Appliance for the treatment of water of a water system which is in proximity of environments frequented by people, particularly humidification systems in air treatment units, water-sanitary circuits, swimming facilities and the like, comprising a base frame supporting: - at least a hydraulic circuit associable with said water system to be treated, comprising at least a first branch (6a, 6b) for taking the water to be treated from said water system and at least a second branch for conveying the treated water to said water system; - ozone production means suitable for transforming the molecular oxygen of the air into ozone, - at least an injection system for injecting said ozone into said first branch (6a, 6b) of the hydraulic circuit, - at least a contact tank placed between said first branch (6a, 6b) and said second branch of the hydraulic circuit suitable for allowing the mixing of said ozone in said water, and - breathing means of said ozone from said contact tank associated with catalyst means suitable for transforming the breathed ozone into oxygen, characterised by the fact that it comprises: - water sensor means suitable for detecting the presence of bacterial load in said water; - at least one processing and control unit (25, 26, 27) which is associated with said ozone production means and with said water sensor means and is suitable for regulating the operation of said ozone production means for the purpose of producing a quantity of ozone substantially proportional to the bacterial load present in said water without substantially switching off said ozone production means; and - air sensor means, associated with said processing and control unit (25, 26, 27), and comprising one or more probes suitable for detecting any possible quantity of ozone released in the air, said probes being distributed in the environments in proximity of the water system, and the processing and control unit (25, 26, 27) being suitable, in case of danger, for emitting specific alarm signals to warn people, and being set to analyse and consider the data coming from said air sensor means, for the purpose of stopping the operation of the appliance in the event of faults and notifying what found. - at least a hydraulic circuit associable with said water system to be treated, comprising at least a first branch (6a, 6b) for taking the water to be treated from said water system and at least a second branch for conveying the treated water to said water system; - ozone production means suitable for transforming the molecular oxygen of the air into ozone, - at least an injection system for injecting said ozone into said first branch (6a, 6b) of the hydraulic circuit, - at least a contact tank placed between said first branch (6a, 6b) and said second branch of the hydraulic circuit suitable for allowing the mixing of said ozone in said water, and - breathing means of said ozone from said contact tank associated with catalyst means suitable for transforming the breathed ozone into oxygen, - water sensor means suitable for detecting the presence of bacterial load in said water; - at least one processing and control unit (25, 26, 27) which is associated with said ozone production means and with said water sensor means and is suitable for regulating the operation of said ozone production means for the purpose of producing a quantity of ozone substantially proportional to the bacterial load present in said water without substantially switching off said ozone production means; and - air sensor means, associated with said processing and control unit (25, 26, 27), and comprising one or more probes suitable for detecting any possible quantity of ozone released in the air, said probes being distributed in the environments in proximity of the water system, and the processing and control unit (25, 26, 27) being suitable, in case of danger, for emitting specific alarm signals to warn people, and being set to analyse and consider the data coming from said air sensor means, for the purpose of stopping the operation of the appliance in the event of faults and notifying what found.