Li Yuefang, head of the Wenshou Kindergarten, said on Thursday the 17 children began to experience the nail problems in December, Xinhua reported.
"No cause has been found," Li said, admitting that 12 kids had been withdrawn from the kindergarten as their parents were worried about their safety.
The educational bureau in Xiacheng district of Hangzhou, which controls the kindergarten, confirmed it and said medical experts have ruled out the possibilities of indoor environmental pollution and fungal infection.
The municipal centre of disease control has entrusted a air testing centre to check the air quality in the kindergarten's classroom. All indicators have proved on par with relevant safety standards.
The Hangzhou No.5 Hospital has also ruled out the possibility of fungal infection for five of the 17 children whose symptoms are more severe than others.