Free Chlorine reacts with the so-called micro pollutants immediately to DBP’s such as combined Chlorine (mono-, di- and tri-chloramines) and strongly irritate eyes, hair, shin, nose, and throat. A THM is even carcinogenic already at low concentrations (0.02 mg per Liter).
According to the WHO guidelines the levels of free chlorine, which are currently considered to be acceptable to achieve adequate depot disinfection, while minimizing the user’s discomfort.
In public pools the minimum dosage varies to a large extend (0.3 to 5.0 mg per Liter). Legally combined chlorine (Chloramine) must not exceed 1 mg per Liter in any public swimming pool and spa pool. Pool operators should ensure as best practice, that combined chlorine never exceed half of the concentration of free Chlorine. In Europe the regulations are much stricter.