Public Fountains Aerosolize Pathogens and Chemicals - and No Unified Safety Standards Govern Them

More than 100,000 public fountains operate worldwide, drawing an estimated 3 billion visitors annually. They cool urban spaces, reduce heat stress during hot weather, and provide sensory environments that draw people outdoors. Most cities treat them as amenities without systematic safety concerns.

An editorial published in Carbon Research by Professor Xiaohui Liu at the Ocean University of China argues that this comfortable assumption deserves scrutiny. Fountains, Liu contends, are not passive water features but active aerosol-generating systems that transform their water content into breathable particles and deliver it directly into the airways of nearby visitors - including approximately 900 million children annually, based on the estimate that 30% of fountain visitors are under 18.

From water to air, with concentration along the way

The specific concern is not that fountain water is dirty in the way a stagnant pond is dirty. It is that the process of atomization - converting water under pressure into fine droplets and aerosols - does not simply disperse contaminants at their source concentration. Certain compounds and microorganisms become concentrated during aerosolization. The aerosol phase can carry substantially higher loads of specific pollutants than the liquid source water.

Liu cites research suggesting that certain perfluoroalkyl acids - a class of synthetic compounds widely used in industrial applications and now found in water supplies globally - can reach air concentrations five orders of magnitude higher than their concentration in the raw fountain water. That is a factor of 100,000. Even if a contaminant is present in fountain water at levels below regulatory thresholds for drinking water, the aerosolization process could produce breathable air concentrations that pose a different order of risk.

The biological hazards are more familiar. Reclaimed water - treated wastewater that many cities use in non-potable applications including ornamental fountains and irrigation, in part for water conservation reasons - can harbor Legionella bacteria even after treatment. Legionella pneumophila, the organism responsible for Legionnaires' disease, thrives in warm water systems and is transmitted almost exclusively by inhalation of contaminated aerosols. Fountain mist creates precisely the conditions for that transmission. The editorial references a Legionnaires' disease cluster in New York in 2025 as an example of fountain-associated respiratory illness reaching clinical scale.

Why the risk has grown with water-saving policy

The increased use of reclaimed water and harvested rainwater in fountain systems, driven by conservation goals, has made the biological and chemical complexity of fountain water substantially greater than it was when these systems used treated municipal water directly. Reclaimed water undergoes treatment, but not to the standard of drinking water. It may contain residual pharmaceuticals, industrial compounds, agricultural chemicals, and microorganisms at levels acceptable for irrigation but not for systems that generate inhalable aerosols in public spaces.

Liu describes this as a cross-media threat - contaminants that exist below concern thresholds in water become a different category of risk when transferred to air. Regulatory frameworks built around water quality standards do not automatically address the aerosolization pathway. A substance that is acceptable in water at a given concentration may require a different standard entirely when the relevant exposure route is inhalation rather than ingestion.

Children as the primary exposed group

The editorial gives particular attention to children because of their differential vulnerability. Pound for pound, children breathe more air than adults - their inhalation rate relative to body mass is higher. They are also more likely to interact physically with fountain water: running through mist jets, touching wet surfaces, accidentally ingesting spray. These behaviors increase exposure across multiple pathways simultaneously.

With 3 billion annual fountain visitors and an estimated 30% being children, the potential scale of exposure is substantial. Even a small per-visit risk, multiplied across billions of visits, can produce meaningful population-level effects that would be difficult to attribute to fountain exposure in routine epidemiological surveillance.

What the editorial calls for

Liu proposes a four-part approach: regular water quality monitoring that covers both pathogens and chemical contaminants, engineering modifications to reduce unnecessary aerosolization, clear public guidance about risk especially for vulnerable populations, and international coordination to develop unified safety standards for urban water features.

The editorial is a call to attention rather than a primary research study - it synthesizes existing evidence and argues for a policy response, but does not present new epidemiological data. The claims about perfluoroalkyl acid concentration ratios and the attribution of specific outbreaks to fountain exposure draw on prior literature that varies in quality and specificity. The 3 billion annual visitors figure requires context: it includes brief passing exposure, not extended contact, which affects the risk calculation substantially.

The core observation is nevertheless worth taking seriously. Fountain systems in most jurisdictions fall between the regulatory gaps of drinking water standards and air quality standards. As urban water management increasingly relies on reclaimed sources, the gap between the chemistry of fountain water and the assumptions underlying any existing safety frameworks will widen.