Combined chlorine forms when free chlorine reacts with nitrogen-containing compounds – in a pool these are mainly urea, ammonium from sweat and urine, skin particles and cosmetic residues. The reaction products are chloramines (monochloramine, dichloramine, trichloramine).
Chloramines have only a greatly reduced disinfecting effect. They are responsible for the typical swimming pool smell as well as for irritation of the eyes, skin and mucous membranes. The smell is therefore not an indicator of excess chlorine / overdosing, but of chlorine that has already reacted and of an organic load in the water.
Combined chlorine is determined as a difference: total chlorine (DPD1+DPD3) − free chlorine (DPD1) = combined chlorine.
In a private pool, the value should ideally be below 0.2 mg/l and at most around 0.5 mg/l. Above that, the smell and irritation increase noticeably. Combined chlorine is broken down mainly by shock chlorination in the optimal pH range (about 7.0–7.4) and partly also by backwashing and adding fresh water. Persistently elevated values indicate insufficient active chlorine reserves, too infrequent or too low chlorine dosing and too much dirt entering the pool.