Exposure assessment

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Exposure at the simplest level is the dose of the pathogen that an individual ingests (eg. number of noroviruses), inhales (eg. numbers of bacteria cells of Legionella), or comes in contact with (eg. numbers of skin bacterial pathogens such as Staphylococcus). This number feeds into the dose-response models to predict the probability of infection. However exposure assessment is very complex and involves a combination of addressing the methods used to measure the microbes and the concentrations in the water or air for example, as well as the timing of the exposure. In most cases exposure can be viewed as a pathway from the source of the pathogen (eg shedding of pathogens by infected individuals, or concentrations in sewage) to the actual exposure (swimming at the beach). This also involves understanding the transport and survival of the microbe.

Exposure is often venue or media specific. Federal agencies such as EPA and FDA have developed long term programs to collect and accrue large amounts of data on populations exposures to drinking water and types of food often separated by age. EPA Exposure Factors Handbook

This section will begin to collect and present the parameters which are associated with exposures for water and fomites and will include pathogen specific parameters.

  • Pathogen excretion from infected individuals or populations
  • Pathogen occurrence and concentrations in various sources
  • Pathogen inactivation over time on various surfaces, in water
  • Amount of air inhaled, water ingested

Exposure Parameters

Drinking Water

Distribution Systems

Recreational Water


Fomites are any nonliving surface that can harbor a pathogen. Fomites are experienced and encountered everyday and are typically more commonly encountered than other exposure routes, such as drinking water. When developing a QMRA for fomites, there are a number of key pieces of information and data that need to be addressed. First the pathogen survival on fomites must be addressed, this available data that has been used in past QMRAs by CAMRA and non CAMRA investigators. In the fomite page the following parameters are summarized and described:

  • Survival of pathogens on fomites are as decay rates of the pathogens, this does not include chemical inactivation of the pathogens. This data is summarized for Category A agents at this moment.
  • Transfer efficiency from fomite to hands, in terms of percent of pathogens transferred from a fomite to a finger.
  • Transfer efficiency from hands to mouth, in terms of percent of microorganisms transferred from human fingers to the mouth.
  • Contact rates for humans, where the number of hand-to-face behavior is quantified

Indoor Air

Outdoor Air

Human Specific Parameters

Fecal Output

Table 1: Fecal Output for Different Populations
Population Age Number of Subjects Mean Standard Deviation References
Healthy people
Healthy Nigerian Children 6 months to 5 years 410 109.3 mL/day 54.07 Akinbami et al. 1995
Healthy Nigerian Adult 23-28 years 37 143.3 g/day 48.5 Ogunbiyi 1978
Healthy British Children and Adult 11-56 years 17 153 g/day 79 Davies et al. 1986
Healthy American Adult Adult 115 123.6 g/day 40.2 Rendtorff et al. 1967
Ill people
Indian Children with diarrheaa 3 to 24 months 70 126 g/kilogram of body weight/hour
during initial 24 hours and rehydration period
33.6 Dutta et al. 2000
Tunisian Adult with acute diarrheab over 18 years 70 499 g/day 284 Hamza et al. 1999
American Adult infected with Enteropathogenic Escherichia coli 18-40 years 11 406 g/day 300 Donnenberg 1993

a In this study, a high isolation rate of different enteropathogens, e.g. Shigella flexneri, Shigella boydii,Salmonella typhimurium, Enteroagrregative E. coli, Enteropathogenic E. coli, Aeromonus sp. and rotavirus was observed.
b Shigella, E. coli, Proteus, Pseudomonas aeruginosa were identified from the subjects.

Akinbami, F., Erinoso, O. and Akinwolere, O. (1995) Defaecation pattern and intestinal transit in nigerian children. African Journal of Medicine and Medical Sciences 24, 337-341. Abstract

Dutta, P., Dutta, S., Manna, B., Chatterjee, M. and De, A. (2000) Hypo-osmolar oral rehydration salts solution in dehydrating persistent diarrhoea in children: Double-blind, randomized, controlled clinical trial. Acta Paediatrica 89, 411-416. Full text

Hamza, H., Ben Khalifa, H., Baumer, P., Berard, H. and Lecomte, J.M. (1999) Racecadotril versus placebo in the treatment of acute diarrhoea in adults. Alimentary Pharmacology and Therapeutics 13, 15-19. Full text

Ogunbiyi, T.A. (1978) Whole-gut transit rates and wet stool weight in an urban Nigerian population. World Journal of Surgery 2(3), 387-392. Full text

