Health officials in the US have almost halved the amount of fluoride in tap water in some areas because of a minor side effect.

White splotches on teeth, a merely cosmetic problem affecting as many as two in five Americans, prompted the change.

Adding the chemical to tap water remains the “best method for delivering fluoride to all members of the community,” Deputy Surgeon General Boris Lushniak said on Monday.

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Previously, US areas with cooler climates added 1.2 milligrams of fluoride per litre of water, but will reduce this to 0.7 milligrams — the same as warmer areas.

Water authorities try to keep fluoride levels in Australia between 0.7 and 1 milligrams per litre, with a maximum of 1.5 milligrams.

Sceptics have long attacked its addition to tap water, claiming the chemical can be harmful to humans.

In developing countries, huge amounts of naturally occurring fluoride (far in excess of added levels in Australia) can damage the body.

“Very, very high levels of fluoride in the water can cause skeletal fluorosis, which is where bones actually become stiffer and people can be hunched over,” said Australian Dental Association spokesman Dr Michael Foley in a recent radio interview.

“Now, that’s a very rare condition … I’ve never seen it reported in Australia,” Dr Foley said.

He said medical experts were: “constantly reviewing all the literature, but it is highly unlikely to change the views of decades that fluoridation is a very effective way to reduce tooth decay”.

Tasmania was the first state to experiment with fluoridation in 1953, and Queensland was the last state to adopt the measure in 2008.