Carbon monoxide hazards in pool and spa systems share one critical trait: When protection fails, there is no warning.

According to the Centers for Disease Control and Prevention, carbon monoxide poisoning causes more than 400 deaths and over 100,000 emergency department visits each year in the United States. Most cases are tied to fuel-burning appliances — furnaces, generators, and heaters.

Pool and spa heaters represent a much smaller subset, but when they are involved, the incidents are often severe because exposure occurs in enclosed or semi-enclosed environments where occupants are unaware of the danger.

Dangerous concentrations of carbon monoxide can accumulate in minutes, and occupants may become disoriented or unconscious before recognizing what is happening. Survivors do not always recover fully. Carbon monoxide deprives the brain of oxygen, and long-term effects can include memory loss, cognitive impairment, headaches, dizziness, and in severe cases permanent neurological damage.

Carbon monoxide (CO) is colorless, odorless, and non-irritating. It gives no signal before it incapacitates. In pool and spa environments, it is produced by gas-fired heaters — natural gas or propane — whenever combustion gases are not properly vented away from occupied spaces.

The safety layers are well established. Heaters must be installed with proper clearances from structures and openings. Exhaust must be directed away from air intakes. Fuel type must match the appliance configuration. Ventilation must prevent accumulation. In occupied structures, CO detection provides a final layer of warning. These requirements are reflected in standards such as the International Fuel Gas Code and NFPA 54, as well as manufacturer installation instructions.

The following recent incidents show what happens when those layers fail.

In June 2025, three members of a family — a mother and her two sons — were found dead inside a home in Nacogdoches, Texas. Investigators traced the source of the carbon monoxide to a newly installed pool heater.

The heater had been configured incorrectly for the fuel supply — set up for natural gas but connected to propane — a mismatch that can dramatically increase carbon monoxide production and directly violates manufacturer specifications and fuel gas code requirements. At the same time, the unit had been installed too close to the home, positioned beneath fresh-air intake vents — a placement that runs counter to basic clearance and exhaust separation requirements. Deficiencies in the home’s duct system then allowed exhaust gases to be pulled inside and distributed throughout the house.

The heater ran overnight. The occupants never woke up.

This was not a single mistake. It was a sequence of failures: wrong fuel setup, improper placement, and no effective barrier between exhaust and the home’s air system.

What should have been done is straightforward and nonnegotiable. Fuel type must be verified at installation — physically confirmed, not assumed — including correct orifice sizing and regulator configuration. Heater placement must meet clearance requirements from walls, windows, and especially air intakes. Exhaust discharge must be evaluated in relation to how the building actually moves air, not just where the unit sits on a pad.

And one step that is often skipped: the heater should be verified under operation before being left in service. That means confirming proper combustion and ensuring exhaust gases are dispersing safely — not being pulled back toward the structure. Standing at the exhaust termination while the heater is running will often reveal problems that are invisible on paper.

Every one of these conditions was visible and preventable before the system was put into operation.

In one reported 2025 incident at a Midwestern hotel, guests were evacuated after symptoms consistent with carbon monoxide exposure were