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An exposure assessment is a two part process that includes 1) evaluation of pathways that allow microorganisms to be transported from the source to the point of contact with human beings and 2) estimation of the amount of exposure that is possible between humans and contaminants. Conducting a realistic exposure assessment is important for good risk characterization as well as risk management.

The goal of chapter is to become familiar with fundamental computations for 1) changes in microbial concentrations, e.g. die-off and reduction, 2) exposure doses from different exposure routes, and 3) risk characterization and management.


Microbial concentrations may vary from the source to the point where a human may contact a pathogen. Many environmental factors, such as temperature, sunlight (ultraviolet (UV) radiation), moisture content, pH, salinity, nutrients, organics, and other chemicals, may affect microbial fate and transportation.

Some microorganisms may grow or survive longer in moisten and nutrient rich environments, e.g. foods and soils. Growth of microorganisms, e.g. Staphylococcus aureus, Salmonella, Clostridium perfringens, Campylobacter, Listeria monocytogenes, Vibrio parahaemolyticus, Bacillus cereus, and Entero-pathogenic Escherichia coli, cause food poisoning and this is a serious problem in the food industry. Regrowth of microbes, e.g. fecal coliform, E. coli, and enterococci in warm environments complicates the use of these microbes as fecal indicators of pollution for recreational water quality assessment. Lower temperatures may support microorganisms to survive longer.

Pathogens may decrease in concentrations due to dilution or natural die-off as they are released into different environments from the original host environments. Sunlight, dry, and low nutrients are major environmental factors that accelerate microbial die-off. Survival rates differ with microorganisms and environmental matrix (water, air, soil, and fomite). In general, microbes survive shorter in seawater compared with freshwater. Microbial inactivation rates in surface water are faster than groundwater.

Transportation of microorganisms can be affected by various environmental conditions, such as distance from the source to the exposure point, flow rate, exchange rate, suspended solid, permeability, and soil sorption. Fate and transportation models for water and air/fomites are discussed in Chapters 7 and 8.