Pool Water Smell: Chloramines, Causes, and the Fix

The most persistent myth in pool care is this: if the pool smells strongly of chlorine, it has too much chlorine. The reality is the exact opposite. That strong, sharp, eye-burning smell is chloramines — a family of compounds that form when chlorine is depleted by reacting with nitrogen-containing contaminants. The fix is not less chlorine. It is dramatically more chlorine, applied correctly.

Understanding chloramines is fundamental to competent pool service. They are the reason swimmers get red eyes, why indoor pool facilities have air quality requirements, and why shock treatments work the way they do.

What Are Chloramines?

Chloramines (also called combined chlorine, CC) form when free chlorine reacts with ammonia and other nitrogen compounds. The nitrogen sources in swimming pools come almost entirely from bathers:

• Urine (the primary source — urea breaks down to ammonia in water)
• Sweat (contains urea and amino acids)
• Body oils and skin cells
• Hair care products and sunscreen with nitrogen-containing compounds

There are three types of chloramines, each progressively more problematic:

Dichloramine and trichloramine are the compounds responsible for the classic "pool smell" and for the eye redness and skin irritation that swimmers experience. These compounds evaporate from the water surface, which is why indoor pool facilities can have poor air quality from chloramine off-gassing and why outdoor pools are less affected (but still produce enough to cause problems).

How Chloramines Are Measured

The standard chemistry panel measures three related values:

• Free chlorine (FC): The active, available chlorine doing the sanitizing work
• Total chlorine (TC): FC plus all combined chlorine forms
• Combined chlorine (CC): TC minus FC = the chloramine load

Target values:

• CC should be 0.0–0.3 ppm. Above 0.3 ppm = noticeable chloramine load.
• CC above 0.5 ppm = significant problem; swimmers will have red eyes and smell the pool from a distance.
• CC above 1.0 ppm = serious issue requiring immediate treatment.

Test kit accuracy matters here. Test strips are notoriously unreliable at distinguishing FC from TC, making CC calculations inaccurate. A FAS-DPD drop test kit (Taylor K-2006, Taylor K-2005, or equivalent) is the professional standard for measuring CC precisely.

The Breakpoint Chlorination Calculation

Breakpoint chlorination is the chemistry concept behind shocking. When you add chlorine to a pool with high CC, the chlorine first reacts with existing chloramines — there is an initial "chlorine demand" phase where added FC goes straight to CC destruction. If you add enough chlorine, you reach the breakpoint: the concentration at which the chlorine has sufficient oxidizing potential to break apart all the chloramine compounds, releasing nitrogen gas and returning the chlorine to its free, active form.

The breakpoint concentration is approximately 10 times the CC reading:

• If CC reads 0.5 ppm → shock to at least FC 5.0 ppm and hold it there for 8 hours
• If CC reads 1.0 ppm → shock to at least FC 10 ppm and hold for 8 hours
• If CC reads 2.0 ppm → shock to at least FC 20 ppm and hold for 8 hours

For a stabilized pool (with CYA), remember that the FC:CYA relationship still applies. High CYA levels require proportionally higher FC to maintain effective sanitation and to reach breakpoint. In a pool with 80 ppm CYA and CC of 2.0 ppm, reaching breakpoint may require 20+ ppm FC — plan your chemical quantities accordingly.

Step-by-Step Breakpoint Chlorination Treatment

1. Test thoroughly. Measure FC, TC (to calculate CC), pH, and TA before starting. Log results in PoolLens.
2. Adjust pH to 7.2–7.4. Lower pH increases hypochlorous acid fraction and makes shock more effective. This is a meaningful difference — at pH 8.0, chlorine is only about 3% hypochlorous acid (the killing form); at pH 7.2, it is about 66%.
3. Calculate shock dose. Target FC = CC × 10. Calculate the pounds of calcium hypochlorite (68%) or granular shock needed to reach that FC level in your pool volume. 1 lb of 68% cal-hypo raises FC about 5.7 ppm per 10,000 gallons.
4. Add shock at dusk. UV from sunlight destroys chlorine rapidly. Evening treatment allows the shock to work overnight without significant UV degradation.
5. Broadcast shock — do not add to skimmer. Shock floated to one location can bleach plaster or vinyl. Broadcast across the water surface while walking around the perimeter.
6. Run filter continuously for 8–12 hours. Circulation ensures the shock reaches all parts of the pool.
7. Test the next morning. CC should be below 0.3 ppm. FC should still be elevated. If CC is still above 0.3 ppm, the breakpoint was not reached — repeat with a higher dose.

