YOU feel hot and sticky and your throat is dry. You ran for a bus and missed it. You took ages to find your keys. You yearn, more than anything, for a long glass of cool, clear water. As soon as you reach the kitchen you turn on the tap . . .

Or do you? Perhaps you are one of the thousands of people who have a vague, unspecified mistrust of the liquid that comes out of your tap. Instead you spend pounds every week buying bottled water - Scottish, English, French, or Italian, reassuringly pristine and nicely labelled.

But isn't it just possible that your prejudice against tap water is entirely unfounded? Millions more Scots, after all, drink the stuff, reassured by that fact that, generally speaking, our water comes from reservoirs, lochs, and streams - which is more than can be said for parts of England.

''The Thames is said to be drunk seven times over every year,'' says laboratory manager Helen Keenan at Strathclyde University's department of environmental health. ''I drink Scottish tap water even though I spend a lot of time looking at what's really in it.''

The ''what's in it'' that upsets people most includes ''wee black bits'' and cloudiness, says Dr Tony Grimason, lecturer in the department. ''The former are usually iron and manganese which have been missed in the aeration process, and the second is because there has, perhaps, been a break

in the water supply and the

water authority has added more chlorine for safety. But neither is health-threatening.''

It's what you can't see that could be more worrying.

Dr Grimason points out that some of the research whose conclusions are used as mean levels of safety is 30 years old, therefore constant updating is vital - and Strathclyde University is recognised as a top testing centre.

From Tiree to Dumfries, West of Scotland Water has 179 water treatment works, 231 water pumping stations, 524 reservoirs, 176 waste treatment works, 404 waste water pumping stations, and 17,300 kilometres of water mains to manage. It would be a miracle if every drop of drinking water always came out 100% perfect.

All the same it claims that the water it supplies not only meets the standards set up by Parliament but that these standards in any case exceed those set by the European Union directive on drinking water.

''It is a criminal offence to supply water unfit for human consumption,'' says Grimason. ''The legal requirement is for wholesome water at time of supply.'' But so many factors - from unusual discharges into treated water to heavy rainfall causing torpidity - can affect our water that the process of getting it to us safely is a constant balancing act.

In the 1970s scientists were pondering the possible association of nitrates in the water being associated with the ''blue baby'' syndrome. Much later, at Camelford, the discharge of aluminium sulphate into the water caused ulceration and other health problems.

One of Grimason's special areas of interest is waterborne cryptosporidiosis. This is the kind of contamination that can cause suppliers to shudder - a protozoan parasite which twice in the last 10 years has caused outbreaks of diarrhoea and enteritis (the first in Ayrshire, the second in Oxfordshire). Following these outbreaks a national workshop was set up in Scotland, as was an expert committee to prevent future disasters.

It remains a concern, but the cost of testing for the parasite can be up to #200 per sample compared with up to #35 for normal faecal indicator organisms.

Cost of testing for all the real and likely - not to mention the merely distantly possible - sources of contamination cannot be ignored. The World Health Organisation recently published the most comprehensive review ever produced on the subject, with input from over 200 scientists worldwide and with recommended guidelines for about 100

different parameters, including substances which were unknown a few decades ago.

If a UK water company adhered rigidly to them all, it could cost millions of pounds extra per annum. A line has to be drawn somewhere - and we who turn on our taps just have to pray the line is in the right place.

Keeping tabs on our water is no easy task. In the distant past, poisonous metals were all analysts had to look for. Now they have to consider (however unlikely it may be to find them) naturally occurring inorganics, industrial contaminants, agricultural contaminants, the by-products of treatment and distribution (including polymers and plasticisers), and the stuff that comes off some home plumbing.

A 1996 study of possible health problems from lead in tap water was concerned with lead exposure to infants either through maternal blood or through bottle feeds.

An estimated 13% of infants were exposed via bottle feeds to tap water lead contamination exceeding the WHO guidelines, although the mothers' blood lead concentrations were found to be safe.

The conclusions were that although lead in Glasgow water (and Glasgow is almost alone in having this problem) is about a third of what it was in 1981 it is still a public health problem for babies who are bottle-fed.

If you have old lead pipes and you are pregnant you should use bottled water and you should consider breast-feeding for your baby's safety.

Even more important, you should try to get the pipes replaced as soon as possible. Check with your local council for the possibility of grants and, if your landlord or your factor is at fault, keep hounding him until he does something.

All the same, Ernest Chambers, chief executive of West of Scotland Water, is of the opinion that the quality of water in Scotland has never been higher.

''This statement is made against a backcloth where our knowledge of the quality of water being supplied, both in extent of sampling and breadth and accuracy of analysis, has never been higher,'' he says. The treatment that encourages this confidence starts with screening for leaves and debris and then goes through

coagulation, clarification,

filtration, adjustment of water quality (pH correction), and disinfection (chlorination).

The regular and random tests - some of them conducted hourly - include those for E-coli, aluminium, lead, nitrates, and a host of other substances.

Vast amounts of scientific know-how, within the water authority itself, in Strathclyde and other universities, and in the

public health sector go into ensuring that standards get higher

and higher.

But so too does public awareness. People notice more and complain more and regulation gets tougher. In 1995 a company which supplied burn water through an illegal connection in Freuchie in Fife was fined #60,000.

Lothian Regional Council also got into hot water, if you'll pardon the pun, for failing to prevent unchlorinated raw water from entering the supply in West Lothian in the same year.

In fact, our water is pretty damned good. But if you don't drink yours for any specific reason, or think it looks or tastes suspicious, let us know. The West of Scotland Water Authority has promised to follow up any complaints.

In fact, since the debacle of the diesel contamination earlier this year, it is more than anxious to foster good public relations.

''We supply 90 million gallons a day to Glasgow from Loch Katrine,'' it points out. In fact only 2% of that is used for drinking and cooking. The rest is for toilet flushing, bathing, washing clothes, cleaning cars, and all the other things we do with water, while taking it completely for granted.

Property owners are responsible for keeping plumbing, tanks, and pipes in good repair.

There have been enough scares in recent years - some of them in water authority areas far from ours - to make people jumpy about turning on the tap. Personally, having seen the desperate lack of potable water in West Africa, and the joy of those (including myself) who finally got connected to a tap instead of a bucket and a well, I consider myself very lucky indeed to be able to drink Loch Katrine water.