The appearance of dark or black discoloration in a hot tub is an unsettling discovery that immediately raises concerns about health and safety. This dark growth, often called “black mold” by the public, signals a severe lapse in water maintenance, creating an environment ripe for microbial growth. The elevated temperature and agitated water of a spa accelerate the development of these contaminants, transforming a relaxing amenity into a potential hazard. This discoloration indicates a complex, underlying issue within the plumbing and water chemistry that requires immediate attention and should not be ignored.
Identifying Hot Tub Contaminants
The black substance observed in a hot tub is typically not the household toxic mold, Stachybotrys chartarum, but rather a robust microbial community known as a biofilm. This biofilm is a slimy, protective matrix of various microorganisms that adhere firmly to surfaces, especially within the hidden plumbing and jet lines of the spa. The warm water and aeration from the jets provide an ideal habitat, allowing these organisms to multiply rapidly and shield themselves from standard sanitizers.
The dark coloration comes from a mix of bacteria, fungi, and algae, including significant bacterial populations like Pseudomonas or Legionella. When sanitation levels drop or organic matter from bathers is introduced, this dark film forms on the spa shell and inside the pipes, often appearing black or greenish-black. This buildup is concerning because the protective slime makes the embedded bacteria highly resistant to chlorine or bromine, effectively neutralizing the water’s sanitizing agents.
Specific Health Hazards of Exposure
Using a hot tub contaminated with a dark biofilm poses health risks, primarily through direct skin contact and the inhalation of contaminated water droplets. The most common danger is the risk of skin infections, particularly folliculitis, often called “hot tub rash.” This infection is caused by the Pseudomonas aeruginosa bacteria, which thrives in the warm water and biofilm and leads to an itchy, red rash with pus-filled bumps that develops after immersion.
More serious consequences arise from the aerosolization of water when the jets are running, releasing fine water mist into the air. If the biofilm contains Legionella bacteria, inhaling this mist can lead to respiratory illnesses such as Pontiac fever or the more severe Legionnaires’ disease. Prolonged exposure to aerosolized fungal elements and bacteria can also trigger hypersensitivity pneumonitis, a serious lung condition sometimes called “Hot Tub Lung,” especially in individuals with compromised immune systems. The presence of these contaminants can also cause generalized allergic reactions, leading to symptoms like a runny nose, eye irritation, or the exacerbation of asthma.
Immediate Steps for Cleaning and Removal
Once dark growth is spotted, the hot tub should be immediately taken out of use to prevent further health exposure and contamination. The first action is to treat the hidden plumbing lines, where the bulk of the biofilm resides, using a specialized plumbing purge product. This cleaner is added to the existing, contaminated water, and the jets are run on high for an extended period, often 30 minutes or more, to physically break apart the slime layer and release it into the water.
Draining and Surface Cleaning
After the purge is complete, the contaminated water must be completely drained from the tub. The visible surfaces, including the shell, headrests, and the underside of the cover, should be thoroughly scrubbed with a soft cloth and a spa-safe cleaner or a diluted bleach solution. The filters must also be removed and deeply cleaned with a dedicated filter cleaner or replaced entirely, as they are a prime location for microbial growth.
Refilling and Shocking
The final step is to refill the hot tub with fresh water and immediately “shock” the new water with a high dose of granular chlorine or bromine. This super-sanitization process helps to kill any remaining residual contaminants and re-establishes a proper sanitizer residual in the water. Consistent preventive maintenance, including weekly water testing and regular filter cleaning, is necessary to prevent the re-establishment of the dark, protective biofilm.