A whiff of pool water - often described as the smell of chlorine -can stir happy thoughts of summer. If strong enough, however, "pool smell" can signify a source of irritation to the eyes, lungs and skin of swimmers.
Pool smell is due, not to chlorine, but to chloramines, chemical compounds that build up in pool water when it is improperly treated.
Chloramines result from the combination of two ingredients: (a) chlorine disinfectants and (b) perspiration, oils and urine that enter pools on the bodies of swimmers. Chlorine disinfectants are added to pool water to destroy germs that can give swimmers diarrhea, ear aches and athlete's foot. Perspiration, oils and urine, however, are unwanted additions to pool water. By showering before entering the pool, and washing these substances from the skin, swimmers can help minimize pool smell.
The Chemistry of Pool Smell
When chlorine disinfectants are added to water, two chemicals are unleashed that destroy waterborne germs: hypochlorous acid, HOCl, and hypochlorite ion, OCl-. A measure of the chlorine in these two chemicals is known as "free available chlorine" or FAC. Pool operators manage the FAC level of pool water for the safety of swimmers. Their challenge comes from the fact that FAC is reduced when it reacts with perspiration, oils and urine from swimmers to form chloramines.
One way that chloramines are formed in pool water is by the reaction of hypochlorous acid with ammonia. Ammonia, NH3, is a component of sweat and urine. Its chemical structure is illustrated in the figure at the right.
There are three chemical reactions that can occur when hypochlorous acid reacts with ammonia, each involving the replacement of hydrogen ions with chlorine ions. When one of ammonia's hydrogen ions is replaced with chlorine, monochloramine is formed:
HOCl + NH3 → NH2Cl + H2O
Replacing one more hydrogen ion with chlorine produces dichloramine,
HOCl + NH2Cl → NHCl2 + H2O
Finally, it is possible to replace all three of ammonia's hydrogen ions with chlorine to form trichloramine, also known as nitrogen trichloride:
HOCl + NHCl2 → NCl3 + H2O
Monochloramine is sometimes intentionally added to water because it is actually a useful disinfectant. Drinking water, for example, is sometimes purified with monochloramine. Dichloramine and especially trichloramine are the chloramines most responsible for pool smell. By showering before entering the pool, swimmers can minimize the formation of these two chloramines.
Managing Chlorine in the Pool
What is a Part Per Million?
A part per million (ppm) refers to
"one in a million". It is equivalent to:
- One drop of dye in 18 gallons
- One second in 12 days
- One penny out of $10,000
As hypochlorous acid combines with ammonia to form chloramines, the FAC of pool water is reduced. Lowering the FAC reduces the ability of chlorine to destroy germs. The amount of chlorine that is "tied up" in chloramine compounds, and is therefore unavailable as free chlorine, is known as combined available chlorine (CAC). The sum of FAC and CAC is the total chlorine (TC).
TC = FAC + CAC
The Association of Pool and Spa Professionals suggests FAC concentrations in pool water should remain in the range 1.0 - NC 4.0 parts per million for chlorine to work effectively (FAC should never fall below 1 part per million). CAC levels should be less than 0.2 parts per million.
Pool managers can use test kits to measure both FAC and TC. CAC is then simply calculated:
CAC = TC - FAC
Minimizing Pool Smell
Swimmers with reddened, irritated eyes have been known to complain that "there is too much chlorine in the pool". In fact, however, when pool water is irritating, there is not enough chlorine in swimming pool water!
You may be surprised to learn that there is no odor to a well-managed pool. Chloramines, which produce pool smell, can be eliminated using chlorine. "Shock treatment" or "superchlorination" is the practice of adding extra chlorine to pools to destroy ammonia and the organic compounds that combine with chlorine to make chloramines. To effectively destroy chloramines through shock treatment, the pool water FAC concentration must be about ten times the CAC.
Properly disinfected pool water is a must for the health and safety of swimmers. Pool managers have the responsibility to adjust the pool water chemistry to reduce the risk of infection for swimmers. But you can use your senses to help you determine whether a pool is safe for swimming.
The "SENSE-ABLE" Swimming Check List
USE YOUR SENSE OF SIGHT.
Does the pool water look clear and blue? You should be able to see through the water down to the drain or stripes painted on the floor of the pool. If the water is cloudy and colored, there may be algae in it. DON'T GO IN!
USE YOUR SENSE OF TOUCH.
Does the pool wall around the water line feel slimy? If it does, there are probably germs living on the wall. DON'T GO IN!
USE YOUR SENSE OF SMELL.
Is there a strong chemical odor around the pool? If there is, the pool manager may have to treat the water. DON'T GO IN!
USE YOUR SENSE OF HEARING.
The sound of pool-cleaning equipment is a good sign!
DON'T USE YOUR SENSE OF TASTE.
Just don't taste the water! If you do get some water in your mouth, don't swallow it.
USE YOUR COMMON SENSE.
Shower before entering the pool to remove the substances that can help form chloramines. Encourage young children to take regular bathroom breaks, and never go swimming when you have diarrhea.
- In the chemical reactions that produce chloramines, what happens to the hydrogen ions that are being replaced with chlorine?
- Explain why pool managers must test the chemistry of the pool water once every hour on hot summer days.
- Pete's dad uses his pool test kit and measures a FAC of 2.5 ppm and a TC of 3.0 ppm. Should Pete and his buddies go swimming in the pool?
For a list of previous "Chlorine Compound of the Month" features, click