<P> Survival of pathogens in waste materials, soil, or water, depends on many environmental factors including temperature, pH, organic matter content, moisture, exposure to light, and the presence of other organisms . Fecal material can be directly deposited, washed into waters by overland runoff, transported through the ground, or discharged to surface waters via sewer lines, pipes, or drainage tiles . Risk of exposure to humans requires: (1) pathogens to survive and be present; (2) pathogens to recreate in surface waters; and (3) individuals to come in contact with water for sufficient time, or ingest sufficient volumes of water to receive an infectious dose . Die - off rates of bacteria in the environment are often exponential, therefore, direct deposition of fecal material into waters generally contribute higher concentrations of pathogens than material that must be transported overland or through the subsurface . </P> <P> In general, children, the elderly, and immunocompromised individuals require a lower dose of a pathogenic organism in order to contract an infection . Presently there are very few studies which are able to quantify the amount of time people are likely to spend in recreational waters and how much water they are likely to ingest . In general, children swim more often, stay in the water longer, submerge their heads more often, and swallow more water . This makes people more fearful of water in the sea as more bacteria will be growing on and around them . </P> <P> Quantitative microbiological risk assessments (QMRAs) combine pathogen concentrations in water with dose - response relationships and data reflecting potential exposure to estimate the risk of infection . </P> <P> Data on water exposure are generally collected using questionnaires, but may also be determined from actual measurements of water ingested, or estimated from previously published data . Respondents are asked to report the frequency and timing and location of exposures, detailed information about the amount of water swallowed and head submersion, and basic demographic characteristics such as age, gender, socioeconomic status and family composition . Once sufficient data are collected and determined to be representative of the general population, they are usually fit with distributions, and these distribution parameters are then used in the risk assessment equations . Monitoring data representing occurrence of pathogens, direct measurement of pathogen concentrations, or estimations deriving pathogen concentrations from indicator bacteria concentrations, are also fit with distributions . Dose is calculated by multiplying the concentration of pathogens per volume by volume . Dose - responses can also be fit with a distribution . </P>

What is the danger from low dose organisms