An NGO has alerted Chinese parents to be cautious in choosing "rubber duckies" for their children, as many toys sold online were found to have an excessive amount of a chemical that could cause damage to children's reproductive systems.

After Dutch artist Florentijn Hofman's inflatable Rubber Duck appeared in Beijing in 2013, the giant rubber duck went viral and yellow toy ducks resembling their bigger brother became an online sensation. The toy duck has been a must in the bathtub for many Chinese families.

What the TOXICs-Free China NGO found, however, may cause many parents to throw away the ducks in their children's tub ASAP.

Many toys sold online were without the China Compulsory Certification, the country's safety guarantee, and most toy ducks the organization sent to test were found to have an excessive amount of plasticizer phthalic acid esters.

The NGO checked 100 shops that sell the toy on each of three major e-commerce platforms, Taobao, Pinduoduo and JD.com, and found about half failed to make public their safety certification according to a report published by the NGO on Wednesday.

The CCCs of some shops have been found to be invalid. On Pinduoduo, for example, the certification for two shops had been cancelled and another two had suspended certification, the report said.

The NGO bought 20 brands of toy ducks with online CCCs, but found 13 of them did not attach certification to their toys as required by law.

TOXICs-Free sent 12 toys it bought online — four from each of the online platforms — to a testing agency and found nine of them had an excessive amount of PAEs. The content of the chemicals in one brand of toy duck, Xidiqi, sold on JD.com was found to have 365 times the national minimum even though the brand has a CCC both online and on its product, the report said.

It said, however, it's still a little bit safer to buy toy ducks with online CCC than those without, as all three brands that meet national standard have certification in their online shops.

According to the NGO, PAEs are prone to release from the toys on contact with hot water or shampoo, then enter the human body through the respiratory and digestive tracts, as well as the skin. The chemical plays a role similar to that of the female hormone, and could lower the quality of sperm and result in infertility.

Liu Jianguo, associate professor of the College of Environmental Sciences and Engineering at Peking University, said the chemicals could result in more damage to children than adults because the density of children's is higher due to their small size.