Which is safer, cold water from the upstairs or downstairs taps?
In theory, both should be equally safe! However, it’s not always as simple as that. (I’m limiting my answer to the case of mains water supply in the UK; things may well be different in other countries or with UK non-standard water supplies such as wells. I’m also making the working assumption that the mains supply itself is safe!)
In ye olden days when running water was first invented, it was often supplied just to one tap in the household, which would almost invariably be in the kitchen. But as households grew wealthier, the supply would be extended to further taps, and also to a hot-water storage tank. (Oh, the luxury!) Many older properties have a direct water supply of this kind.
Then the trend in house plumbing design changed, and it became much more common to have an indirect water supply, with the mains water pipe supplying a cold water tank in the loft, which in turn would provide cold water to the toilet cisterns and to all the other taps in the house, upstairs and down. Nevertheless, the kitchen tap would still be plumbed direct to the mains as it was thought better to have fresh drinking water – without any risk of contamination in the tank – available for cooking, washing food and, well, drinking!
In recent years the trend has switched back to direct plumbing. But if you have a cold water tank in the loft your water elsewhere may well still be safe to drink, as long as your tank is kept clean.
Under the water regulations, a storage tank has to be fitted with warning and overflow connections (to let you know when the tank was overfilling) which are designed in such a way that they prevent insects from getting into the tank – generally by means of screening. The lid of the tank must be close-fitting (though not airtight), must exclude light (to prevent algal growth) and insects, and must be made of a material which doesn’t contaminate any condensation which may form on it. The tank itself should be insulated to minimise the risk of freezing and heat gain (water is best stored below 20°C/68°F).
Kits to ensure that storage tanks comply with the regulations are commonly known in England and Wales as “Byelaw 30” kits, named for the relevant byelaw in the old water byelaws. (The legislation on water fittings is now known as Water Regulations in England & Wales (in full, the Water Supply (Water Fittings) Regulations 1999). In Scotland there are still Byelaws, updated in 2003, and the kits are known as “Byelaw 60” kits.
Anyway, to cut a long story short… As long as the mains supply itself is clean:
The kitchen tap will always be safe.
The rest of the house (upstairs and down) will be safe as long as either:
(a) the taps are on a direct water supply; or
(b) the inside of your cold water storage tank is kept scrupulously clean.