**Core Concept**
Disinfection of swimming pool water requires the maintenance of a specific level of residual free chlorine to inactivate pathogenic microorganisms, preventing the spread of waterborne diseases. The recommended level of residual free chlorine is crucial to ensure effective disinfection without causing harm to bathers.

**Why the Correct Answer is Right**
The Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) recommend maintaining a residual free chlorine level of at least 1.0 mg/L to ensure effective disinfection of swimming pool water. This level is sufficient to inactivate most pathogens, including bacteria, viruses, and protozoa, without causing eye and skin irritation to bathers. The recommended level is based on the oxidation-reduction potential (ORP) of the water, which measures the ability of chlorine to inactivate microorganisms.

**Why Each Wrong Option is Incorrect**
**Option A:** 0.5 mg/L is below the recommended level and may not provide adequate disinfection.
**Option B:** 2.0 mg/L is higher than the recommended level and may cause eye and skin irritation to bathers.
**Option C:** No value specified is not a valid answer, as a specific level of residual free chlorine is required for effective disinfection.

**Clinical Pearl / High-Yield Fact**
It's essential to regularly test and maintain the residual free chlorine level in swimming pool water to prevent the spread of waterborne diseases. A level of 1.0 mg/L is considered safe and effective for disinfection.

**Correct Answer: A. 1.0 mg/L**