Some food containers have traces of plasticisers while some brands are not suitable for microwave ovens, the Consumer Council has found.
Seven of the 25 brands of plastic food boxes tested contained plasticisers, used to make the product more durable or flexible.
The amount is within safety limits set by the European Union but experts have advised against using them.
"Plasticiser amounts within safety standards means that even daily exposure should be safe, considering just this one single product. But we are exposed to plasticisers through various ways in daily life, so we should try our best to avoid them when given a choice," said City University biology and chemistry associate professor Michael Lam Hon-wah.
The study, released yesterday, also found that the colours could leach in three brands of containers and five had covers that were not safe for microwave use.
Professor Ron Hui Shu-yuen, a member of the council's publicity and community relations committee who announced the report yesterday, said seven brands were safe to use when following guidelines on their labels.
He said plastic containers should not be used when microwaving food with a high fat or a high sugar content. Tests on animals have shown that prolonged exposure to some plasticisers may interfere with the hormone system and damage the liver, kidney and reproductive system.
Plasticisers are also found in eating utensils and clothes. Heating the plastic causes more of the toxic chemical to be released, Lam said.
The study, published in Choice magazine, said two brands, Tark and Lock & Lock, manufactured in Korea and Vietnam respectively, were found with Di(2-ethylhexyl)phthalate, or DEHP, a more toxic kind of plasticiser, in the box and lid.
The lids of six brands - Lustroware, Super, New Ocean, Unix ware, Otuku and Jamka - were made of polyethylene or polystyrene, which can resist heat up to 110 degrees Celsius and 90 degrees Celsius respectively. They may not withstand higher temperatures in microwaves.
Hui reminded consumers to follow guidelines on the labels. If there was any colour or texture change suggesting ageing, the boxes should not be used. It would be better to transfer food to suitable containers for reheating, he said.