Laundry scent boosters did not help with cleaning at all, the consumer watchdog said on Friday, as it found "varying types and concentrations" of fragrance allergens in all 12 samples it tested.

"Except for one model with only one type of fragrance allergen, the remaining 11 models had three to eight types of fragrance allergens. The test also revealed that models with two-coloured scented granules generally had more types of fragrance allergens," the Consumer Council said.

The council also said a third of the scent boosters contained a musk compound, galaxolide, which is not readily biodegradable and highly toxic to aquatic organisms, potentially posing a threat to marine ecosystems in the long run.

The 12 in-wash scent boosters tested were priced ranged from HK$38 to HK$179 per bottle.

Half of the models claim to help with softening fabrics.

"The council considers these in-wash scent boosters as unnecessary add-ons for laundry as the contained fragrances have no effect on cleaning, while the cost per wash could reach a maximum of HK$21 based on the recommended usage," the watchdog said.

"As such, consumers should consider the long-term need for these products, especially if they or their family members are prone to skin or respiratory allergies. Should consumers choose to use these products, they should exercise caution when using, and at the same time keep them out of young children's reach to avoid accidental ingestion."