The washing of hands after playing with modelling dough and clay toys is a crucial habit for children to prevent an allergy from developing, the Consumer Council said on Monday.

The watchdog made the remarks after testing 15 modelling dough and clay toys and found that one-fifth of the samples contained two allergenic preservatives.

The level of one of the preservatives in a product made by Creation Nation exceeded the European Union's standard for children under 36 months by up to about 11 times.

According to Angel Cheung, director of the council's research and survey, getting children to also wash hands before playing can prevent the contamination of the dough and clay toys with germs.

"If children have an adverse effect or the skin has a redness or allergic reaction during or after playing with dough, then they need to stop playing with it," she said.

"And if the situation is serious, they need to seek consultation from a doctor."

Cheung also suggested disposing of the toys if children feel unwell after playing.

As for toys that do not come with an expiry date, the watchdog said they should be discarded once they become dusty.

The council has handed the test results to the Customs and Excise Department for follow-up action.