www.epa.gov/research
technical BRIEF
BUILDING A SCIENTIFIC FOUNDATION FOR SOUND ENVIRONMENTAL DECISIONS
Assessment of Relative Potential for Biological Threat
Agent Exposure during Uses of Drinking Water
Deliberate or accidental contamination of drinking
water supplies could pose a major public health
risk for water customers. Models based on
HSRP's water security modeling and simulation
research indicate that pathogens (disease-causing
microbes) injected directly into the drinking water
system could result in significant exposures to
end-use water customers, especially from
ingestion.
Although potential exposure risks from ingestion of
contaminated water are well known, knowledge
gaps for non-ingestion hazards from inhalation or
skin contact are still being addressed.
U.S. EPA's Homeland Security Research Program
(HSRP) develops products based on scientific
research and technology evaluations. Our products
and expertise are widely used in preventing, preparing
for, and recovering from public health and
environmental emergencies that arise from terrorist
attacks. Our research and products address biological,
radiological, or chemical contaminants that could affect
indoor areas, outdoor areas, or water infrastructure.
HSRP provides these products, technical assistance,
and expertise to support EPA's roles and
responsibilities under the National Response
Framework, statutory requirements, and Homeland
Security Presidential Directives.
A screening level microbial exposure assessment was conducted to assist HSRP and other
stakeholders in better understanding potential exposures associated with pathogen
contamination of drinking water supplies (EPA, 2011 a). The results of the preliminary
assessment could be used to support the development of response strategies to protect human
health and the environment in the event of a contamination event.
The goals of the research included:
• Understanding potential drinking water exposure pathways during common
water uses
• Estimating exposure doses from ingestion and inhalation
• Developing a method for prioritizing exposure pathways that require
additional or future assessment
From a comprehensive list of possible exposure sources, a set of quantifiable pathways was
identified for assessment. The exposure pathways included but were not limited to:
• Ingestion from drinking water or from incidental ingestion while bathing,
showering or swimming
• Inhalation from exposure to devices including humidifiers, heating and air
conditioning systems, toilets, faucets, or hoses
• Inhalation from activities such showering, swimming, or watering lawns
U.S. Environmental Protection Agency
Office of Research and Development, Homeland Security Research Program
EPA/600/S-12/643
December 2012
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LEGIONELLA SPECIES
To exemplify the process utilized for the overall microbial exposure assessment, the methods,
model, and results for the Legionella species assessment are provided below. The Legionella
species are important waterborne pathogens in terms of disease morbidity and mortality.
Legionellosis, also known as Legionnaires' disease or Pontiac fever, is a waterborne disease
transmitted by inhalation of respirable water particles or aspiration of water contaminated with
Legionella species. The most common potential reservoirs identified for transmission of
legionellosis include heat-rejection equipment (e.g., cooling towers, evaporative condensers),
plumbing systems (e.g., showers, faucets, hot water tanks), nebulizers, humidifiers, whirlpool
spas, and public fountains.
This exposure assessment developed emission factors to model aerosolization, quantitatively
assess inhalation exposures of aerosolized Legionella species or Legionella species surrogates,
and rank six common in-home uses of water for potential Legionella species inhalation
exposure.
A targeted literature search was conducted to inform development of the conceptual model
(Figure 1), including identification of exposure pathways and input values for the exposure
assessment. Exposure pathways from in-home use of potable water that could be associated
with reported transmission of legionellosis or aerosolization of similar waterborne bacterial
pathogens were identified and evaluated. If Legionella species data were unavailable from the
published literature and respirable aerosol generation was documented, data for bacterial
surrogates were also considered. Exposure sources that could be considered common sources
for large numbers of individuals (e.g., cooling towers) were not evaluated in this exposure
assessment.
Source
Human Water
Uses
Contaminant
Release
Mechanism
Medium of Exposure
Receptor
•Cool Mist Humidifier
• Faucet
• Shower
•Therapy Pool
•Toilet
• Ultrasonic Humidifier
Figure 1. Conceptual model for inhalation of Legionella species from identified uses of
contaminated water.
Exposure doses were calculated using Equation 1. Events were not assessed cumulatively and
assumed to occur one time per day only (e.g., shower) or data were aggregated for multiple
events (e.g., toilet, faucet), if available.
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(1)
D = [EF x Cwater] xIRxETxCF
where:
D = Exposure Dose (Colony Forming Unit (CPU) or Direct Count)
EF = Emission Factor (L/m3)
Cwater = Water Concentration (CFU or Direct Count/L)
IR = Inhalation Rate (m3/minute)
ET = Exposure Time (minutes or hours)
CF = Conversion Factor (Factor of 60 minutes/hour to convert ET to minutes
when necessary)
Air and water concentration data identified from the literature search were used in the
development of emission factor values using Equation 2.
(2)
Ei ~ ("air ' Cwater
where:
EF = Emission Factor (L/m3)
CAir = Air Concentration (CFU or Direct Count/m3)
Cwater = Water Concentration (CFU or Direct Count/L)
The starting water concentrations were informed by the results of the literature search and
assumed the Legionella species concentration as measured at the point of use (e.g., at faucet,
showerhead). A low end starting water concentration value of 103 CFU/L was chosen based on
the reported level of Legionella species associated with human legionellosis identified in
retirement home nurses (Hautemaniere et al., 2011). The high end value of 10s CFU/L was
identified from the high value reported using culture-based measurements from monitoring of
hot and cold water outlets in six European countries (Lee et al., 2011).
