KEEP it capped is the message from local swimming pools.
Managers are worried that people using the eight pools without swimming caps are putting extra chemicals into the water.
These come from mousses, gels and conditioners - and the effect of each one has to be countered by more chemicals in the filtration and cleaning processes.
The pools are controlled by Kirklees Active Leisure.
Holmfirth Pool operations manager Jane Donarski said: "We are planning to reaffirm our Keep Swimming Fresh campaign which encourages all swimmers, particularly pupils, to wear caps in time for the next set of lessons which begin in the summer.
"I explained to parents why it is so important, and many of them went and bought caps straight away.
"Our swimming teachers say that when wearing a cap, children are more focused and in the right frame of mind to swim, like wearing a uniform.
"Most children wear hair gel, mousse and conditioner. For every chemical that goes into the pool, we have to combat that with another chemical.
"Obviously that does not benefit either the swimmers or the environment. We are very keen to keep the chemicals to a minimum and, by wearing caps, we can reduce the chemicals added to the water.
"If the water is cleaner to start with, the pool filters can work more efficiently. And wearing a cap does protect your hair from the pool water."
She added: "We hope that our swimmers understand why it is far more hygienic for themselves and for other pool users if they wear a cap and hope it is a habit they get into."
Damon Shaw, marketing manager, said: "We hope people take up our fantastic offer and enjoy a free swim on us. We are very keen to highlight the message Keep Swimming Fresh, which focuses on water hygiene.
"This includes measures such as wearing a cap and having a shower before entering the pool."
All Kirklees Active Leisure pool receptions sell swimming caps for both adults and children priced from £1.30 to £4.20.
KAL pools are: Huddersfield Sports Centre, the Stadium Pool, Holmfirth Pool, Scissett Baths, Batley Baths and Recreation Centre, Batley Sports and Tennis Centre and Spenborough Pool. Dewsbury Pool is currently closed for refurbishment.