“Not for use in outdoor pools.”

That warning — now required on sodium bromide-based algicides as part of the U.S. Environmental Protection Agency’s ongoing pesticide registration review — marks a significant shift for a product long relied on to treat some of the most stubborn algae problems in the field.

The 2023 label change stems from a growing regulatory concern: When sodium bromide is used in combination with chlorine, it can form bromate — a disinfection byproduct associated with cancer.

For service professionals, the restriction creates an immediate challenge. Sodium bromide has been widely used precisely in outdoor pools, where algae problems are most persistent. Now, its use in those environments is explicitly prohibited.

Regulatory concern is not new. Earlier laboratory and environmental studies — including research cited by regulators and industry groups in the early 2000s — showed that bromide can be converted to bromate under oxidizing conditions, sometimes at high rates.

But those findings were largely based on laboratory or drinking water conditions, not outdoor swimming pools.

And until recently, the matter had not been studied under real-world conditions in outdoor pools.

The gap between laboratory assumptions and field conditions has left considerable room for debate. Pool chemistry — where sunlight, organic matter, cyanuric acid, and fluctuating sanitizer levels all interact — is far more complex.

Now, a newly released field study is attempting to close that gap.

Conducted in outdoor test pools in Southern California and released March 31, 2026, by United Chemical, the study examined how bromate forms under conditions designed to simulate real pool environments. Eight aboveground pools were treated with sodium bromide and sodium hypochlorite across multiple phases, including scenarios with algae, without algae, and under repeated superchlorination.

The results confirm a key point: Bromate does form in outdoor pools treated with sodium bromide and chlorine.

But the extent — and the mechanism — appear to differ significantly from earlier expectations.

Equally notable — and central to the study’s design — is what appears to suppress bromate formation.

Pools containing organic material — such as algae — showed little to no bromate formation during initial treatment. The presence of ammonia and nitrogen-based compounds, including those associated with swimmer waste, also reduced formation. Even simulated swimmer load — created using artificial human sweat — produced a measurable, if modest, decrease. In other words, the “dirtier” the water, the less favorable the conditions for bromate formation — at least initially.

That finding runs counter to a common assumption that more contamination necessarily leads to more byproducts. In this case, those same contaminants appear to compete for chlorine, reducing the amount available to fully oxidize bromide into bromate.

One of the study’s other notable findings is how quickly bromate forms when it does occur. Approximately 78 to 94 percent of total bromate formation took place within the first hour after chlorine dosing. After that, formation slowed dramatically.

That timing matters. If most bromate forms immediately, then formation is tied directly to how chemicals are applied — not just to long-term exposure to sunlight or normal pool operation.

In practical terms, the issue shifts from being purely environmental to partially procedural — something that can vary from pool to pool depending on dosing sequence, concentration, and timing.

The study also points to a different controlling factor than previously emphasized: the ratio of chlorine to bromide in the water.

Regulatory concern has often focused on sunlight and UV exposure as drivers of bromate formation. In this study, however, UV showed little correlation with bromate levels. Instead, the dominant factor appeared to be the chemical relationship between chlorine and bromide at the time of dosing.

That does not mean UV plays no role. It does mean that an oxidizer — chlorine — is required for bromate to form, and that the intensity and timing of that oxidation may matter more than sunlight alone.

In practical terms, higher chlorine levels relative to bromide led to greater conversion to bromate. Conversely, increasing bromide relative to a fixed chlorine level reduced the percentage converted.

As Scott Hamilton — CEO of United Chemical and a longtime pool industry chemistry expert — explained, “introducing a higher ratio of bromide to chlorine actually reduces overall bromate formation” because the available oxidizing capacity is distributed across more bromide ions.

Chemically, bromate formation proceeds through a series of oxidation steps, moving bromide from a reduced state toward higher oxidation states. If the available oxidizer is limited, that progression is less likely to go to completion. In that sense, the ratio — not just the absolute amount — becomes the controlling factor.

That dynamic becomes increasingly pronounced under more aggressive treatment conditions: In the third phase of the study, pools were subjected to repeated high-dose chlorination. Under those circumstances, bromate levels increased significantly, in some cases exceeding 1 ppm after multiple dosing cycles.

Whether that is alarming depends on how those conditions translate to the field.

The study’s high-end results were produced under repeated, closely spaced superchlorination events designed to stress the system. That scenario may not reflect routine maintenance — but it is not unrealistic either.

Service professionals and homeowners can encounter similar conditions when dealing with persistent algae, unknown treatment history, or pools that have been heavily dosed over time. In those situations, repeated shocking — especially in water that already contains bromide — can create conditions where bromate accumulates more rapidly than expected.

That aligns with practical guidance. When asked how service professionals should respond, Hamilton said it is best to “follow recommended shock doses and avoid excessive shocking.”

What constitutes “excessive” is not a single number, but a pattern. Standard superchlorination typically ranges from about 5 to 10 ppm free chlorine for residential pools. Repeated high-dose applications in short succession — particularly without allowing levels to drop between treatments — are where the study suggests bromate formation can increase more significantly.

The increase observed in Phase III reflects repeated oxidation of the same bromide already present in the water. Each additional chlorine dose creates another opportunity for bromide to be further oxidized into bromate.

Even so, the overall conversion rates observed in the study remained well below earlier worst-case assumptions. Rather than approaching full conversion, observed rates generally ranged from low single digits to around 20 percent under most conditions tested.

The study also attempted to place those findings in a healthrisk context. Using the EPA’s SWIMODEL framework and a conservative exposure scenario, the authors calculated an excess lifetime cancer risk of approximately 4.58 × 10 -5 — or about 1 in 21,800.

In this context, “conservative” means the model assumes relatively high exposure — including frequent swimming, consistent ingestion of small amounts of water, and longterm use — to ensure the estimate errs on the side of caution.

That estimate is based on a modeled bromate concentration of approximately 0.47 ppm. For comparison, a typical sodium bromide treatment might introduce roughly 2 to 4 ppm of bromide into a pool. If 10 to 20 percent of that were converted, resulting bromate levels would fall in the range of roughly 0.2 to 0.6 ppm — consistent with the study’s modeled scenario.

Under those assumptions, the calculated risk falls within the EPA’s generally accepted range for carcinogens — 1 in 1,000,000 to 1 in 10,000 — though closer to the higher end than the lower.

Still, important questions remain. The study was funded by United Chemical, a manufacturer of sodium bromide-based products including Yellow Treat, a relationship disclosed in the report. It also excluded certain real-world variables — most notably cyanuric acid — and was conducted over a relatively short timeframe.

Hamilton acknowledged both the limitation and the company’s position, stating that the guiding principle behind the study was “don’t be like the cigarette companies,” while emphasizing that the data would be released regardless of outcome.

He also noted that pools using cyanuric acid would likely produce less bromate, because CYA slows the reaction between chlorine and bromide, and that the study was designed to represent a “worst-case” scenario.

What is clear is that the conversation is shifting.

The issue is no longer whether bromate can form. It can. The question now is how much forms under typical conditions — and whether that amount represents a manageable byproduct or a regulatory problem.

That answer will not come from a single study. But this one adds something the industry has lacked: field data under conditions that begin to resemble reality.

This is Part 1 of a two-part series examining the regulation of sodium bromide. In the next issue of Service Industry News, Part 2 will examine whether the science behind current restrictions tells the full story — or whether key pieces of that story are still missing.