PORTLAND — The Portland Water District has temporarily suspended the addition of fluoride to water because of a supply shortage.

The suspension began Monday, but a shipment of fluoride from the district’s supplier is expected to arrive Friday.

The withdrawal of a major fluoride supplier has caused a national shortage since 2005, but the Portland area has been somewhat insulated because of its contract with Monson Chemicals, according to the water district.

Fluoride is added to water to promote dental health. Its absence does not pose a health risk.