Pool sanitizer is any chemical or system that kills bacteria, viruses, and algae in swimming pool water, keeping it safe to swim in. Chlorine is by far the most common pool sanitizer, but bromine, salt chlorine generators, and chlorine-free options all serve the same basic purpose. The CDC recommends maintaining a free chlorine level of at least 1 ppm in pools and at least 3 ppm in hot tubs, with a pH between 7.0 and 7.8.
How Pool Sanitizers Work
Pool sanitizers do two jobs at once. They kill disease-causing organisms like bacteria and viruses, and they oxidize organic contaminants, breaking down things like sweat, body oils, sunscreen, and urine that swimmers introduce into the water. Without a sanitizer working continuously, a warm pool becomes a breeding ground for harmful microorganisms within hours.
The active form of chlorine, called free chlorine, handles both tasks. When chlorine reacts with contaminants, it gets “used up” and forms compounds called chloramines, sometimes called combined chlorine. Chloramines are what produce that strong “pool smell” many people associate with too much chlorine. Ironically, that smell means the pool needs more sanitizer, not less. At high enough levels, chloramines irritate eyes, skin, and airways. In one documented incident at an indoor hotel pool, children exposed to elevated chloramine levels experienced coughing (84% of cases), eye irritation (78%), and skin rash (34%). The optimal combined chlorine level is zero.
Chlorine: The Most Common Option
Chlorine comes in several forms, each with a different concentration of available chlorine and a different use case.
- Tablets (trichlor): The most popular choice for residential pools. These slow-dissolving tablets sit in a floating dispenser or an inline feeder and release chlorine steadily. They contain up to 90% available chlorine and include a built-in stabilizer that protects chlorine from sunlight breakdown. The CDC recommends keeping chlorine at 2 ppm or higher when using stabilized products like these.
- Granular (calcium hypochlorite): A powder commonly sold as “pool shock.” Standard formulations contain about 65% available chlorine, while higher-potency versions reach 73%. Granular chlorine dissolves quickly and is typically used for weekly shock treatments or to raise chlorine levels fast.
- Liquid (sodium hypochlorite): Essentially a stronger version of household bleach. Pool-grade liquid chlorine is standardized at 12.5% available chlorine, compared to the 5–6% found in household bleach. It works immediately but degrades quickly in sunlight, so it’s best added in the evening.
- Granular (dichlor): Contains 56–62% available chlorine depending on the formulation. Dichlor dissolves fast, is pH-neutral, and includes a stabilizer, making it popular for hot tubs and as a quick-dissolve shock.
Why pH Matters So Much
Chlorine’s germ-killing ability depends heavily on your pool’s pH. The active ingredient that actually disinfects, hypochlorous acid, exists in balance with a much weaker form. At a pH of 7.5, roughly half the chlorine in your pool is in its strong, active form. Let the pH drift up to 7.8 and only about 35% remains active. At 8.2, that drops to under 18%. You could have plenty of chlorine showing on a test strip and still have a poorly sanitized pool if the pH is too high. This is why testing pH is just as important as testing chlorine levels.
Salt Water Pools
Salt water pools still use chlorine as the sanitizer. The difference is how it gets there. A salt chlorine generator uses electrolysis to split dissolved salt (sodium chloride) into its components, releasing chlorine directly into the water. The salt then reforms and the cycle repeats, so you rarely need to add salt after the initial dose.
The salt concentration is low, typically 2,800 to 3,500 ppm, which is roughly one-tenth the salinity of ocean water. Most people describe it as barely noticeable. The generator cell typically lasts about five years before needing replacement. One quirk of salt systems is that the electrolysis process makes the water trend alkaline over time, so you’ll need to monitor and adjust pH more frequently than with a traditional chlorine pool.
Bromine
Bromine is the second most popular halogen sanitizer and is especially common in hot tubs and spas. It stays stable at higher temperatures where chlorine breaks down more quickly, and it works effectively across a broader pH range. This makes it lower-maintenance in heated water where pH can fluctuate.
Bromine also produces fewer harsh-smelling byproducts than chlorine, which matters in enclosed spaces like indoor hot tubs. The tradeoff is cost: bromine is more expensive than chlorine and degrades rapidly in direct sunlight, which is why it’s rarely used in outdoor pools without a cover.
Biguanide: A Chlorine-Free Alternative
Biguanide (sold under brand names like Baquacil and SoftSwim) is a non-halogen sanitizer registered with the EPA. It’s completely unrelated to chlorine or bromine and appeals to swimmers who are sensitive to those chemicals.
Biguanide has some genuine advantages. Its performance doesn’t depend on pH, it isn’t degraded by sunlight, and it stays stable in hot water. It doesn’t produce the eye and skin irritation associated with chlorine byproducts. However, biguanide can’t oxidize contaminants on its own, so you need to add hydrogen peroxide regularly as a separate oxidizer. You’ll also need a compatible algaecide since biguanide alone doesn’t fully prevent algae growth.
The biggest limitation is compatibility. Biguanide cannot be mixed with chlorine, bromine, copper-based algaecides, mineral systems, or salt chlorine generators. Switching a pool to or from biguanide requires a complete chemical transition. Filters also need more frequent cleaning because biguanide coagulates certain impurities into particles that clog filter media faster. Plan on chemically cleaning your filter at least twice per season.
Supplemental Sanitizing Systems
UV light and ozone generators don’t replace your primary sanitizer, but they reduce how much of it you need. Both are installed inline with your pool’s plumbing and treat water as it circulates through the equipment.
UV systems expose water to ultraviolet light, which destroys microorganisms without adding any chemicals to the pool. Ozone generators inject ozone gas into the water, a powerful oxidizer that breaks down contaminants and kills pathogens on contact. Both systems allow you to significantly reduce the amount of chlorine or bromine in the water, which means less chemical smell, less skin irritation, and lower ongoing chemical costs. You still need a residual level of sanitizer in the pool itself, since UV and ozone only treat water passing through the equipment at that moment.
Testing and Maintaining Sanitizer Levels
A pool’s sanitizer level changes constantly. Sunlight, heat, rain, swimmer load, and organic debris all consume free chlorine. Testing your water two to three times per week with test strips or a liquid test kit is the minimum for a residential pool. You’re checking three critical numbers: free chlorine (or your sanitizer of choice), pH, and combined chlorine.
For chlorine pools without stabilizer, the minimum free chlorine is 1 ppm. For pools using stabilized chlorine products like trichlor or dichlor, aim for at least 2 ppm because the stabilizer (cyanuric acid), while protecting chlorine from sunlight, also slightly reduces its killing power. Hot tubs need at least 3 ppm regardless. When combined chlorine rises above 0.5 ppm, it’s time to shock the pool, a process where you add a large dose of oxidizer to break apart those irritating chloramine compounds and restore the free chlorine level.