During examination of an 8-year-old child there were spots detected on the vestibular surface of the 11th, 12th, 21st and 22nd teeth. The spots are pearly-white, glossy, painless when probed, and gradually merge with non-changed enamel. Ultraviolet irradiation revealed the spots to fluoresce blue. The child has been living up to the age of 3 in the region, where fluorine content of drinking water was 2 mg/l. What kind of non-carious lesion is it?