A positive Legionella result in Thailand: what to do in the first week

A positive result is common and manageable. Thai Department of Health sampling in 2025 found Legionella in 28% of the hotels it tested, and earlier surveys found 32% of hotels in the North-east (2012–13) and 44% of hotel cooling towers in Patong (2009–10). What separates a managed incident from a crisis is what happens in the first week, and whether it is written down.

Day 1: read the result before you act

Ask the laboratory for three things: the sampling point, the count, and the method. Counts in Thailand are reported in CFU per litre from a seven-day culture. The Thai Department of Health guideline treats anything under 100,000 CFU/L as “may not be satisfactory”, 100,000 to 1,000,000 as a potential hazard, and above 1,000,000 as a serious situation requiring shutdown. The European and UK benchmark is far stricter: above 1,000 CFU/L in a hot or cold water system calls for disinfection and resampling, and above 10,000 CFU/L in a cooling tower calls for immediate action. If European guests stay with you, work to the stricter figure.

Note which system the sample came from. A positive shower in one wing, a positive storage tank and a positive cooling tower are three different problems with three different fixes.

Day 1 to 2: isolate what you can, disinfect what is affected

Take out of service anything that can be isolated without cutting water to occupied rooms: a spa pool, a single cooling-tower cell, a decorative fountain, a disused wing. Then disinfect the affected system, not the whole building. The Thai Department of Disease Control protocol is 50 mg/L free chlorine held for at least one hour, or 20 mg/L for two hours, or thermal disinfection. Cooling towers are shock-dosed at 10 mg/L for three to six hours, drained, cleaned of scale and slime, refilled and re-dosed.

Keep a record of the dose, the contact time, the readings and who did it. Investigators and auditors will ask.

Day 2 to 5: fix the cause

A count is a symptom. The causes Thai investigators find again and again are:

• A chlorine dosing point that has failed: an uncovered rooftop doser degraded by sunlight, an auto-chlorinator broken for months, tablets dropped in by habit without a measurement.
• Hot water stored or returned below 50 °C, usually a heat pump set low to save energy.
• Storage tanks not cleaned for years, with sediment and biofilm.
• Rooms, wings or floors closed for the low season and never flushed.
• Groundwater or tanker water entering the system without disinfection.

Measure free chlorine from the incoming supply through the tanks to the furthest tap, and temperatures at the store, the return and the furthest rooms. The point where the residual or the temperature drops is usually the point where the bacteria grew.

Day 5 to 7: resample independently

Resample after disinfection, at the same points and at an accredited laboratory, with the sample taken by someone other than the contractor who treats the water. This is the recommendation Thai investigators made after the March 2025 Phuket investigation, where a certified hotel with two contractors testing regularly had clean results on file and heavily contaminated cooling towers. In Thailand the options include TEST TECH (ISO 11731:2017), IMC Thai (AS 3896:2017) and the Department of Medical Sciences’ Regional Medical Sciences Centers (CDC-based method). Ask for the laboratory’s certificate with the report.

The record that matters

By the end of the week you should have, on one or two pages: the original result; what was isolated and when; the disinfection method, dose, time and readings; the cause found and the fix made; the resample result with the laboratory named; and who did each step. That record answers the questions on the European two-week and six-week reports, satisfies a tour-operator auditor, and is the evidence an insurer or a claimant solicitor will ask for a year later.

Afterwards: make the next result a check, not a surprise

A positive result often points to a control scheme that did not exist or was not being followed. The fix is a risk assessment of the whole site and a written scheme: control limits, daily and weekly checks, a logbook, a named responsible person and a routine for low-season closure. With that in place, the six-monthly sample confirms what the logbook already shows.

If you have a positive result now, contact us. If you want the scheme in place before the next one, ask for a quote.

Sources

• Bureau of Environmental Health guideline
• Department of Health national study, Pensuk et al. 2026
• North-east hotel survey, JHS 2017
• Patong cooling-tower study, Yanpisitkoul & Srichai 2013
• Phuket March 2025 investigation, Tangkuha et al. 2026
• Department of Disease Control disease page
• ECDC Form A and Form B
• HSE HSG274