Powdered composition comprising: (a) 100 parts by weight of at least one polyhydroxyalkanoate (PHA) in the form of particles having an average diameter (d50) from 1 μm to 100 μm, preferably from 5 μm to 60 μm; (b) from 0.1 to 10 parts by weight, preferably from 0.5 to 5 parts by weight, of at least one silicate or silica. The powdered composition, having a relatively low abrasivity index and high flowability, can be used in air-polishing without causing excessive abrasion of the enamel and of the root cementum and without causing sticking in the device. Furthermore, the presence of PHA, being a polymer characterized by high biodegradability, makes the powdered composition particularly suitable for use in the oral cavity.