Free Chlorine 3 ppm, Pool Still Green: What Cyanuric Acid Does to That Number

If you test your pool, get a free chlorine reading inside the range every chart tells you to hold, and the water still looks dull or goes green anyway, you are not testing wrong. You are testing something that stops meaning much once one other number gets high enough.

That number is cyanuric acid, sold as stabiliser or conditioner. Most home test strips include a pad for it, most people never look at it, and if you dose with tablets it climbs on its own whether you buy stabiliser or not. Here is what it does to your chlorine, how to work out whether yours has gone too far, and the arithmetic for getting it back down.

Your test kit measures the wrong thing, sort of

The standard DPD test gives you free chlorine, which is a total. It counts every form of unbound chlorine in the water together, and it does not tell them apart.

The form that actually kills things is hypochlorous acid, HOCl. Cyanuric acid binds chlorine reversibly, so in a stabilised pool most of what your kit reports as free chlorine is sitting bound to cyanurate rather than available as HOCl at any given moment.

The 2019 study by Richard Falk and colleagues in Water puts a hard number on why this matters. Reanalysing published inactivation data for Cryptosporidium, they found the correlation between log inactivation and Chick-Watson Ct was far better when the concentration term meant HOCl rather than free chlorine, at r = -0.96 against r = -0.06.

In plain terms: how fast a pool kills things tracks HOCl almost perfectly and tracks your free chlorine reading essentially not at all. That is the whole problem in one line.

The ratio is the number, not the chlorine

The useful part of that paper is that you do not need to measure HOCl, which no consumer kit does. You can infer it from two numbers you can measure.

The authors show that HOCl concentration stays nearly constant, varying by about 1.8%, whenever the ratio of cyanuric acid to free chlorine is held constant across the range pools actually run at. Their table lists 20 ppm CYA with 1.0 ppm free chlorine giving 0.01962 mg/L HOCl, and 90 ppm CYA with 4.5 ppm free chlorine giving 0.02031 mg/L. Same ratio, same disinfecting power, wildly different chlorine readings.

So the ratio is the real reading. Divide your cyanuric acid by your free chlorine. Their recommendation is that the result should not exceed 20.

That gives you a minimum free chlorine for whatever your stabiliser happens to be. At 30 ppm CYA you need 1.5 ppm free chlorine. At 50 ppm you need 2.5. At 80 ppm you need 4.0, and at 90 ppm you need 4.5. It is just CYA divided by 20, and the paper prints the same list as a table for pool operators.

Set against that, a pool sitting at 90 ppm cyanuric acid with 2 ppm of free chlorine is at a ratio of 45. The reading looks fine and the ratio is more than twice what the paper recommends. Their conclusion is that the ratio works as an inverse proxy for how much HOCl is actually present, so the higher it climbs, the less the chlorine number is telling you.

How the stabiliser got there without you buying any

This is the part that catches people who never bought a bag of conditioner in their lives. Trichlor tablets, the slow-dissolving pucks that go in a floater or feeder, are made of trichloroisocyanuric acid. Cyanuric acid is most of the molecule by weight, and it stays in the water after the chlorine is spent.

The Pool & Hot Tub Alliance fact sheet on trichlor states the conversion plainly: "When trichlor is used 0.6 ppm of cyanuric acid is added to the water for each ppm of available chlorine added." The same sheet notes trichlor carries the highest available chlorine content of any solid pool sanitiser at 90%.

Run that through the dosing maths our own calculator uses. For a 50,000 litre pool, about 13,200 US gallons, raising free chlorine by 1 ppm with a 90% product means 56 grams of tablet. That single ppm brings 0.6 ppm of cyanuric acid with it, which is 30 grams of CYA left permanently in the water.

Now multiply by a season. If your pool consumes and replaces 2 ppm of free chlorine a day, an illustrative figure rather than a measured one, then across 90 days you have added 180 ppm of free chlorine and, with it, 108 ppm of cyanuric acid. From tablets alone, starting from nothing.

Real pools land lower than that because splash-out, backwashing and rain dilute as you go. The direction is what matters. Nobody decided to run the pool at 108 ppm of stabiliser, and at that level the ratio rule wants 5.4 ppm of free chlorine, which is above the top of the target band in our own tool and above what most home charts print.

A review in Frontiers in Public Health on cyanuric acid health risk gives a sense of the real-world spread, reporting a survey of 21 pools in Kunshan City where concentrations ran from 2 to 79 mg/L with a median of 20.0.

