Introduction
Added too much chlorine to the pool or did a shock treatment and now levels are too high? Or are your children complaining of red eyes after swimming? Excess chlorine is a common problem, especially after shock treatments or when using poorly calibrated automatic dosers.
While chlorine is essential for keeping water safe and clean, excessive levels can cause discomfort to swimmers, damage equipment and linings, and even discolour swimwear. The good news is there are several ways to lower chlorine quickly.
In this article, we explain what ideal levels are, the symptoms of high chlorine, how to lower the level safely, and how to avoid excess in the future.
Chlorine Levels: What's Normal
Reference Values
| Type of Chlorine | Ideal | Acceptable | High | Dangerous |
|---|---|---|---|---|
| Free chlorine | 1-3 ppm | 0.5-5 ppm | 5-10 ppm | >10 ppm |
| Combined chlorine | <0.5 ppm | 0.5-1 ppm | >1 ppm | >2 ppm |
| Total chlorine | 1-3 ppm | 1-5 ppm | >5 ppm | >10 ppm |
Important difference:
- Free chlorine: What's available to disinfect
- Combined chlorine (chloramines): "Spent" chlorine that causes smell and irritation
- Total chlorine: Sum of free + combined
💡 Dica Profissional
The strong "chlorine" smell at the pool doesn't mean too much chlorine – it means excess chloramines. They're what irritate eyes and skin. If you smell this, the pool needs more chlorine (shock) to eliminate the chloramines, not less.
Factors That Affect Chlorine
| Factor | Effect on Chlorine |
|---|---|
| Sun/UV | Degrades quickly (loses 1-2 ppm/day) |
| High temperature | Accelerates degradation and consumption |
| Number of swimmers | Consumes chlorine |
| Organic matter | Consumes chlorine |
| pH | High pH reduces effectiveness |
| Cyanuric acid | Stabilises against UV |
Symptoms of High Chlorine
In Swimmers
| Symptom | Typical Level | Severity |
|---|---|---|
| Slightly red eyes | >3 ppm | Mild |
| Very irritated eyes | >5 ppm | Moderate |
| Dry skin/itching | >5 ppm | Moderate |
| Hair discolouration | >5 ppm prolonged | Moderate |
| Respiratory irritation | >10 ppm | Severe |
| Nausea, dizziness | >15 ppm | Severe |
Note: These symptoms can also be caused by chloramines (high combined chlorine), incorrect pH, or unbalanced alkalinity.
On Pool and Equipment
| Symptom | Likely Cause |
|---|---|
| Liner discolouration | Chlorine >5 ppm prolonged |
| Metal corrosion | Low pH + high chlorine |
| Faded swimwear | Chlorine >5 ppm |
| O-ring degradation | Chronic high chlorine |
| Strong chemical smell | Chloramines or very high chlorine |
How to Test Correctly
| Method | Accuracy | Cost | Notes |
|---|---|---|---|
| Test strips | Low-medium | €10-20 | Practical, less accurate |
| Drop kit (DPD) | Medium-high | €20-40 | More accurate |
| Digital photometer | High | €100-300 | Professional |
Important: Test both free AND total chlorine to calculate combined (Total - Free = Combined).
Causes of High Chlorine
Dosing Error
| Cause | How to Prevent |
|---|---|
| Wrong volume calculation | Measure/calculate correctly |
| More concentrated product | Read label carefully |
| Dose accumulation | Test before adding |
| Poorly calibrated doser | Check regularly |
After Shock Treatment
Shock purposely raises chlorine to 10-30 ppm. This is normal and necessary, but:
- Don't use pool for 8-24 hours
- Wait until it drops to <5 ppm (ideal <3 ppm)
- If it takes too long, use reduction methods
Automatic Chlorinator
| Problem | Solution |
|---|---|
| Setting too high | Lower to 1-2 |
| Too many tablets | Reduce quantity |
| Timer too long | Reduce hours of operation |
| Summer accumulation | Adjust seasonally |
How to Lower Chlorine
Method 1: Wait (The Simplest)
Chlorine degrades naturally with:
- Sunlight (UV)
- Heat
- Pool use
- Time
Typical degradation rate:
| Condition | Daily Loss |
|---|---|
| Direct sun, no stabiliser | 3-5 ppm |
| Direct sun, with stabiliser | 0.5-1 ppm |
| Covered, with stabiliser | 0.1-0.3 ppm |
To accelerate:
- Remove cover
- Turn off chlorinator
- Maximum sun exposure
- Run pump to circulate
Estimated time:
| Current Level | To 3 ppm | To 5 ppm |
|---|---|---|
| 10 ppm | 2-3 days | 1-2 days |
| 15 ppm | 3-4 days | 2-3 days |
| 20 ppm | 4-5 days | 3-4 days |
Method 2: Dilution (Effective)
Replace part of the water with fresh water:
Approximate calculation:
- Drain 10% = lowers chlorine ~10%
- Drain 25% = lowers chlorine ~25%
- Drain 50% = lowers chlorine ~50%
Procedure:
- Calculate % of water to remove
- Drain to sewer/garden (if chlorine <5 ppm)
- Refill with tap water
- Test and adjust chemistry
Advantages:
- Also renews the water
- Reduces other accumulated chemicals
- No product addition
Disadvantages:
- Wastes water
- Need to rebalance all chemistry
- Not practical for very high levels
Method 3: Chlorine Neutraliser
Sodium thiosulphate (most common):
| Dosage | Effect |
|---|---|
| 60g per 10,000L | Reduces ~1 ppm |
| 120g per 10,000L | Reduces ~2 ppm |
| 180g per 10,000L | Reduces ~3 ppm |
Procedure:
- Calculate required dose
- Dissolve in bucket with water
- Add slowly, distributing
- Circulate 30 minutes
- Test again
- Repeat if necessary
💡 Dica Profissional
Always add less than you calculate and test. It's easier to add more neutraliser than to add more chlorine if it drops too much. Thiosulphate acts quickly – results in 15-30 minutes.