Pathogen Excretion Rate

Table 2. Concentration of microbes in stools of an infected person and incidence of microbes in the United States
Organism Concentration in stool per gram Reference Incidence(%) Notes Reference
Giardia 1-5*106 Jakubowski, 1984 3.8 All age groups Howell and Waldron, 1978
18-26 Children in day care centers Sealy and Shuman, 1983
29-54 Developed countries Black et al, 1977
Cryptosporidium 106-107 Robertson, 1994 0.6-20 Children in day care centers Soave and Weikel, 1990
27-50 Ungar, 1990
Hepatitis A 108 Coulepis, 1980 0.0097 Reported cases of clinical illness MMWR, 1988
8.2 Occurrence of virus in stools of healthy person DeFilippes et al, 1987
Rotavirus 1010-1012 Flewett, 1982 10.4 Annual rates of clinical infection Monto et al, 1983
29 Children under 2 years of age Champsaur et al, 1984
Poliovirus 103-106.5 Melnick and Rennick, 1980
Coxsackie and echo virus 102-105.5(max. 107.2) Melnick and Rennick, 1980
Fecal coliform 107-109 Feachem et al, 1983
Enterovirus 10 Occurrence in fecally soiled diapers Peterson, 1972; 1974
30-40 During the summer months Fox and Hall, 1980

Black R.E., Dykes A.C, Sinclair S.A., and Wells J.G. (1977) Giardiasis in day-care centers: evidence of person to person transmission. Pediatrics. 60: 193-197. Full text

Champsaur H. Questiaux E., Prevot J. et al. (1984) Rotavirus carriage, asymptomatic infection, and disease in the first years of life. I. Virus shedding. J Infect Dis. 149: 667-674. Full text

Coulepis A.G., Locarnini S.A., Westway E.G., Tannock G.A., Gust I.D. Biophysical and biochemical characterization of hepatitis A virus. J Infect Dis 141: 151-156. Summary

Feachem R.G., Bradley D.J., Garelick H. , Mara D.D.. 1983. Sanitation and disease (John Wiley and Sons, Inc. NY)

Flewett T.H.. Clinical features of rotavirus infections, in virus infections of the gastrointestinal tract, edited by D.A.J. Tyrell, A.Z. Kapikian (Marcel Dekker Inc. NY, 1982). 125-146.

Fox J.P., Hall, C.E. Viruses in families(PSG Publishing Company, Inc. Littleton, MA. 1980).

Howell R.T.k, Waldron B.S.. Intestinal parasites in Arkansas. J Arkansas Med Soc. 75: 212-214.

Jakubowski W. Detection of Giardia cysts in drinking water: State-of-the-art, in Giardia and Giardiasis, Biology, Pathogenesis and Epidemiology, (Plenum Press, NY, 1984). 263-285.

Melnick J.L. (1957) Special publication of the New York Acad of Science. 5: 365-381.

Melnick J.L., Rennick V. (1980) Infectivity of enterovirus as found in human stools. J Med Virology. 5: 205-220. Full text

Monto A.S., Koopman J.S. Longini I.M., Isaacson R.E.. (1983) The Tecumseh study XII. Enteric agents in the community, 1976-1981. J Infect Dis. 148: 284-291. Full text

MMWR. (1988) Morbidity and mortality weekly, summary-cases of specific notifiable diseases, United States. MMWR. 36: 840

Peterson M.L. (1972) The ocurrence and survival of viruses in municipal solid waste. Dissertation abstracts. 33/3, 2232-B-2233-B. Ann Arbor, MI.

Peterson M.L. (1974) Soiled disposable diapers: A potential source of viruses. Amer J Public Hlth. 64: 912-914. Full text

Robertson L.J., Smith H.V., Paton C.A.. Occurrence of Giardia and Cryptospordidium oocysts in sewage effluent in six sewage plants in Scotland and the prevalence of cryptosporidiosis and giardiasis diagnosed in the communities served by those plants, in Protozoan Parasites in Water.. 45-49

Sealy D.P, Shuman S.H. (1983) Endemic giardiasis and day care. Pediatrics. 72: 154-158. Full text

Soave R., Weikel C.S.. Cryptosporidum and other protozoa including Isospora, Sacryocysts, Blantidium coli and Blastocysts, in Principles and Practice of Infectious Disease. (Churchill Livingstone Inc., NY, 1990) 2122-2130

Ungar B.L. Cryptosporidiosis in humans (homo sapiens), in Cryptosporidiosis in man and animals. (CRC Press, Boca Raton, FL, 1990). 59-82


Pathogen Excretion Rate

Droplet Spray

Besides exposure assessment, the other major components of microbial risk assessment are hazard identification, dose response assessment, and risk characterization