What the regulators allow, and what the research says about it

The CDC's Model Aquatic Health Code, 5th edition regulates the two numbers separately. Section 5.7.3.1.1.2.2 requires a minimum of 2.0 ppm free chlorine in venues using cyanuric acid, against 1.0 ppm for venues without it. Section 5.7.3.1.3.2 caps cyanuric acid at 90 ppm.

Hold both limits at once and you get a permitted ratio of 45, more than double what Falk and colleagues recommend. They say so in the paper: "The MAHC currently allows a CYA/FC ratio up to 45."

Their risk model quantifies the gap. Swimmers in pools at a ratio of 45 are described as approximately two and five times as likely to be infected with Giardia and E. coli O157:H7 respectively as swimmers in pools at a ratio of 20.

The Giardia figure is the one that crosses a regulatory line. Modelling a high bather load pool at 2 mg/L free chlorine and 90 mg/L cyanuric acid, they get an annual infection probability of 0.071, against the 0.036 the US EPA accepts for untreated recreational water like a beach. The same model puts a pool at 2 ppm free chlorine with no cyanuric acid at 0.00052.

Worth being precise about what has and has not been measured here. That work is about pathogen inactivation, not about your green water. On algae the evidence is thinner, and the trichlor fact sheet is careful about it, saying the effect of cyanuric acid on kill rates of bacteria, viruses and algae "has been demonstrated, primarily in controlled laboratory studies." Same chemistry, less field data.

One genuinely useful side effect of the same chemistry: cyanuric acid flattens out the pH problem. Falk and colleagues report that HOCl drops by 53% going from pH 7.5 to 8.0 with no cyanuric acid present, but only by 15% with 30 mg/L of it. If you are stabilised, a pH drift costs you far less chlorine strength than the standard advice implies. It cost you much more up front, when the stabiliser bound the chlorine in the first place.

You need much less stabiliser than you think

The tempting reading of all this is to drop cyanuric acid to zero. Outdoors that fails, because unstabilised chlorine burns off in sunlight fast. The trichlor sheet puts the benefit at free chlorine lasting three to four times longer with cyanuric acid in the recommended 30 to 50 ppm band than without it.

But the protection saturates early. Falk and colleagues note that a cyanuric acid to free chlorine ratio of 5 to 10 "provides most of the stabilization with diminishing returns in additional stabilization at higher CYA/FC ratios."

At 2 ppm free chlorine, a ratio of 10 means 20 ppm of cyanuric acid. That is well under what most pools carry, and it buys most of the sun protection while leaving your chlorine reading close to honest.

This is also the argument for what you dose with. Calcium hypochlorite and liquid sodium hypochlorite add no cyanuric acid at all, so with those you set your stabiliser once and it stays where you put it. Tablets set it for you, upward, every day.

The only way down is water

Cyanuric acid does not break down usefully on a pool season's timescale. The trichlor sheet describes it as "a very stable molecule" that "does not readily degrade in swimming pools and spas," and recommends a partial drain and refill above 100 ppm.

So the fix is dilution, and the arithmetic is a single line. The fraction of water you have to replace is 1 minus your target divided by your current level.

Going from 108 ppm down to 40 ppm means 1 minus 40 over 108, which is 0.63. You are replacing 63% of the pool. Going from 100 to 50 is exactly half. There is no chemical shortcut, which is why it pays to know the number before it gets there rather than after.

The regulators treat the far end as an emergency rather than a maintenance problem. MAHC section 5.7.3.1.3.3 requires a venue at 300 ppm or higher to stay closed until remediation has happened. The same edition also stops spas and therapy pools using cyanuric acid or stabilised chlorine products for new construction, substantial alteration or disinfection equipment replacement, and gives those venues four years from adoption of the code to stop using it entirely.

What to do this week

Test cyanuric acid, even if you have never bothered. If you dose with tablets and have not tested it this season, that is the single most likely reason your chlorine reading and your water quality disagree.

Then divide. Cyanuric acid over free chlorine, and aim for 20 or below. If the answer is 30 or 40, you have a decision between holding a much higher chlorine level than you are used to, or replacing water.

Our Pool Chemical Dosage Calculator will size the doses once you have decided. It has a cyanuric acid field under the advanced readings, flags the standard 30 to 80 ppm band, and lists trichlor at 90% alongside calcium hypochlorite and liquid chlorine so you can see what a switch of product does to the grams. Its chlorine target band runs 1.0 to 3.0 ppm, which under the ratio rule covers stabiliser up to 60 ppm and no further, so above that you are deliberately running the high end.

None of it needs the calculator, though. A test strip, the ratio, and one division will tell you whether the number you have been trusting all summer has been quietly drifting away from what it is supposed to represent.