By Kelly A. Reynolds, MSPH, PhD

In a recent Drexel University news blog, Charles Haas, PhD, a well-known expert in waterborne disease and microbial risk assessment, posted a commentary on the need to consider aerosol transmission of water microbes, in addition to ingestion routes.1 Legionella cases (known to spread almost exclusively by water) have increased dramatically over recent years. This and other microbes that grow in the water distribution system (and are transmitted via the inhalation route) pose a unique challenge to maintaining tap water quality. The commentary called for an amendment to the 1974 Safe Drinking Water Act to include ‘Safe Breathing Water’ provisions.

Aerosol pathogens
Waterborne pathogens may be transmitted via ingestion, inhalation or direct contact. Microbes such as E. coli, Cryptosporidium and norovirus are examples of enteric waterborne pathogens that typically originate in animal or human feces, contaminate water from an external source and cause infection following ingestion. Control of waterborne pathogens largely involves elimination of the contaminant source followed by municipal treatment. Contamination post-treatment occurs if there is a breach in the distribution system, such as a leaky cross connection or broken pipe.

In contrast, microbes such as Legionella, mycobacteria, Acinetobacter baumannii and Aspergillus are examples of water-based pathogens. They are indigenous to their environment, meaning that water is a natural part of their life cycle, not a result of a contamination event. Municipal water treatment cannot fully manage water-based pathogens as they persist and grow within distribution systems (which includes residential plumbing and fixtures), protected by biofilms or within other organisms such as free-living amoeba. Many water-based pathogens are spread by inhalation, resulting in respiratory infections such as pneumonia. The elderly and immunocompromised populations are at increased risk of infection and more serious health outcomes, including hospitalization and death.

Aerosols are the suspension of inhalable droplets of water in the air. Droplets carrying infectious bacteria, fungi or viruses may become lodged in the respiratory tract of exposed individuals. A wide range of droplet sizes are inhalable but tap water particles are typically less than 10 microns. One study of Legionella-contaminated shower heads and hot-water faucets recovered the bacteria from nearby air spaces up to three feet away. When taps were running hot water, 6/19 (32 percent) of samples collected were positive for Legionella aerosols.2 Particle sizes ranged from one to eight microns.

Water-based outbreaks
Pneumonia is the leading cause of disease in the US and water-based agents such as Legionella species and Mycoplasma pneumoniae are major causative agents that are almost entirely transmitted by water. Legionnaire’s disease cases result in an estimated 8,000 to 18,000 hospitalizations per year in the US and 18,000 to 108,000 pneumonia cases requiring hospitalizations due to Mycoplasma pneumoniae.3 The estimated case fatality rate in patients with Legionnaire’s disease is five to 30 percent and higher.

During the most recent surveillance summary published by the CDC, 15 legionellosis outbreaks occurred in 2011-2012, resulting in 254 illness cases and 10 deaths.4 Sites included hotels, hospitals, long-term care facilities and an office building in addition to an industrial setting, mobile-home park and a resort. Ornamental fountains were the most common identified source. For 10 of the outbreaks, the water source could not be determined, sometimes because multiple water sources tested positive. Legionella’s ideal growth occurs in a temperature range of 35-46°C (95-114°F) but it can survive at 55°C (131°F). Thus, hot water should be maintained above 60°C (140°F) and not permitted to stagnate in the system.

Mycobacterium avium has been associated with hot-tub lung disease, producing a pulmonary hypersensitivity with symptoms of shortness of breath, fatigue, chills and cough. Like many water-based bacteria, it is resistant to common drinking water disinfectants, including chorine and UV light. This organism can grow in distribution biofilms despite residual chlorine use.

With the goal of prioritizing waterborne disease prevention efforts, one study looked at healthcare costs, as evidenced by health insurance claim databases from private insurance companies, Medicare and Medicaid patients.5 Costs incurred for treatment of 13,000 annual cases of Legionella and 16,000 annual cases of mycobacteria total $859 M. Such a significant health burden warrants additional safeguards.

Challenges in controlling aerosol transmission
Legionella concentration limits are not regulated by US EPA’s Safe Drinking Water Act. Instead, control of other microbes using specific treatment techniques is assumed to be effective for controlling Legionella in the source water. Unfortunately, water-based pathogens persist post-treatment. Control is typically accomplished by increasing the temperature of the hot water heater, reducing biofilms by flushing the system, addition of biocides or use of UV irradiation, filters or other interventions. Routine pipe maintenance and flow management aimed at elimination of stagnant zones or dead spaces are other important actions.

