What chlorine does and doesn't do
TL;DR
- Chlorine is a disinfectant, not a cleaner. It works over minutes, not instantly.
- At 1 ppm free chlorine, E. coli O157:H7 dies in under a minute; inactivating Cryptosporidium takes about 10.6 days.
- Chlorine gets used up by sweat, dirt, pee, poop and sunlight, so what you bring in decides how much is left to work with.
- If you have had diarrhoea, stay out of the water. For Crypto, that means two weeks after it has fully stopped.
Contents
The rule
Chlorine disinfects pool water. It does not clean it, and it does not work the moment it touches something. Held at the right level, it kills most germs within a few minutes. One of the commonest takes about ten days.
Why this rule exists
The number that matters is free chlorine, the fraction still available to do the killing. The CDC recommends at least 1 ppm of it in a pool, and a pH between 7.0 and 7.8. Chlorine loses potency as pH climbs, especially past 8.0.
At 1 ppm free chlorine, pH 7.5, and 25 degrees C, the CDC's disinfection timetable reads:
- E. coli O157:H7: under 1 minute
- Hepatitis A: about 16 minutes
- Giardia: about 45 minutes
- Cryptosporidium: about 15,300 minutes, which is 10.6 days
(Those times assume no cyanuric acid, the chlorine stabiliser. With it, they get longer.)
The last line is the one to carry around. Crypto survives more than seven days in properly disinfected water, and diarrhoea is the most common illness linked to the water we swim in.
India's own standard, IS 3328:1993, reads differently. It allows pH 7.5 to 8.5, and puts total residual chlorine between 0.2 mg/l at the outlet and 0.5 mg/l at the inlet. Total residual is free chlorine plus chlorine that has already reacted, so free chlorine can never exceed the total. A pool can clear the Indian tolerance across its whole circuit and still sit below the concentration the CDC recommends.
What happens if you don't follow it
Chlorine is a budget, and every swimmer spends some. Sweat, dirt, pee, poop and sunlight all consume it. A pool with twenty unrinsed swimmers has less left for the germ that arrives on the twenty-first.
Swallow a mouthful and you can pick up the standard recreational water illness: watery diarrhoea, sometimes for weeks. Where several pools share one filtration system, water mixes, and so do the germs.
Quick reference
- A one-minute rinse before entry removes most of the dirt, sweat and oils that eat the chlorine.
- Do not swallow the water. A small mouthful can be enough to make you ill.
- Stay out if you have diarrhoea. After Crypto, wait two weeks from the day it fully stops.
- Take children to the toilet every hour. Swim nappies are not leak-proof; they delay leaks by a few minutes.
- Read the water, not the smell. The sharp smell is chloramines: chlorine bound to what swimmers brought in. Cloudy water is the warning. IS 3328 asks that a black disc at the deepest point stay visible from nine metres away.
- Ask the pool for its log. CDC tells even home pool owners to test chlorine and pH twice a day. A society pool should manage a sheet.
Chlorine buys the pool time. Everything you do before you get in decides how much of it is left.
Quick check
1. A pool tests within the Indian standard: pH 8.2, total residual chlorine 0.25 mg/l at the outlet. The manager says the water is compliant. Is he right, and is the water doing what you assume it is?
Select an answer.
2. Your niece had a bout of watery diarrhoea, later confirmed as Cryptosporidium. She has felt fine for three days. The society pool is chlorinated daily and there's a birthday swim on Sunday. What's the call?
Select an answer.