Cleaner Air May Be Fueling the Rise in Legionnaires’ Disease, Study Finds

A puzzling and rapid increase in Legionnaires’ disease—a severe bacterial lung infection—may be linked to improvements in air quality, according to a new U.S. study analyzing trends in sulfur dioxide (SO₂) pollution.

Researchers from two U.S. universities and the New York State Department of Health set out to investigate the long-term rise in Legionnaires’ disease, a form of pneumonia caused by Legionella bacteria. Their study focused on identifying environmental factors that could explain this growing public health concern.

Legionnaires’ Disease on the Rise

Since 2000, the incidence of Legionnaires’ disease in the U.S. has surged nine-fold, rising from approximately 1,100 cases to nearly 10,000 in 2018. Similar spikes have been reported across Europe and parts of Canada, with cases increasing five- to seven-fold.

Historically, major outbreaks of Legionnaires’ disease—first identified in 1976—have been linked to contaminated cooling towers, air conditioning units, and commercial ventilation systems. These systems can disperse airborne water droplets carrying Legionella, allowing the bacteria to spread over considerable distances. However, in the majority of sporadic cases, no clear source of infection has been identified.

Given this uncertainty, the research team sought to determine whether broader environmental conditions could be influencing Legionella proliferation.

The Role of Sulfur Dioxide in Legionella Growth

New York state emerged as a focal point for the study, as it reported the highest number of Legionnaires’ disease cases in CDC data from 1992 to 2019. Initial analyses of climate variables—such as humidity, temperature, precipitation, and UV radiation—failed to account for the dramatic rise in cases. However, a surprising trend emerged: as atmospheric sulfur dioxide (SO₂) levels declined over the past two decades, Legionnaires’ disease cases increased at a similar rate.

Using a model of water-based chemistry, researchers demonstrated that SO₂ in the air can dissolve into water droplets and convert into sulfuric acid, increasing acidity and creating an inhospitable environment for Legionella bacteria. As SO₂ emissions dropped, water droplets in the air became significantly less acidic—by at least a factor of ten—potentially allowing Legionella to survive longer and spread more easily through cooling towers and ventilation systems.

Intriguingly, the study also found a one-week lag between declines in SO₂ levels and increases in Legionnaires’ disease cases—closely matching the incubation period from exposure to symptom onset.

Proximity to Cooling Towers as a Key Factor

Further analysis revealed a strong geographic correlation between Legionnaires’ disease cases and cooling tower locations in New York state. Given that cooling towers generate large volumes of water vapor, researchers suggest that less acidic conditions may be increasing Legionella’s ability to persist in the air and infect those who inhale contaminated droplets.

This study highlights the complex, unintended consequences of environmental changes on public health. While reducing air pollution remains a critical goal, understanding its broader impacts—including potential risks for waterborne diseases—will be essential in shaping future public health strategies.

Protect Your Facility from Legionella Growth

Legionnaires’ disease remains a serious risk for commercial and industrial facilities, particularly those with cooling towers, HVAC systems, or water distribution networks. Implementing a comprehensive Legionella water management plan can help mitigate risks and ensure compliance with industry regulations. Contact our team today to learn how our advanced water treatment solutions can help safeguard your facility.