Note: Thiosulphate also consumes oxygen from the water. In cases of very high doses, it may be necessary to aerate the water.
Method 4: Hydrogen Peroxide
Alternative to thiosulphate:
| Dosage (35% peroxide) | Effect |
|---|---|
| 100ml per 10,000L | Reduces ~1 ppm |
Advantages:
- Leaves no residue
- Adds oxygen
- Doesn't affect other parameters
Disadvantages:
- More expensive
- Less available
- Reacts violently with very high chlorine
Method Comparison
| Method | Speed | Cost | Difficulty | Ideal For |
|---|---|---|---|---|
| Wait | Slow | Zero | Easy | Small excess |
| Dilution | Medium | Water | Medium | Moderate excess |
| Thiosulphate | Fast | Low | Easy | Large excess |
| Peroxide | Fast | Medium | Medium | Alt. to thiosulphate |
When Is It Safe to Swim?
Safe Levels
| Free Chlorine | Safety | Action |
|---|---|---|
| <1 ppm | Sanitary risk | Don't swim, add chlorine |
| 1-3 ppm | Ideal | Can swim |
| 3-5 ppm | Acceptable | Can swim, sensitive may feel |
| 5-10 ppm | Not recommended | Wait for it to drop |
| >10 ppm | Prohibited | Don't swim, reduce chlorine |
After Shock Treatment
| Type of Shock | Wait Until |
|---|---|
| Light shock (10 ppm) | 8-12 hours or <5 ppm |
| Normal shock (15 ppm) | 12-24 hours or <5 ppm |
| Strong shock (20-30 ppm) | 24-48 hours or <5 ppm |
Golden rule: Always test before swimming after any treatment.
Preventing High Chlorine
Correct Dosing
Basic formula for pools with stabiliser:
- Maintenance: 1-2 ppm
- For each 10,000L, ~17g of calcium hypochlorite raises 1 ppm
Steps:
- Test current level
- Calculate how much you need to add
- Add half the calculated dose
- Wait 4 hours
- Retest and adjust
Automatic Chlorinator
| Action | Frequency |
|---|---|
| Check chlorine level | 2x per week |
| Adjust setting | As needed |
| Check tablet quantity | Weekly |
| Calibrate timer | Monthly |
Seasonal adjustment:
| Season | Typical Setting |
|---|---|
| Winter | 1-2 (low) |
| Spring | 2-3 (medium) |
| Summer | 3-4 (high) |
| Autumn | 2-3 (medium) |
Best Practices
| Practice | Benefit |
|---|---|
| Test before adding | Avoids overdose |
| Use cyanuric acid | Stabilises chlorine in sun |
| Keep pH 7.2-7.4 | Chlorine more effective = less needed |
| Add at night | Less UV loss |
| Use cover | Reduces loss and need |
Costs
Products for Reduction
| Product | Typical Packaging | Cost |
|---|---|---|
| Sodium thiosulphate | 1 kg | €8-15 |
| Hydrogen peroxide | 5 L (35%) | €15-25 |
Water (Dilution)
| Region | Cost per m³ |
|---|---|
| Margem Sul typical | €2-4/m³ |
| Renew 10% (5 m³) | €10-20 |
| Renew 25% (12.5 m³) | €25-50 |
Frequently Asked Questions
Can high chlorine damage the pool?
Yes, chronically high levels (>5 ppm for weeks) can:
- Discolour and weaken liner
- Corrode metal parts
- Degrade seals and O-rings
- Accelerate equipment ageing
Do red eyes mean high chlorine?
Not always. In fact, red eyes are more often caused by:
- Chloramines (combined chlorine) – "pool" smell
- Incorrect pH (especially high)
- Unbalanced alkalinity
Can I swim with chlorine at 5 ppm?
Technically yes, but it's not ideal. Sensitive people may experience:
- Slight eye irritation
- Dry skin after swimming
- Strong smell
Does cyanuric acid affect high chlorine?
Yes and no. Cyanuric acid (stabiliser):
- Protects chlorine from UV
- Keeps level more stable
- But also "holds" chlorine, making it harder to lower
How long does shock take to drop?
| Stabiliser Level | Time to Drop from 20 to 3 ppm |
|---|---|
| No stabiliser | 1-2 days (sun) |
| 30-50 ppm | 3-5 days |
| >50 ppm | 5-7 days |
If you're in a hurry, use neutraliser or dilution.
Conclusion
High chlorine is a common problem but easily solved. The main points to remember are:
- Ideal levels are 1-3 ppm – more doesn't mean cleaner
- Symptoms may have other causes – test all parameters
- Several methods to lower – wait, dilute, or neutralise
- Don't swim above 5 ppm – wait for it to drop
- Prevention is better – test before adding, calibrate equipment
If you need help with the chemical balance of your pool in Margem Sul, ManutençãoPiscinas is here to help. Contact us for a professional water analysis.
