Green water in a swimming pool signals a severe failure in the water’s sanitation system. This discoloration is not merely an aesthetic issue but a visual symptom that the water chemistry has broken down, creating an environment where harmful microorganisms can thrive. A properly maintained pool relies on a precise balance of disinfectants and pH to remain safe; green water indicates this delicate balance is absent. Swimming in green pool water is highly discouraged, as the conditions that cause the color allow pathogens to multiply rapidly.
Immediate Safety Assessment
Green water confirms a significant loss of free chlorine, the active sanitizer required to neutralize bacteria and viruses. Free chlorine levels should be maintained between 1 and 3 parts per million (ppm) for constant disinfection, and a visible color change suggests this level is at or near zero. Without sufficient chlorine, the water cannot kill contaminants introduced by the environment or swimmers, exposing individuals to organisms that cause recreational water illnesses.
The color also presents a physical safety hazard by severely reducing visibility below the water line. Murky or dark green water makes it impossible to see the bottom of the pool, which is a significant risk for identifying a struggling swimmer or other emergencies. The ban remains in place until chemical testing confirms the required sanitation and pH levels have been restored and stabilized.
Identifying the Source of the Green Color
The most frequent cause of green pool water is an algae bloom, which occurs when disinfectant levels drop too low or the water’s pH balance shifts out of the ideal range of 7.4 to 7.6. Algae spores, carried by wind or rain, multiply quickly in warm water lacking adequate sanitizer, turning the water a cloudy, deep green. This growth directly consumes the remaining chlorine and creates a fertile breeding ground for other, more dangerous microorganisms.
A second, less common cause is the oxidation of dissolved heavy metals, primarily copper or iron, which can leach into the water from source water, corroded equipment, or certain algaecides. When these metals are exposed to chlorine, they precipitate out of the solution, often resulting in a bluish-green tint. Unlike algae, metal-stained water may remain relatively clear, but the presence of metals indicates a serious chemical imbalance that can stain pool surfaces and equipment.
A third possibility is an extremely high concentration of airborne pollen. Pollen can tint the water a yellowish-green, and the particles are often too small for standard filters to catch. Pollen itself is generally harmless to swimmers, but its presence can be mistaken for the more dangerous algae, necessitating professional chemical testing to differentiate the source.
Specific Health Consequences of Contaminated Water
The greatest health risk in green pool water stems from the proliferation of pathogenic bacteria and parasites. When sanitation fails, waterborne pathogens like Escherichia coli (E. coli), Giardia, and the highly chlorine-resistant Cryptosporidium can survive and multiply. Ingestion of this contaminated water is the primary route of infection, leading to severe gastrointestinal distress, including diarrhea, vomiting, and fever.
Beyond ingested pathogens, the environment of a green pool promotes surface infections. Contact with the water can result in skin irritations, rashes, and dermatitis, while bacteria entering the ear canal can lead to external ear infections. The presence of certain types of algae, specifically cyanobacteria (blue-green algae), can release potent toxins that pose serious risks to the liver and nervous system.
A chemically imbalanced pool also presents risks from irritants that are a byproduct of failed sanitation. When chlorine reacts with organic matter like sweat, urine, and cosmetics but does not fully oxidize them, it forms compounds called chloramines. These volatile compounds are responsible for the irritating chemical smell often associated with “dirty” pools. Chloramines cause eye redness, skin irritation, and respiratory problems, including coughing and asthma exacerbation.
Required Steps to Restore Water Safety
Restoring a green pool to a safe, swimmable condition requires a deliberate and aggressive chemical treatment protocol. The water must first be professionally tested to determine the exact levels of pH, alkalinity, cyanuric acid (stabilizer), and free chlorine. This precise chemical profile is required to calculate the necessary dosage for treatment, as simply guessing the amount of chemicals can be ineffective or even dangerous.
The restoration process must include super-chlorination, which involves adding a large, calculated dose of chlorine to kill all living organisms. This treatment is often followed by the addition of a potent algaecide to prevent immediate regrowth. Throughout this process, the pool filter must run continuously to remove the dead organisms and contaminants. Until subsequent testing confirms that the water has achieved a balanced pH and maintains the required minimum free chlorine residual, the pool remains a public health hazard and should not be used.