Almost half of all coin-operated water vending machines nationwide are below standard with the water unsafe for general consumption, the Department of Health said Wednesday.
The department released data showing around 40% of the machines dispense water which fails to meet acceptable water-quality standards.
Deputy director-general Danai Theewanda said the department carried out a study in 2016 on the quality of water provided by the dispensers.
The study found much of the water contained too much bacteria while the amounts of acid and basic salt materials failed to meet standards, he said.
Dr Danai said drinking contaminated water poses a risk of diarrhoea, digestive problems and amoebic dysentery.
He urged consumers to exercise caution and only buy drinking water from machines that are well-maintained and regularly disinfected.
People are urged to check the machine for labels indicating how often it is cleaned. The label should include the company's name and that of the inspector, Dr Danai added.
The machine also must be located in a safe and clean place that is a "decent" distance from any waste-collection point, he said.
The water diffuser inside the machine should be made of non-toxic material, such as stainless steel, and the water must be relatively clear and colourless, he added.
Dr Danai said the Department of Public Health strictly monitors vending machine suppliers as the water they dispense could pose a health risk.
Operators are required to apply for a licence to run the business, Dr Danai said.