are better because the Water Supplies Department imposed the requirement in 1995 that pipes should be made of a new flexible, non-toxic carbon material. Buildings over six storeys also need water tanks, and although the Water Supplies Department says they should be cleaned, not everyone complies, so the tanks may get dirty and contaminate the water further.'

Boiling water is a traditional remedy that is probably slightly better than doing nothing at all. 'If you boil the water then let it settle for a while, then some contaminants will sink to the bottom,' says Ho.

An alternative is bottled water, although this may not always be the healthy solution the price would suggest.

'There have been tests on bottled water that showed the microbial content was higher than previously suspected,' says Walker. 'It may also have chemicals added so that the composition is unhealthy.'

And not all brands are equally good. 'Is it natural spring water, or has it been filled with tap water and called natural spring water? Just because it is well-marketed doesn't mean you're getting the best quality,' says Walker. 'Well-known, global brands are probably a good bet because they need to be of a certain reproducible quality, as their reputation depends on it.'

In hot weather, distilled water may actually be unhelpful. 'Mineralised or spring water is best because you need to replace the minerals you lose through sweat and every time you go to the toilet,' says Walker.

Investing in a water filter may be another option. Simple jug filters can remove some of the chlorine, whereas reverse osmosis and granular activated carbon filters fitted to your water system are more expensive but more effective. 'A good water filter is a good investment, because you are not going to cook in bottled water, or shower in it,' says Walker.

But if you do have a filter fitted to your tap it's important to change the filter cartridge regularly. 'The lifespan is two to three months, but some people use theirs for half a year,' says Ho. 'The contaminants can colonise on top of the filter and make the water even dirtier than it was before.'