In these systems the danger is linked to the use of chlorine to purify water. There are many different supplying resources: underground, superficial, or brackish water, if properly treated. In most aqueducts water is disinfected. Disinfection always implies the use of chemical substances (except for ultraviolet disinfection). Chlorine derivates are usually used for this purpose.
Chlorine (Cl): it is the most used element; because of its high toxicity, it easily eliminates many biological risks.
CI: chlorine irritates the breathing system, mainly in children and old people. When in gas state it irritates the mucosa, and in liquid state, it causes skin burns. Chlorine odour can be smelt at a 3.5 ppm concentration.