Duration and Frequency of Exposure

In order to assess the potential extent of human exposures and health effects, members of dairy farm families who consumed raw dairy products known to be contaminated with heptachlor epoxide were studied (Stehr-Green et al. 1986). These individuals and an unexposed urban reference population were compared with regard to serum pesticide levels and liver toxicity. The farm family members had significantly higher mean serum levels of heptachlor epoxide (0.81 0.94 ppb), oxychlordane (0.70 0.75 ppb), and transnonachlor (0.79 0.60 ppb) than the unexposed population. This study is limited because exposure level, duration, and frequency of exposure are not known. There was no increase in prevalence of abnormal liver function tests in the dairy farm families... 

Studies using an appropriate route, duration, and frequency of exposure in relation to the expected route(s), and frequency and duration of human exposure have greater weight. 

Exposure is considered as single events, or series of repeated events, or as continuous exposure. The duration and frequency of exposure, the routes of exposure, human habits and practices, as well as the technological processes need to be considered. Furthermore, the spatial scale of the exposure (e.g., personal/local/regional level) has to be taken into account. 

If the hazard assessment indicates that the compound is potentially hazardous, the next step is to evaluate the various possibilities for exposure. What is the most likely route of exposure oral, inhalation or skin How much absorption is expected from the different routes of exposure Information is also needed on amount, duration, and frequency of exposure. Is exposure occurring in the home, workplace, school, or other areas This information helps to define the population of concern. Exposure information may also be important for designing appropriate studies on hazard assessment and certainly for the next step of establishing dose-response relationships. 

When choosing a study for applications in occupational toxicology, it is important that the exposure protocol be relevant to the exposure scenario in the workplace. The route, duration, and frequency of exposure can have a significant effect on the toxicity of a xenobiotic agent. 

It is of interest that the same person may have an immediate-onset response on one occasion, a delayed-onset reaction on another, and, under other exposure conditions, exhibit a dual response starting with immediate-onset symptoms that resolve within an hour and followed several hours later by a second set of symptoms. The underlying mechanisms for such effects are not known. However, clinical and experimental evidence has indicated that this process is like many other toxicologic effects in that the response is related to concentration, duration, and frequency of exposure. 

This process helps set up a system of pathways from the source of contamination, to the mechanism of transport through environmental media, to routes of exposure, and to the exposed or potentially exposed population. The duration and frequency of exposure, chemical classes associated, and the significance of each pathway are documented as well. 

Duration and frequency of exposure, chronic criteria, acute and chronic measurements, types of exposure, and information available in IRIS database. 

Exposure assessment estimates the number of exposed persons together with the magnitude, duration and frequency of exposure. A direct possibility to measure the human exposure to toxic substances via ambient air, for example, is the utilization of personal monitors. 

JACOBSON The lettuce plants in the greenhouse were exposed to rain from a nozzle which simulated both the intensity and the droplet diameter of ambient rainfall. We found in previous experiments that it s not only the acidity of rain that s important in determining the development of foliar lesions or growth effects in addition, it s the duration and frequency of exposure, the intensity of rainfall, and, of course, the cultural conditions of the plant, which change their predisposition to injury. 

It s use results in a duration and frequency of exposure which is not greater than the range of exposures that could be reasonably experienced by consumers when used for the purpose intended. 

Employees who are required to work with hazardous chemicals in a manner that result in a duration and frequency of exposure greater than what a normal consumer would experience have a right to know about the hazards of those chemicals. In these cases, the HazCom standard treats those consumer products as any other workplace chemical hazard. 

Tabie 5.103 provides a basic overview of the resistance of elastomers. The supplement to DIN EN 12115 specifies chemical resistance of elastomers used as inner layers of hoses. Hose properties can be influenced by the type of chemical, temperature, flow rate, abrasion, duration, and frequency of exposure and by contaminants in the chemicals conveyed by the hose. Thus, the resistance data in Table A.41 in the Appendix are only guidelines with limited temporal validity. In essence, they are based on laboratory tests by hose manufacturers that are usually performed on test sheet at room temperature, as well as based on data in the literature and from experience. Chemical suitability tests were not performed in every case. 

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