Legionella is a bacterium, not a virus, that proliferates in stagnant water at temperatures between 20°C and 50°C. It spreads through inhalation of contaminated aerosol produced by poorly maintained water or air-conditioning systems, and never from person to person. Summer is the highest-risk season: high temperatures, reduced water turnover, and irregular system maintenance favor bacterial growth in cooling towers, showers, spas, and centralized air-conditioning units.
Although the topic tends to come up mostly during the hottest months, prevention should be treated as an ongoing, year-round activity, especially in buildings with complex systems. Proper system management significantly reduces the risk of contamination and helps maintain high safety standards indoors.
What legionella is, and why air conditioning is being talked about these days
Legionella (Legionella pneumophila, the strain responsible for most cases) is an environmental bacterium naturally found in lakes and waterways at low concentrations, but which finds ideal conditions to multiply in artificial water systems. Its link to air conditioning stems from the fact that many systems use water circuits to dissipate heat. When these systems are not cleaned and checked regularly, they can become a favorable environment for bacterial growth and the spread of contaminated aerosol.
Legionella pneumophila: where it really proliferates
The bacterium does not live in the cold air emitted by air conditioners: it proliferates in stagnant water at lukewarm temperatures, typically between 20°C and 50°C — conditions typical of tanks, poorly circulated piping, and cooling towers that are not regularly disinfected. The risk therefore does not come from simply using the air conditioner, but from the water component of the system, when it is not properly maintained and periodically disinfected.
By contrast, household direct-expansion units, which have no water circuits for cooling, present a much lower risk. It is still advisable to regularly clean filters and condensate collection trays, ensuring efficient operation and good indoor air quality.
The Manhattan outbreak of July 2026 and its link to cooling systems
In mid-July 2026, a legionella outbreak hit Manhattan’s Upper East Side (the Carnegie Hill and Yorkville neighborhoods): 74 cases recorded as of 19 July, according to city health authorities, including 2 deaths, 8 people hospitalized, and 51 discharged. Investigations identified more than 20 buildings positive for the bacterium, including the Guggenheim Museum, tracing the outbreak’s origin to the cooling towers of centralized air-conditioning systems. This is not an isolated case: a similar outbreak a year earlier hit the Harlem neighborhood, with around a hundred cases and seven deaths. The pattern is consistent: high-density buildings, aging or poorly maintained cooling systems, summer season.
These episodes show how critical it is to quickly identify the source of contamination and carry out immediate remediation. Health authorities constantly monitor these events, because prompt maintenance and disinfection of systems is decisive in limiting new infections.
Why summer is the highest-risk season in Italy
Summer weather conditions favor legionella proliferation in water and air-conditioning systems, especially when maintenance is inadequate or systems sit unused for long periods.
Rarely used tanks, poor maintenance, stagnant water circulation
In the summer months, heavier use of air-conditioning systems, combined with seasonal heat, creates ideal water-temperature conditions for bacterial growth. Tanks and sections of piping rarely used during the year, such as those in tourist accommodations, are particularly at risk when restarted without preventive disinfection.
Reopening hotels, holiday villages, campsites, and similar facilities after long periods of inactivity requires specific sanitization procedures. Simply restarting systems without a preliminary check can, in fact, favor the presence of the bacterium in water circuits.
ISS data on legionellosis notifications
According to the latest publicly available data from Italy’s National Institute of Health, 3,111 cases of legionellosis were notified in Italy in 2022, up 14% from the previous year: a growth trend linked both to the wider use of air-conditioning systems and to improved diagnostic capacity. Summer months remain the ones with the highest incidence of notifications. Experts also point out that the actual number of infections could be higher than reported cases, since some mild forms are mistaken for other respiratory infections and are not always investigated with specific diagnostic tests.
How it spreads and who is most at risk
Legionellosis does not spread like a common respiratory infection. Understanding how it is transmitted and which groups are most vulnerable is essential for adopting effective prevention measures and recognizing symptoms promptly.
Inhalation of contaminated aerosol
Infection occurs exclusively through the respiratory route, by inhaling tiny droplets of contaminated water (aerosol) generated by showers, cooling towers, decorative fountains, whirlpool tubs, or misting systems. Drinking contaminated water does not cause infection.
It is important to remember that legionella does not spread through normal contact between people, nor by sharing spaces with someone who is ill. For this reason, prevention strategies focus on properly managing systems rather than isolating patients.
Vulnerable groups and forms of the disease
Legionellosis presents in two main forms: Pontiac fever, a mild, flu-like form that resolves on its own, and Legionnaires’ disease, a form of pneumonia that can be serious, especially in the elderly, smokers, people with chronic respiratory conditions, or a weakened immune system.
In more severe forms, symptoms can also include intense fatigue, confusion, gastrointestinal disturbances, and respiratory failure, requiring hospitalization. Early diagnosis and targeted antibiotic therapy improve the prognosis.
Prevention: regulatory requirements and best practices
Legionella prevention relies on regular checks, scheduled maintenance, and proper system management. Italian regulations set out specific procedures to reduce risk in the most exposed public and private facilities.
Periodic maintenance and disinfection of systems
In Italy, Ministry of Health guidelines for legionellosis prevention and control require facility managers (hospitality, healthcare, sports facilities, etc.) to follow maintenance protocols, periodic water sampling and disinfection of water networks and cooling towers. Beyond scheduled interventions, it is essential to keep up-to-date documentation of inspections, microbiological analyses, and maintenance activities carried out. This makes it possible to verify, over time, the effectiveness of the measures adopted and to respond quickly in the event of anomalies.
Monitoring air quality in at-risk environments
Alongside water system maintenance — which remains the first line of defense and the only measure able to address the source of the problem — monitoring and treating air quality indoors makes it possible to act as an additional layer of biological risk management.
Indoor air quality control also includes monitoring key parameters such as ventilation, humidity, temperature, and particulate concentration: factors that contribute to occupant wellbeing and help reduce overall risk.
As part of an integrated approach to healthier indoor environments, this is also where e4shieldtm technology comes in, contributing to active indoor air sanitization.
Frequently Asked Questions (FAQ)
What is legionella and how does it spread?
Legionella is a bacterium that spreads exclusively through the respiratory route, by inhaling contaminated water aerosol produced by poorly maintained water or air-conditioning systems. It does not spread by drinking contaminated water or from person to person.
Does air conditioning cause legionella?
Not the cold air itself, but the water component of the system: if the cooling towers or water circuits of centralized air-conditioning systems are not periodically disinfected, they can become a reservoir for the bacterium, which spreads through the aerosols the system generates.
What are the symptoms of legionellosis?
Legionellosis can present as Pontiac fever (mild, flu-like symptoms that resolve on their own within a few days) or as Legionnaires’ disease, a form of pneumonia that requires medical care and can be severe in vulnerable people: high fever, cough, breathing difficulties, muscle aches.
How is legionella prevented in air-conditioning systems?
Through periodic maintenance and water sampling, disinfection (thermal or chemical shock) of cooling towers and water networks, and compliance with Ministry of Health guidelines for managers of hospitality, healthcare, and sports facilities.
Is legionella a virus or a bacterium?
It is a bacterium (Legionella pneumophila being the most common strain), not a virus. The distinction matters because it changes prevention approaches compared with the viral respiratory infections covered in other blog articles.
Which environments are most at risk in summer?
Facilities with poorly maintained cooling towers and centralized air-conditioning systems, hotels and seasonal accommodations restarted without preventive disinfection, nursing homes, hospitals, gyms, and spas with whirlpool tubs.
