58% of Sampled Merchant Vessels Test Positive for Legionella

A 2018 study found Legionella in 58% of water samples taken from 360 non-passenger merchant vessels docked at UK ports — and 27% of those samples exceeded the upper action limit, the threshold demanding immediate remediation. For offshore and maritime crews, that number frames an uncomfortable truth: the water flowing from a shower dozens of miles into the Gulf of Mexico depends on a chain of filtration, disinfection, storage and monitoring that fails quietly when neglected.

Legionella pneumophila is the headline threat. It causes Legionnaires' disease, a severe pneumonia that kills approximately 10% of those infected, with workers over 50, smokers and the immunocompromised at heightened risk. The bacteria become dangerous when contaminated water is aerosolized — through showers, cooling towers or any spray-producing device — and inhaled. One contaminated system can sicken multiple workers simultaneously. The disease doesn't spread person to person, but with incubation running 2 to 14 days and symptoms including fever, cough, shortness of breath and muscle aches, an outbreak on a rig miles from shore, with limited medical facilities, can turn life-threatening fast.

How do offshore installations get their water?

Two ways: bunkering from shore, or producing it onboard. Bunkering loads water from land-based supplies via supply vessels and dedicated hoses — hoses that must be capped when idle and disinfected if unused for more than 14 days, with samples tested before transfer begins. Risk persists regardless: water that passed quality standards onshore can deteriorate in transit and storage if hoses aren't flushed or connections aren't sealed.

Reverse osmosis (RO) lets platforms and vessels make their own fresh water. Seawater, drawn from roughly 20 to 30 feet below the surface to dodge surface contaminants, passes through pre-treatment before high-pressure pumps force it through membranes that block salts, minerals, bacteria and viruses. Fresh water emerges on one side; brine discharges back to the sea. Modern offshore RO units produce anywhere from a few hundred to tens of thousands of gallons per day, consuming far less energy than older thermal distillation. But the systems are complex. Membrane fouling, mineral buildup and biological growth can all degrade performance and quality.

Where does the system break down?

Storage and distribution — this is where many problems begin. Tanks, ranging from a few thousand to tens of thousands of gallons, are commonly steel. Rust is the issue: as steel oxidizes, it consumes oxygen from the water and creates conditions favorable for bacterial growth, including Legionella. Newer installations use stainless steel, copper or plastic piping, but older platforms often run on decades-old steel pipe that demands constant maintenance.

Temperature control matters as much as materials. The CDC recommends hot water stored above 140°F and cold water stored and circulated below 77°F — though Legionella can grow at temperatures as low as 68°F. Chlorination is the primary defense, with offshore installations typically maintaining 0.2 to 0.5 milligrams per liter of chlorine. Holding those levels across a complex distribution system, especially one with dead legs or rarely used fixtures, requires diligent monitoring.

Beyond Legionella, coliform bacteria such as E. coli signal fecal contamination and gastroenteritis. Cruise ship norovirus grabs headlines, but gastrointestinal illness can devastate offshore crews living in close quarters, sharing facilities, far from medical support.

Who regulates this?

A patchwork. The U.S. Coast Guard holds primary responsibility for vessel sanitation and potable water quality, enforcing standards for production, storage and distribution and inspecting for compliance. The Maritime Labour Convention 2006, in force since 2013, requires under Regulation 3.2 that vessels provide adequate supplies of safe drinking water for consumption, food preparation and hygiene — water that must not cause immediate or long-term harm.

Fixed offshore platforms sit in a regulatory gray area. BSEE regulates offshore safety and environmental protection broadly, but potable water quality on fixed installations lacks the clear federal oversight that applies to vessels. Industry guidelines and company safety management systems must fill that gap — and often don't.

What should operators do now?

The prescribed playbook has four parts: mandatory, proactive water quality testing for Legionella, coliform, chemicals and other contaminants — not waiting until problems surface; systematic upgrades of aging steel piping and tanks, regular flushing of dead-end sections and infrequently used taps, and strict temperature control; training all workers on waterborne illness risks, hygiene and reporting procedures; and a comprehensive, regularly updated Water Safety Plan identifying every contamination point, with monitoring procedures and clear response protocols.

The technology exists. RO systems desalinate reliably, modern materials resist corrosion and bacterial colonization, and monitoring equipment detects contamination before it sickens anyone. What's missing, as the piece bluntly puts it, is the commitment to implement and maintain these systems rigorously over time — a gap the UK vessel data suggests remains wide across the merchant fleet.