Total exposure

The early concept was that these were values which must not be exceeded in other words, they were truly maximum values. As the understanding of limits and the development of the philosophy for such limits grew, it was realized that short-term exposures to somewhat higher concentrations could be permitted without undue harm if the total exposure during the day was sufficiently below the maximum limit. This led to the development of the concept of Threshold Limit Values (TLV s) by the ACGIH. 

Data files on the THERdbASE CD are 1990 Bureau of Census Population Information, California Adult Activity Pattern Study (1987-88), AT T-sponsored National Activity Pattern Study (1985), Chemical Agents from Sources, Chemical Agent Properties, Air Exchange Rates, Information from EPA s TEAM (Total Exposure Assessment Methodology) Studies, Information from EPA s NOPES (NonOccupational Pesticides Exposure Study) Studies, Information from EPA s AIRS (Aerometric Information Retrieval System), and Human Physiological Parameters. 

In densely populated areas, traffic is responsible for massive exhausts of nitrous oxides, soot, polyaromatic hydrocarbons, and carbon monoxide. Traffic emissions also markedly contribute to the formation of ozone in the lower parts of the atmosphere. In large cities, fine particle exposure causes excess mortality which varies between one and five percent in the general population. Contamination of the ground water reservoirs with organic solvents has caused concern in many countries due to the persistent nature of the pollution. A total exposure assessment that takes into consideration all exposures via all routes is a relatively new concept, the significance of which is rapidly increasing. 

Failure rates are computed by dividing the total number of failures for the equipment population under study by the equipment s total exposure hours (for time-related rates) or by the total demands upon the equipment (for demand-related rates). In plant operations, there are a large number of unmeasured and varying influences on both numerator and denominator throughout the study period or during data processing. Accordingly, a statistical approach is necessary to develop failure rates that represent the true values. 

Rates of equipment failure are calculated by dividing the number of failures for an equipment population by its total exposure hours or total number of demands. The following key types of information, therefore, are needed to develop plant-specific failure rate data ... 

Note that die total exposure hazard index is calculated separately for clironic, subchronic, and shorter-term exposure periods. 

The nuclear explosions that devastated Hiroshima and Nagasaki killed 100,000 to 200,000 people instantaneously. Probably an equal number died later, victims of the radiation released in those explosions. Millions of people were exposed to the radioactivity released by the accident at the Chernobyl nuclear power plant. The full health effects of that accident may never be known, but 31 people died of radiation sickness within a few weeks of the accident, and more than 2000 people have developed thyroid cancer through exposure to radioactive iodine released in the accident. Even low levels of radiation can cause health problems. For this reason, workers in facilities that use radioisotopes monitor their exposure to radiation continually, and they must be rotated to other duties if their total exposure exceeds prescribed levels. 

Total exposures vary considerably with human activities as well. Frequent flyers, for example, receive higher doses of radiation because the intensity of cosmic radiation is significantly greater at high altitude than it is at ground level. Residents in locations such as Montana and Idaho, where there are uranium deposits, receive higher doses of radiation from radon, one of the radioactive decay products of uranium. 

Wallace LA, Pellizzari ED, Hartwell TD, et al. 1986a. Total exposure assessment methodology (TEAM) study Personal exposures, indoor-outdoor relationships, and breath levels of volatile organic compounds in New Jersey. Environ Int 12 369-387. 

Wallace LA, Pellizzari ED, Sheldon L, et al. 1986d. The total exposure assessment methodology (TEAM) study Direct measurement of personal exposures through air and water for 600 residents of several U.S. cities. In Cohen Y, ed. Pollutants in a multimedia environment. New York, NY Plenum Publishing Corp., 289-315. 

Figure 1 Mass spectrometer (MS) intensities versus surface temp ature in a rqnesentadve temperature programmed desorpticm (TPD) profile of methyl on a clean Ni(lOO) surface. Total exposure was 1 L. Adsorption temperature was 105 K and heating rate was 3 K/s. Methane is produced near 225 K.

Note The values are not corrected for background or recovery. TX (total exposure) values represent the range of values of the total exposure obtained for each individual operator. The lower value represents one half the sum of the limits of detection. Individual operator results are provided in the next section. 

Human populations are likely to be exposed to a pollutant through more than one exposure route at a time. Total exposure may combine intake through ingestion of different substances, dermal absorption from surface water and water supply, and inhalation at different locations in the study area (e.g., work, home, recreational areas, commuting routes). Calculation of total exposure requires that the pharmacokinetics (absorption, metabolism, storage, excretion) for different exposure routes are understood for the pollutant of concern. Otherwise, only exposures by route can be combined. 

Particularly in the last two examples, fate models provide a useful tool not only for estimating concentrations, but also for tracing back the relative contributions of various sources to total exposure. 

Figure 3.25 A sequence of STM images taken at 673 K of oxygen spillover from Pd clusters in an ambient pressure of 5 x 10-8 mbar of 02 (doubled before image (f)).The total exposures to oxygen from image (a)-(f) were (in units of L) 114, 178, 237, 282, 344, and 531. The spillover begins with the formation of one layer...

Interroute extrapolation. The IEUBK Model includes an exposure module that simulates age-specific lead exposures via inhalation, and ingestion of lead in diet, dust, lead-based paint, soil, and water. The total exposure from each route is defined as the total lead uptake ( pg/day) over a 1-month period. Other routes of exposure may be simulated by the IEUBK Model pending available information from which to characterize both the exposure and media-specific absorption variables. Values for variables in the biokinetic component of the IEUBK Model are independent of the route of exposure. 

Angle CR, Marcus A, Cheng I-H, et al. 1984. Omaha childhood blood lead and environmental lead A linear total exposure model. Environ Res 35 160-170. 

McKone TE (1993) CalTOX, a multimedia total exposure model for hazardous-waste sites. Part I. Executive summary. A report written for The Office of Scientific Affairs Department of Toxic Substances Control California Environmental Protection Agency Sacramento, California by the Lawrence Livermore National Laboratory, Livermore... 

Interaction with a chemical indicator Can be highly specific, if suitable indicator. Can measure total exposure over time (dosimetry), if a non-reversible reaction is used. Can allow operation at a convenient wavelength, when gas has no convenient absorption in that spectral range. Poisoning can occur, and is easily fouled. Sensitive to groups of chemicals, e.g. acid gases, rather than to a specific gas. May exhibit non-reversible behaviour, which, in many cases, may be undesirable. May need water vapour present, to act as a catalyst, if dry reaction is too slow. 

Figure 6. UV absorption spectra before and after 20 msec exposure of a thin film of an equimolar TTDBM/SEGDVE mixture to a Fusion D bulb (I = 1.4 W cm"2 line speed = 45 m min"1 total exposure 28 mJ cm"2).

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