Cooling Tower Best Practices: Recommendations from CDC, OSHA, CTI, and ASHRAE
Cooling towers are frequently listed by CDC among the water systems associated with Legionnaires’ disease outbreaks. CDC, ASHRAE, and OSHA each publish guidance on keeping them under control.
New York City has seen the stakes this year. A cluster on the Upper East Side, identified in early July 2026, reached 92 cases, 79 hospitalizations, and 7 deaths, and 59 of the 183 cooling towers the Health Department sampled tested positive for live Legionella. The city ordered 83 buildings to remediate their towers, and the source of the cluster had not been confirmed when the exposure period was declared over. In September, the city ordered cleaning and disinfection of cooling towers at 10 buildings in the South Bronx after another cluster. It follows the 2025 Central Harlem outbreak, which the Health Department traced to cooling towers at two city buildings and which caused 114 cases and 7 deaths.
Legionella grows best in water between 77°F and 113°F (25–45°C), a range many towers sit in for much of the year, and the bacteria spread through the fine mist a tower releases.
Start with the foundation: a written Water Management Program
A Water Management Program (WMP) is the foundation, and the first building block of Legionella control. Every best practice below builds on it. ASHRAE Standards 188 and 514 call for a program team and seven core elements: system descriptions with flow diagrams, a hazard analysis, control measures with limits and corrective actions, verification, validation, documentation, and communication. ASHRAE 188 focuses on Legionella, while 514 extends the framework to physical, chemical, and microbial hazards. CDC’s toolkit builds on 188 with site-specific log sheets, test procedures, and service reports for tracking and improving operation and maintenance.
New York City, New York State, and New Jersey each require a written program for cooling towers that builds on ASHRAE 188:
| Jurisdiction | Written program | Inspection and testing | Records and reporting |
|---|---|---|---|
| New York City | Maintenance program and plan consistent with ANSI/ASHRAE 188, certified annually by a qualified person (§8-03) | Weekly visual checks; qualified-person inspection at least every 90 days; Legionella sampling every 31 days (§8-04, §8-05) | Annual certification filed with the Department of Buildings; records kept at least 3 years; notify the Health Department within 24 hours of a result at or above 1,000 CFU/mL (§8-07) |
| New York State | Maintenance program and plan per ASHRAE 188 in place before startup (10 NYCRR 4-1.4) | Inspection before seasonal startup and every 90 days in use; sampling through ELAP-certified labs every 90 days (4-1.5, 4-1.8) | Registry registration; annual certification by November 1; notify the local health department within 24 hours of a result at or above 1,000 CFU/mL (4-1.3, 4-1.6, 4-1.8) |
| New Jersey | Written program under ASHRAE 188 for buildings with cooling towers or evaporative condensers (P.L.2024, c.66) | Follow guidance of Water Management Program and Legionella Sampling Plan | Records kept 5 years; public notice of compliance; civil penalties of up to $2,000 for a first violation |
Prevent stagnation and plan for shutdown
Stagnant water is where biofilm and Legionella take hold. CDC’s toolkit recommends several steps to avoid it:
- Flush low-flow pipes and dead legs at least weekly.
- Balance operating time among multiple cooling towers so no tower sits idle for long.
- Monitor for water service disruptions and have a response plan ready.
- Operate the system at the lowest water temperature practical, below the 77–113°F (25–45°C) growth range.
CTI also treats shutdown and startup (seven or more consecutive days idle) as a planned procedure in its Legionellosis Guideline, GDL-159:
- Wet lay-up (water left in the system): keep the microbial control program active and circulate water for at least an hour every 5 to 7 days with the fans off. Test with dip slides at least every other week.
- Dry lay-up (system drained): drain completely, then on restart clean the basin, refill, and start the microbial control program before the fans.
Before a wet lay-up system returns to service, run the recirculation pumps for at least three hours under the microbial control protocol before the fans come on, and return to normal operation once dip slides read 10,000 CFU/mL or less.
Clean and disinfect on a schedule
Scheduled cleaning removes the scale, sediment, and biofilm that shelter Legionella and make disinfectant less effective. OSHA recommends cleaning and disinfecting at least twice a year, typically before seasonal startup and after shutdown, with additional cleanings when biofouling is heavy. CDC recommends offline cleaning and disinfection at least once a year. CDC adds that cleaning should cover all accessible components, including fill pack, drift eliminators, basins, strainers, and bypass components.
