Dosing rate estimation, worked example

Artificial sources of radiation are commonly used in industry, research, medicine, nuclear power plants (NPP), etc. Some workers are exposed to natural sources, for example, in mines and other conditions where the radon concentration in air might be higher than in normal cases. Relatively high dose rates are measured during air travel due to the elevated levels of cosmic rays at high altitudes. This means that many people are exposed in their work. Some of them are monitored individually, for example, by a small photographic film, thermoluminescent material, or portable electronic devices. These types of detectors on the body register the dose due to the external sources and yield an estimate of the dose received by the wearer. 

Finally, a chemical-specific dose is estimated for each receptor and complete exposure pathway identified in the conceptual site model. In this step, assumptions are made regarding the rate at which exposure could occur. For example, it is typically assumed that the average American adult drinks two liters of water daily from the tap, weighs 70 kg, and incidentally ingests 100 mg of soil each day. It is further assumed that a resident lives at the same house for thirty years, and a worker works at the same location for twenty-five years. These assumptions can change if there is more specific information available about the site. Using the gas station example, the owner of the station could be contacted to obtain records about the job duration for a typical employee, and the number of hours a typical employee is present at the station each day. 

The time necessary for removing one monolayer during a SIMS experiment depends not only on the sputter yield, but also on the type of sample under study. We will make an estimate for two extremes. First, the surface of a metal contains about 1015 atoms/cm2. If we use an ion beam with a current density of 1 nA/cm2, then we need some 150 000 s - about 40 h - to remove one monolayer if the sputter yield is 1, and 4 h if the sputter rate is 10. However, if we are working with polymers we need significantly lower ion doses to remove a monolayer. It is believed [4] that one impact of a primary ion affects an area of about 10 nm2, which is equivalent to a circle of about 3.5 nm diameter. Hence if the sample consists, for example, of a monolayer film of polymer material, a dose of 10n ions/cm2 could in principle be sufficient to remove or alter all material on the surface. With a current density of 1 nA this takes about 1500 s or 25 min only. For adsorbates such as CO adsorbed on a metal surface, we estimate that the monolayer lifetime is at least a factor of 10 higher than that for polymer samples. Thus for static SIMS, one needs primary ion current densities on the order of 1 nA/cm2 or less, and one should be able to collect all spectra of one sample within a quarter of an hour. 

Pharmacists and technicians play a major role in medication safety in modern pharmacy practice. After summarizing several studies performed in hospitals and long-term care facilities, Allan and Barker (1990) estimated that medication errors occur at a rate of about 1 per patient per day. In a more recent study performed in ambulatory pharmacies, they found an overall dispensing accuracy rate for prescription medications of 98.3 percent (Allan, Barker, and Carnahan, 2003). While most of these errors probably have minimal clinical relevance and do not affect patients adversely, many experts believe that medication error rates may be higher in the ambulatory care setting because errors may not always be evident to the health professionals who work there. For example, medication errors can occur when a patient purchases nonprescription medications without speaking with the pharmacist about any potential interactions with his or her prescription medications or if patients fail to verify the appropriate dose of the over-the-counter (OTC) medication. 

Human biological exposure indices are guidance levels of determinants for assessing worker dose from occupational exposures. They differ from other occupational exposure limits (OELs) for chemicals, which typically are measured in air, in that their determinants are measured in biological materials from the workers. BEIs consider the dose that has entered a worker s body by all routes. Thus, these measurements can provide more complete estimates of exposure, especially for chemicals that may be absorbed by routes other than inhalation and when inhalation rates are altered because, for example, of increased work rates. 

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