Pool sanitizer is any chemical or system that kills bacteria, viruses, algae, and other harmful organisms in swimming pool water. Chlorine is by far the most common type, but bromine, biguanide, and salt-based systems all serve the same core purpose: making the water safe to swim in. Understanding how these sanitizers work helps you choose the right one and keep it at effective levels.
How Pool Sanitizers Work
Every pool sanitizer does two jobs. First, it kills disease-causing organisms like E. coli, Streptococcus, and various viruses. Second, it oxidizes organic waste, breaking down things like sweat, urine, body oils, and sunscreen that swimmers constantly introduce into the water. Without a sanitizer performing both functions, pool water becomes a breeding ground for illness within hours on a warm day.
The killing action happens at the molecular level. In chlorine-based systems, the active ingredient (hypochlorous acid) is a small, neutrally charged molecule that slips through the outer membranes of bacteria and attacks their internal proteins, lipids, and DNA. This causes rapid, irreversible cell damage. Hypochlorous acid is roughly 80 times more effective at killing germs than the other form of chlorine (hypochlorite ion) that exists in the same water, which is why keeping your water chemistry balanced matters so much. The ratio between these two forms shifts with pH: as pH rises, more of your chlorine converts to the weaker form, and your sanitizer becomes less effective even though a test kit shows the same chlorine level.
Chlorine: The Standard Sanitizer
Chlorine dominates residential and commercial pools for good reason. It is inexpensive, widely available, and effective against a broad range of pathogens. You’ll find it sold as liquid sodium hypochlorite (essentially concentrated bleach), granular calcium hypochlorite, and stabilized tablets or sticks (trichlor and dichlor). All of these dissolve in water and produce hypochlorous acid as the active germ-killing agent.
The CDC recommends maintaining free chlorine between 1 and 4 parts per million (ppm) with a pH between 7.0 and 7.8. “Free chlorine” refers to the chlorine that is still available to kill germs, as opposed to chlorine that has already reacted with contaminants. Checking these levels at least twice per day is standard practice, and more often during heavy use. Calcium hypochlorite is also used for shock treatments, which involve adding a large dose all at once to clear algae blooms or reset water chemistry after a period of heavy contamination.
One important distinction: DPD test kits measure free chlorine specifically, telling you how much active sanitizer remains in the water. Older OTO kits only measure total chlorine, which includes spent chlorine that is no longer working. If you rely on an OTO kit, you could think your pool is properly sanitized when it isn’t.
What Chloramines Are and Why They Matter
When free chlorine reacts with nitrogen-based waste from swimmers (sweat, urine, skin cells), it forms compounds called chloramines, also known as combined chlorine. Chloramines are what produce the strong “pool smell” that most people mistakenly associate with clean water. That smell actually signals the opposite: it means chlorine is being used up by contaminants faster than it can sanitize.
Chloramines irritate the skin, eyes, and respiratory tract. They are heavier than air and settle just above the water’s surface, which is why indoor pools with poor ventilation can trigger coughing, red eyes, and nasal irritation in both swimmers and spectators. Most health departments limit combined chlorine to 0.4 ppm or less. You can calculate your combined chlorine by subtracting free chlorine from total chlorine. When it creeps above that threshold, the fix is superchlorination: raising the chlorine level high enough to break apart the chloramine molecules and convert them to nitrogen gas, which dissipates harmlessly.
Saltwater Pools Still Use Chlorine
A common misconception is that saltwater pools are chlorine-free. They aren’t. A saltwater system dissolves a small amount of salt (sodium chloride) into the pool water, then passes that water through an electrolytic cell. The cell uses electricity to split the salt molecules apart, releasing chlorine into the water. That chlorine then forms hypochlorous acid and sanitizes the pool the same way traditional chlorine does.
The practical difference is in delivery. Instead of manually adding chlorine tablets or liquid, the generator produces a steady, low-level supply automatically. This tends to keep chlorine levels more consistent throughout the day, and many pool owners find the water feels softer on the skin. The trade-off is that the electrolysis process makes the water more alkaline over time, so you’ll need to monitor and adjust pH more frequently. The generator cell typically lasts about five years before it needs replacement.