Cleaning is also a worker-safety task. For routine maintenance, OSHA recommends at least an approved N95 respirator, chemical goggles or a face shield, gloves, and suitable clothing. For tower cleaning, it recommends a half-face air-purifying respirator with N-100 cartridges plus chemical protective equipment, along with hazard assessment, fit-testing, medical clearance, and PPE training.
Design for cleanability and match the biocide program to the water
OSHA and CDC recommend towers with sumps and drains, high-efficiency drift eliminators, and components be designed for easy cleaning and Legionella control. CTI’s guideline sets drift targets of 0.005% of circulating flow for crossflow towers and 0.002% for counterflow towers, basins sloped to a central drain, and siting away from building entrances and air intakes. CDC also recommends automating blowdown and the addition and monitoring of corrosion, scale, and disinfectant treatments.
Chlorine stays effective above 0.5 ppm when pH is below 8.0, bromine performs better at the alkaline pH common in cooling towers. CTI recommends continuous chlorine or bromine feed holding 0.5 to 1.0 ppm free residual in the return water, periodic nonoxidizing biocides to avoid resistance, and a biodispersant to improve biofilm penetration. It also describes periodic hyperhalogenation, for example 5 ppm for 6 hours, for heavy biofouling, extended stagnation, or HPC counts that regularly exceed 100,000 CFU/mL. Your program should reflect your water chemistry, not a default.
Test the water: HPC dip slides and Legionella sampling
Dip slides and Legionella sampling answer different questions, and a well-run program uses both. Heterotrophic plate count (HPC) dip slides give a quick read on general bacterial activity, and a rising count is an early sign that disinfectant control is slipping. Under NYC’s rules, an HPC result at or above 10,000 CFU/mL requires disinfectant monitoring three times daily until target residuals hold for 24 hours, a treatment adjustment, and a retest after 48 hours.
| Test | CTI guideline (voluntary) | NYC | NYS |
|---|---|---|---|
| HPC (dip slide) | Typically at least monthly; 10,000 CFU/mL or less is a generally accepted operating range; a rise of more than two logs should trigger a program review (§7.3.2) | At least weekly while operating; corrective action at 10,000 CFU/mL or higher (§8-05(f)(2), Table 8-2) | Bacteriological culture at intervals not exceeding 30 days (4-1.4(b)(1)) |
| Legionella culture | Frequency set by risk; once control is confirmed, at least once a year in the warmest three months, with quarterly or more frequent testing for ongoing confirmation (§7.3.4, §11.2.2) | At least monthly, no more than 31 days between samples; four action levels from 10 to 1,000 CFU/mL (§8-05(f)(3), Table 8-1) | At intervals not exceeding 90 days, and within 14 days of seasonal startup (4-1.4(b)(2)) |
For Legionella, CDC and ASHRAE Guideline 12 treat cooling tower growth below 10 CFU/mL as well controlled, and says any detection should trigger a review of sample collection, equipment, and the WMP.
CTI adds that an HPC result does not show whether Legionella is controlled, so it complements Legionella testing rather than replacing it. Send Legionella samples to a certified laboratory (CDC Elite, ISO, NY ELAP) and file results with your WMP records.
Plan your response before you need it
OSHA’s outbreak response guidance notes that most outbreaks tied to cooling towers involve concentrations of at least 1,000 CFU/mL. It recommends an emergency cleaning and disinfection of the entire cooling system, following CTI’s emergency disinfection procedure. CTI also calls for emergency disinfection above 1,000 CFU/mL or when Legionnaires’ disease is known or suspected to be associated with the tower. Write the plan down now, and decide what your action response levels are.
A quick self-check
- A written WMP and named team are in place and meet NYC, NYS, and NJ requirements where they apply.
- Disinfectant residual, pH, and temperature are monitored and logged routinely.
- HPC dip slides and Legionella cultures run on your jurisdiction’s schedule, with results logged against control limits.
- Offline cleaning and disinfection happen at least twice yearly per OSHA and at least annually per CDC.
- Visual inspections happen routinely, and low-flow pipes and dead legs are flushed weekly.
- Written shutdown and startup procedures cover wet and dry lay-up.
- Biocide choice fits your pH and water chemistry.
- Workers have the right PPE, fit-testing, and training.
- An outbreak response plan is written down.
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