How "Dirty" Are Public Pools? — A Complete Guide to Bacteria, Parasites, Chemicals, and Enjoying Them Safely

Introduction: Questioning the "Myth of Safety" of Clear Water Surfaces

"Clear water = safe" is not always true. Chlorine treatment is not foolproof, and its disinfecting effect diminishes if the contact time or pH is not appropriate. Moreover, some pathogens are resistant to chlorine. Phys.orgCDC

1. The "Safety Line" from Water Quality Standards in Japan and the World

Japan's "Hygiene Standards for Swimming Pools" specifies that free residual chlorine should be at least 0.4mg/L (preferably not more than 1.0mg/L), general bacteria should be below 200 CFU/mL, and E. coli should not be detected. Ministry of Health, Labour and Welfare

The CDC states that if recommended concentrations and pH are maintained in home pools, many bacteria, including E. coli O157:H7, can be inactivated within one minute. CDC

2. How "Dirty" Is It Really? The Reality in Numbers

In 160 pools in the Atlanta area, E. coli was detected in 58% of the filters. Contamination from what is colloquially known as "fecal origin" is more frequent than expected. CBS News

Furthermore, CDC analysis shows that there were over 200 pool-related outbreaks from 2015 to 2019, affecting more than 3,600 people. Phys.org

3. Main Pathogens and Symptoms in Pools

3-1 Cryptosporidium (Crypto)

Highly resistant to chlorine, it can survive up to 10 days even in well-managed pools. Even a small intake can cause mass infections. Symptoms include watery diarrhea, abdominal pain, and vomiting. Phys.orgCDC

3-2 Pseudomonas aeruginosa

Thrives in warm water environments, causing "hot tub rash" or "swimmer's ear (otitis externa)." Most cases resolve naturally, but it can be severe in immunocompromised individuals. Phys.orgCleveland ClinicSkinsight

3-3 Norovirus and Adenovirus

Although reduced by chlorine, these viruses can persist for a certain period. They cause vomiting, diarrhea, fever, and conjunctivitis. Phys.org

4. "Smell ≠ Clean": The Limits of Chlorine and Byproducts

Nitrogen compounds in sweat and urine react with chlorine to produce byproducts like chloramines. The pungent smell is due to these byproducts and may be a sign of poor hygiene. Phys.orgTIMEScience News

5. Who Is at High Risk?

Infants, pregnant women, the elderly, immunocompromised individuals, and those with open wounds are more susceptible to infections and chemical exposure. Cryptosporidium, in particular, can be severe in immunocompromised individuals. Phys.orgCDC

6. Typical Symptoms and Common Mistakes

• Diarrhea, Abdominal Pain, Vomiting: Possible Crypto or norovirus. It is recommended to wait two weeks after symptoms disappear before swimming again. Phys.org

• Skin Rash, Itching: Suspect Pseudomonas folliculitis; shower and maintain cleanliness. Cleveland Clinic

• Strong Odor Causing Eye Irritation: Sign of rising chloramine levels. Report to management. Phys.orgTIME

7. Ten Self-Defense Actions Users Can Take from Today (+α)

Most pool-related diseases can be prevented with basic manners and hygiene. Here are actions recommended by U.S. infectious disease experts and the CDC. Phys.orgCDC

1. Shower for More Than a Minute Before Entering (to remove sebum and sweat that hinder chlorine's effectiveness). Phys.org

2. Do Not Swim When Experiencing Diarrhea, Fever, or Open Wounds. Phys.org

3. Do Not Swallow Water (Make a Habit of Keeping Your Mouth Closed). Phys.org

4. Do Not Use the Pool for Two Weeks After Crypto Diagnosis. Phys.org

5. Take Toilet Breaks and Change Diapers Every Hour, Away from the Pool. Phys.org

6. Dry Ears Well (to prevent swimmer's ear). Phys.org

7. Completely Cover Wounds with Waterproof Bandages. Phys.org

8. Shower Off After Swimming. Phys.org

9. If You Smell Strong Chlorine, Check the Situation or Confirm with Management. Phys.orgTIME

10. Incorporate "No Drinking Water" and "Toilet Breaks" into Children's Play. Phys.org
    
    +α: Protect mucous membranes with goggles and earplugs, and remember to hydrate before and after swimming (dehydration may reduce chlorine effectiveness). ##