If not properly controlled, contaminated aerosol mists may be generated from cooling towers, hot tubs, humidifiers, water fountains, showers and other plumbing fixtures. Many studies have detected water-based pathogens in a variety of aerosols but finding Legionella in the distribution system does not necessarily correlate with the incidence of disease and may not be predictive of future cases.6 One study of single-family homes in Pittsburgh found that more than six percent (14/218) of homes tested positive but none of the residents showed elevated immunity antibodies in their blood, suggesting a lack of previous infection.7 Spread from water distribution systems to susceptible individuals appears to involve a number of confounding variables which are not fully understood.

In the absence of federal regulations, several states or counties and the CDC have established required or recommended monitoring of hospitals, particularly in wards with highly susceptible patients (i.e., transplant units). The benefit of routine monitoring for water-based pathogens, however, is not well-established, particularly in non-outbreak events. A better understanding of water systems, contaminant levels and correlated exposure risks or health outcomes is needed to create a more effective policy.

POU/POE safeguards
A number of studies have targeted the use of POU filtration devices for control of Legionella and other aerosolized, water-based pathogens, as reviewed in a recent US EPA draft document.8 POU filters reduced Pseudomonas cases from 2.71 per 100 patient days to zero in a Hungarian hospital with a history of repeated cases and eliminated both Legionella and Pseudomonas from tap water samples previously testing positive in 5/7 (71 percent) of taps.9 Similarly, POU filters reduced persistent Pseudomonas colonizations and infections in patients from an intensive care unit by 85 percent and 56 percent, respectively.10

Another study of single family residences in two German cities found that homes using instantaneous water heaters were free of Legionella but 12 percent (54/452) of homes with hot-water storage tanks tested positive.11 The evidence supported the use of POU filters to reduce the risk of community-acquired legionellosis from residential sources.

Evaluation of 594 water samples from seven hospital faucets showed 100 percent and 39 percent were positive for Legionella and Mycobacterium, respectively. POU filter devices with 0.2-micron filters effectively eliminated Legionella from the water samples, whereas not a single sample was positive post-filtration.12 Repeatedly, studies show that use of POU filters can help to minimize the risk of water-based pathogens and, given their ubiquitous nature, the use of these devices can provide significant cost savings and risk reductions to exposed populations at increased risk of infection.


  1. Haas, C. We need a safe ‘breathing water’ act. Drexel News Blog (2016). at <>
  2. Bollin, G. E., Plouffe, J. F., Para, M. F. and Hackman, B. Aerosols containing Legionella pneumophila generated by shower heads and hot-water faucets. Appl. Environ. Microbiol. 50, 1128–31 (1985).
  3. Marston, B. J. et al. Incidence of Community-Acquired Pneumonia Requiring Hospitalization. Arch. Intern. Med. 157, 1709 (1997).
  4. Beer, K. D. et al. Surveillance for Waterborne Disease Outbreaks Associated with Drinking Water–United States, 2011-2012. MMWR. Morb. Mortal. Wkly. Rep. 64, 842–8 (2015).
  5. Collier, S. A. et al. Direct healthcare costs of selected diseases primarily or partially transmitted by water. Epidemiol. Infect. 140, 2003–13 (2012).
  6. O’Neill, E. and Humphreys, H. Surveillance of hospital water and primary prevention of nosocomial legionellosis: what is the evidence? J. Hosp. Infect. 59, 273–279 (2005).
  7. Stout, J. E. et al. Legionella pneumophila in residential water supplies: environmental surveillance with clinical assessment for Legionnaires’ disease. Epidemiol. Infect. 109, 49–57 (1992).
  8. US EPA. Office of Ground Water. Draft–Technologies for Legionella Control: Scientific Literature Review. (2015).
  9. Barna, Z. et al. Infection control by point-of-use water filtration in an intensive care unit–a Hungarian case study. J. Water Health 12, 858–67 (2014).
  10. Trautmann, M., Halder, S., Hoegel, J., Royer, H. and Haller, M. Point-of-use water filtration reduces endemic Pseudomonas aeruginosa infections on a surgical intensive care unit. Am. J. Infect. Control 36, 421–9 (2008).
  11. Mathys, W., Stanke, J., Harmuth, M. and Junge-Mathys, E. Occurrence of Legionella in hot water systems of single-family residences in suburbs of two German cities with special reference to solar and district heating. Int. J. Hyg. Environ. Health 211, 179–185 (2008).
  12. Sheffer, P. J., Stout, J. E., Wagener, M. M. and Muder, R. R. Efficacy of new point-of-use water filter for preventing exposure to Legionella and waterborne bacteria. Am. J. Infect. Control 33, S20–5 (2005).

About the author
Reynolds_Kelly_mugDr. Kelly A. Reynolds is an Associate Professor at the University of Arizona College of Public Health. She holds a Master of Science Degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds is WC&P’s Public Health Editor and a former member of the Technical Review Committee. She can be reached via email at [email protected]


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