Average daily exposure, long-term

These exposure histories can be used in a number of ways. The doses from all of the events in a day can be summed to give an estimate of the daily dose. The daily doses can be averaged to produce long-term estimates of dose (7day, seasonal, annual, and, in some cases, lifetime). The models can also be used to look at doses when the simulated individual was a child or during certain seasons of the person s life. 

It is difficult to determine how much "dirtiness" is too much. Nearly everyone would agree that if more than half of the daily exposures exceed the standard (e > %) or if the long-term average exposure i s greater than the standard (x > 1), then the workplace is too dirty. There is much less agreement on when an environment is clean enough to be considered acceptable. 

Figure 1. Nine charts showing how the probability density function, pdf(x), and the long-term average exposure, x, vary as a function of e, the fraction of daily exposures that exceed the standard and GSD, the variability of the work environment.

For any case-study built around Equation 1, we have to consider, for model input, parameters that provide emissions or environmental concentrations, intermedia transfer factors, ingestion (or other intake) rates, body weight, exposure frequency and exposure duration. For our specific case-study below, we are interested in concentrations in surface waters due to deposition from the atmosphere. The relevant intermedia transfer factor is the bioconcentration factor for fish concentration from surface water concentrations. The intake data we need are the magnitude and range of fish ingestion in our exposed population. Because PBLx is a persistent compound that accumulates in fat tissues, we will focus for this case not on exposure frequency and duration but on long-term average daily consumption. 

In contrast to acute or short-term estimates of exposure, for purposes of chronic or long-term exposure and risk assessments, such as evaluation of potential lifetime excess cancer risk, regulatory agencies typically set the averaging time at that of an average lifetime (e.g., 70 or 75 years times 365 days year ) to obtain the lifetime average daily dose. 

Trace dioxins have been detected in soil, sediment, vegetation, fruits, fish and mammalian tissne, and bovine milk. 2,3,7,8-snbstitnted isomers and other polychlorinated dibenzo-p-dioxin and polychlorinated diben-zofurans have been detected in the snrface samples at many PCBs disposal facilities. The major source of human exposure to dioxin is the food chain. Dioxins bioaccumulate in adipose tissue in humans. According to an estimate, the long-term average daily intake of dioxin in humans is 0.05 ng/day (Hattemer-Frey and Travis 1987). 

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