This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, Subject name: Social & Preventive Medicine
The recommended concentration of free chlorine is 
Select and explain. 
A. 0.2mg/dl, B. 0.5mg/dl, C. 5mg/dl, D. 10mg/dl
B: 0.5mg/dl
Answer: B: 0.5mg/dl 
Explanation: The chlorine demand of water is the difference between the amount of chlorine added to the water and the amount of residual chlorine remaining at the end of a specific period of contact (usually 60 min)The point at which chlorine demand of water is met is called the &;breakpoint chlorine&;Chlorine added beyond breakpoint chlorine is free chlorine (HOCl and OCl). The minimum recommended concentration of free chlorine is 0.5 mg/L for one hour.The free chlorine provides a margin of safety against subsequent microbial contamination such as may occur during storage and distribution.Park 23e pg: 715