benefits of carbon dioxide as ph reducer in chlorinated indoor swimming pools carbon dioxide is seldom used as ph reducer in swimming pools nevertheless it offers two interesting advantages first its use instead of the usual hydrochloric acid avoids the characteristic and serious accident of mixing the disinfectant with that strong acid which forms a dangerous chlorine gas cloud and second it allows the facility to become slightly a depository of that greenhouse gas this work introduces the experience of using co2 as ph reducer in real working swimming pools showing three more advantages lower chlorine consumption lower presence of oxidants in the air above the swimming pool and a diminished formation of trihalomethanes in the swimming pool water experiments lasted 4years and they were run in three swimming pools in the barcelona area where the conventional system based upon hcl and a system based upon co2 were consecutively exchanged