Bromine: Better for Hot Water
Bromine is the second most popular halogen sanitizer and is especially common in hot tubs and spas. It works through a similar mechanism to chlorine, producing hypobromous acid as its active killing agent. The CDC recommends maintaining bromine between 3 and 8 ppm, a higher range than chlorine because bromine is somewhat less potent on a per-molecule basis. In controlled comparisons, it takes roughly 1.0 ppm of liquid bromine to match 0.6 ppm of chlorine against E. coli, and about 2.0 ppm of bromine to match the same chlorine level against more resistant bacteria like Streptococcus faecalis.
Bromine’s main advantage is stability at higher temperatures. Chlorine breaks down quickly in hot water, which is why maintaining adequate levels in a 104°F hot tub with chlorine alone can be a constant battle. Bromine remains effective across a wider temperature and pH range. It also produces fewer volatile byproducts, so the air above a bromine-treated spa tends to be less irritating. The downside: bromine costs more, degrades rapidly in direct sunlight (making it impractical for most outdoor pools without a stabilizer), and cannot be stabilized the way chlorine can with cyanuric acid.
Biguanide: The Chlorine-Free Option
For swimmers who experience persistent skin or eye irritation from chlorine, biguanide-based sanitizers offer a halogen-free alternative. The active ingredient is polyhexamethylene biguanide (sold under brand names like Baquacil and SoftSwim). Rather than oxidizing organisms the way chlorine does, biguanide works by altering the surface of bacterial cells and disrupting their membranes.
Biguanide pools feel noticeably different. The water is silky, there is no chlorine smell, and eye irritation is minimal. However, biguanide has real limitations. It struggles against certain viruses, particularly non-enveloped types like adenovirus. Research has shown that biguanide can cause viral particles to clump together rather than fully destroying them, which may reduce its germ-killing effectiveness in some situations. Biguanide is also incompatible with chlorine, bromine, and most oxidizing chemicals, so switching to or from a biguanide system requires a complete water change and equipment cleaning. The ongoing chemical costs run higher than chlorine, and algae management can be more challenging since you need a separate hydrogen peroxide-based oxidizer.
UV, Ozone, and Advanced Oxidation
Ultraviolet light and ozone systems are supplemental sanitizers, meaning they work alongside a chemical residual (usually low-dose chlorine) rather than replacing it entirely. UV systems expose water to germicidal UV-C light as it passes through a chamber, damaging the DNA of microorganisms so they can no longer reproduce. Ozone generators inject ozone gas into the water, which is a powerful oxidizer that breaks down organic contaminants on contact.
When UV and ozone are combined in the same system, they produce hydroxyl radicals, one of the strongest oxidizing agents available. This combination, called advanced oxidation, destroys bacteria, viruses, and organic contaminants that chlorine alone handles slowly or incompletely. The practical benefit for pool owners is that you can run a much lower chlorine residual (often 0.5 ppm or less) while achieving cleaner water with fewer chemical byproducts. These systems are particularly valuable for addressing chlorine-resistant parasites like Cryptosporidium, which can survive for days in normally chlorinated water but are neutralized by UV exposure in seconds.
The limitation is that UV and ozone only treat water as it passes through the equipment. They leave no lasting residual in the pool itself, which is why a small amount of chemical sanitizer is still required to handle contamination between filtration cycles.
Keeping Sanitizer Levels Effective
Regardless of which sanitizer you use, the principles of maintenance are the same. Test your water at least twice daily during swimming season, using a DPD-based test kit that can distinguish free chlorine from combined chlorine. Keep pH in the 7.0 to 7.8 range, since sanitizer effectiveness drops sharply outside this window. In chlorine pools, cyanuric acid (stabilizer) protects chlorine from UV degradation in outdoor pools but should be kept below 50 ppm to avoid reducing chlorine’s killing power.
Swimmer load is the biggest variable in how quickly your sanitizer gets consumed. A pool party with 15 people will burn through chlorine far faster than a quiet afternoon with two swimmers. Testing after heavy use and adjusting accordingly prevents the gap between “the water looks fine” and “the water is actually safe,” which are not always the same thing.