Exposure assumptions for inhalation rate and exposure time were determined for an adult
mostly from the U.S. Environmental Protection Agency's Exposure Factors Handbook (EFH)
(EPA, 2011b) for each exposure pathway identified in the conceptual model shown in Figure 1.
Peer reviewed literature was consulted if values were not available in the EFH. If data were not
available in the peer reviewed literature or were of questionable quality, then best professional
judgment was used to select exposure assumptions.
Considerable variability in the calculated exposure dose was identified between the six identified
exposure pathways, with the doses differing by over five orders of magnitude in each of the
evaluated exposure scenarios (Table 1). The assessment of exposure pathways that have
been epidemiologically associated with legionellosis transmission (ultrasonic and cool mist
humidifiers) produced higher estimated inhalation exposure doses than pathways where
epidemiological evidence of transmission has been less strong (faucet and shower) or absent
(toilets and therapy pool). With consideration of the limited precision of the assessment
process, a relative ranking of exposure pathways from highest to lowest exposure doses was
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produced using culture-based measurement data. In this ranking, the ultrasonic and cool mist
humidifier exposure pathways were estimated to produce the highest exposure doses, followed
by the shower and faucet exposure pathways, and then the toilet and therapy pool exposure
pathways (Table 1).
Table 1. Daily calculated dose and by exposure pathway sorted from highest to lowest
values for an adult exposed to Legionella species or surrogates of Legionella species.
Pathway
Ultrasonic Humidifier
Cool Mist Humidifier
Shower
Faucet
Toilet
Therapy Pool
Shower
Therapy Pool
Inhalation Rate for
Central Tendency Exposure
Low Water
Concentration
(103CFU7L)
High Water
Concentration
(105CFU7L)
Inhalation Rate for
Reasonable Maximum Exposure
Low Water
Concentration
(103CFU7L)
High Water
Concentration
(105CFU/L)
Daily Calculated Dose for Culture-based Measures (CPU/Day)
8.1E+0
5.5E+0
8.8E-2
5.9E-2
1 7E-4
1.3E-5
8.1E+2
5.5E+2
8.8E+0
5.9E+0
1 7E-2
1.3E-3
3.2E+1
2.2E+1
2.6E-1
1.3E-1
3.4E-4
1 .6E-4
3.2E+3
2.2E+3
2.6E+1
1.3E+1
3.4E-2
1 .6E-2
Daily Calculated Dose for Direct Count (# Microorganisms/Day)
5.7E-1
6.4E-3
5.7E+1
6.4E-1
1.7E+0
8.0E-2
1.7E+2
8.0E+0
CPU - colony forming unit
EF - Emission Factor
OVERALL EXPOSURE ASSESSMENT FINDINGS
Although, the overall screening-level exposure assessment exampled with Legionella species
data above was associated with high levels of statistical uncertainty, some general conclusions
could be made.
1. Adult and child exposures with potential human health consequences can occur when a
drinking water system is contaminated with pathogens at concentrations similar or
greater to those evaluated in this exposure assessment.
2. Exposure can result from ingestion of water or from inhalation of aerosolized pathogens
from common water uses.
3. Exposure pathways that included the ingestion route of exposure consistently resulted in
the highest calculated exposure doses.
Identifying and addressing the major knowledge gaps remains an area of ongoing research
including:
• Refinement of exposure assessment methodologies, including exposure through skin
and eye contact
Understanding factors that affect the aerosolization of biological agents (e.g., room
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size, water pressure, air and water temperature)
• Understanding how aerosolization changes the viability of microbes and how that
affects the methods used to measure the number of microbes (e.g., if microbes enter
a viable-but-nonculturable state, how can the microbial exposure dose best be
measured?)
• Understanding how long microbes remain viable in contaminated drinking water and
in the specific devices that might produce contaminated aerosols (e.g., shower
heads, faucets, toilets, humidifiers)
CONTACT INFORMATION
For more information, visit the EPA Web site at www.epa.gov/nhsrc
Technical contact: Sarah Taft (taft.sarah@epa.gov)
General Feedback/Questions: Kathy Nickel (nickel.kathv@epa.gov)
REFERENCES CITED
Lee, J.V., Lai, S., Exner, M., Lenz, J., Gaia, V., Casati, S., Hartemann, P., Luck, C., Pangon, B., Ricci,
M.L., Scaturro, M., Fontana, S., Sabria, M., Sanchez, I., Assaf, S., Surman-Lee, S., 2011. An international
trial of quantitative PCR for monitoring Legionella in artificial water systems. Journal of Applied
Microbiology 110(4), 1032-1044.
Hautemaniere, A., Remen, T., Mathieu, T., Deloge-Abarkan, M., Hartemann, P., Zmirou-Navier, D., 2011.
Pontiac fever among retirement home nurses associated with airborne legionella. Journal of Hospital
Infection 78(4), 269-273.
U.S. Environmental Protection Agency. 2011 a. Microbial Exposure Assessment for Use of Water from
Contaminated Drinking Water Systems. FOR OFFICIAL USE ONLY (FOUO). Washington DC.
U.S. Environmental Protection Agency, 2011b. Exposure Factors Handbook: 2011 Edition. Washington,
DC, EPA/600/R-09/052F.
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