Understanding Residual Chlorine Standards for Hot Spring Baths

Free residual chlorine is the most critical parameter for disinfection in hot spring baths. Setting an appropriate standard is essential to protect bathers' health while preserving the natural therapeutic properties of the hot spring water. Since January 2025, China has implemented the updated national standard "Hot Spring Service – Water Quality Requirements for Hot Springs" (GB/T 41875-2025). 

This standard specifies that free residual chlorine in hot spring bathing water should be maintained within the range of 0.3 mg/L to 0.6 mg/L. Additionally, the industry recommended standard "Hot Spring Water Quality and Facility Requirements" (LB/T 081-2024) suggests a slightly broader range of 0.2 mg/L to 0.4 mg/L for free residual chlorine, with an absolute upper limit of 1.0 mg/L permitted only under extreme conditions.

Maintaining the correct chlorine level is a balancing act. If the level falls below 0.2 mg/L, the water may not be effectively disinfected, allowing bacteria and viruses to survive. If it exceeds 0.6 mg/L, bathers are likely to experience eye irritation, dry skin, and a strong chlorine odour. Excessive oxidation can also degrade the unique minerals and trace elements that give hot spring water its therapeutic value. 

Beyond free residual chlorine, the combined residual chlorine should be kept at or below 0.4 mg/L, as high levels of chloramines are the primary source of unpleasant odours in bath water.

Certain local governments and special facility types have adopted more stringent standards. For example, Chongqing’s local standard (DB50/T 471-2024) sets a maximum free residual chlorine limit of 1.0 mg/L for bathing pools. For wading pools or splash pools where children or sensitive individuals may accidentally ingest water, the national standard applies a stricter free residual chlorine limit of 0.3–0.6 mg/L, with a combined residual chlorine limit of 0.4 mg/L. Internationally, Japan, which has a highly developed hot spring industry, recommends maintaining free residual chlorine in the range of 0.2–0.4 mg/L.

To consistently meet these standards, hot spring operators should equip their facilities with portable or online residual chlorine detectors, measure free chlorine levels at two‑hour intervals, and keep detailed records. Dosage rates should be adjusted based on real‑time bather load and fresh water replenishment. 

When chlorine levels exceed the limit, a combination of back‑washing the filtration system and adjusting disinfectant feed is more effective than simply pausing chlorine addition or diluting the water. By adhering to these standards and management practices, operators can provide a safe and comfortable experience while preserving the therapeutic integrity of their hot spring water.