Half the baby pacifiers checked by the city's quality watchdog failed tests, with some having serious problems such as excessive volatile substances.
Shanghai Quality and Technical Supervision Bureau tested 20 batches of baby pacifiers produced in Shanghai or sold in the city or online, and found 10 of them were substandard.
The test covered general safety requirement, mechanical and chemical properties, and product information.
Mouth shields protect infants from sucking the entire product into their mouths, but three batches of pacifiers labeled "hibb", "Lea&Chille" and "Qinduo" failed the mouth shield test, having serious quality problems, the bureau said.
"The aim of the test is to check whether the mouth shield can protect babies from swallowing the pacifiers into mouth, which could lead to suffocation," said Zhang Yating, an inspector.
The three substandard batches were all sold on Tmall, the online sales platform of e-commerce giant Alibaba.
The "hibb" and "Lea&Chille" batches were found to be made of soft materials that were not hard enough to prevent children from sucking the pacifier into mouth. The "Qinduo" batch failed for an air vent on the mouth shield being too small.
If a pacifier is sucked inside a child's mouth accidentally, the air vent can help children to continue breathing and prevent suffocation, said Zhang.
Seven batches of infant pacifiers were found to have excessive volatile substances, which included acetone, butanone, ethyl acetate and absolute ethyl alcohol. "These substances are potentially harmful to people's health, and could cause chronic poisoning," said Zheng Yi, another inspector.
They included three batches of pacifiers branded "Bearo," "bobble" and "Beichen." The first two were sold on Suning's Yigou online shopping unit.
Investigation is under way.