Non-Chlorine Shock as an Alternative

Potassium monopersulfate (MPS), sold as Leisure Time Renew, Biolab Oxy-Brite, or BioGuard Oxysheen, is an oxidizer that destroys chloramines without adding to the chlorine level. This is useful in specific situations:

• When you need to shock but the pool is in active use (MPS allows swimming 15–30 minutes after treatment vs. 8+ hours for chlorine shock)
• When CYA is already high and adding more chlorine-based product is undesirable
• For routine weekly oxidation in well-maintained pools with low CC

Limitation: MPS is not as effective as chlorine shock for high CC levels (above 1.0 ppm). For serious chloramine problems, chlorine-based shock reaches breakpoint more reliably. MPS is best for maintenance shocking when CC is below 0.5 ppm.

Indoor vs. Outdoor Pool Chloramine Problems

Chloramines are a worse problem in indoor pools because they off-gas from the water surface and accumulate in the air above the pool — they cannot dissipate into the open atmosphere. This is why indoor aquatic facility operators measure air quality (trichloramine levels in air) as well as water chemistry, and why indoor pool buildings often have special ventilation systems.

For service technicians managing indoor commercial pools:

• CC must be maintained at or near zero at all times — the standard 0.3 ppm tolerance is inadequate for indoor facilities
• Superchlorination (breakpoint) should be performed weekly at minimum, and more often during high-use periods
• UV disinfection systems (Pentair UV, Hanovia UV) dramatically reduce chloramine formation by photolytically destroying chloramines as water passes through — a common commercial solution
• Ozone injection is another commercial approach that reduces chloramine load before water returns to the pool

Preventing Chloramine Buildup

The most effective prevention strategy reduces the nitrogen input into the pool water:

• Pre-swim showers. A 60-second rinse before entering the pool removes most sweat, body oil, and urine residue from skin. Research shows pre-swim showers reduce nitrogen input by 40–70%.
• Weekly shocking. Regular oxidation prevents CC from accumulating between service visits. A non-chlorine shock weekly in summer keeps CC consistently near zero.
• Enzyme products. Monthly enzyme treatments (Natural Chemistry Pool Perfect, Biolab Natural Clarifier) break down organic nitrogen compounds before chlorine can react with them.
• Adequate free chlorine. Maintaining FC at the correct level for CYA ensures that nitrogen compounds are oxidized as they enter rather than accumulating for later chlorine to react with.
• Regular CC testing. Test CC every service visit, not just FC. A rising CC trend is an early warning that gives you time to intervene before the problem becomes a complaint.

PoolLens makes CC tracking a standard part of every service visit log. When you have CC readings over time, you can spot the trend — the pool where CC slowly creeps from 0.1 to 0.5 to 1.0 ppm over a summer because bather load increased, or because the shock frequency was not adjusted for a new client's usage patterns.

Answering the Client Question: "Why Does My Pool Smell?"

Pool owners almost universally believe that pool smell = too much chlorine. Correcting this understanding is one of the most valuable things a service technician can do. The accurate explanation:

"The smell you're noticing is actually from chloramines — that's what happens when the chlorine reacts with sweat and other things swimmers bring into the water. The fix is actually to add more chlorine, not less. We shock the pool with enough chlorine to break apart those compounds. After treatment, a clean pool should have almost no smell at all — free chlorine is nearly odorless."

This explanation builds trust, corrects a widespread misconception, and justifies the professional service. It also sets the right expectation: a well-maintained pool with proper chemistry should smell like water, not like a locker room.