Exposure frequency interval

In general, chronic toxic effects occur when the agent accumulates in the biological system, when an agent produces irreversible toxic effects, or when there is insufficient time for the system to recover from the toxic effect within the exposure frequency interval. For many agents the toxic effects of acute exposure are quite different from those produced by chronic exposure. According to the site of action, chronic toxic effects can be divided into two groups local, and systemic effects. For some substances both local and systemic effects can be observed. 

When there is insufficient time for the system to recover from the toxic effect within the exposure frequency interval... 

Eyes have the greatest danger from microwaves. Microwaves often have a cumulative effect on the lens of the eyes, ultimately producing cataracts. The onset of cataracts depends on frequency, power density, duration of exposure, and intervals between exposures. There is some evidence that microwaves also affect the central nervous system in various ways. They also affect the performance of some types of cardiac pacemakers. 

In Figure 3, the series of spectra of AI taken from 30 cm" to 80 cm" show the lowest frequency features. Figure 3a shows the spectrum from an untreated dipeptide sample. When the dipeptide crystals were treated by exposure to vacuum at 1.33 x 10 Pa (lO" Torr) for 5 days, the spectrum of Figure 3b was obtained. The two strong peaks in this frequency interval are shifted from 43.31 cm" to 44.24 cm" and from 61.22 cm to 62.33 cm. In addition to shifts in the line centers, the apparent linewidths changed with dehydration from 4.95 to 4.24 cm" and from 4.20 to 5.45 cm", respectively. 

Figure 4 provides a comparison to spectra obtained from the retro-analogue, lA. This dipeptide loses its co-crystallized water within a few minutes when crystals are removed from solution (75) so that little if any water is expected to remain trapped in the pores of lA. Figure 4a shows a spectrum of the untreated sample while Figure 4b shows a spectrum from a sample treated by exposure to 1.33 X 10 Pa (10" Torr) vacuum for four days. The latter spectrum shows little change in the peak positions, with shifts of the three lines in this frequency interval measuring 0.2 cm or less. 

If the assessment shows that the exposure approaches the limit values, so that the OEL is met but there is a probability of exceeding the limit values, subsequent measurements at appropriate intervals must be taken to ensure that the assessment situation continues to prevail. The frequency of these measurements will depend on the previous results, so that the nearer the concentration recorded comes to the limit value, the more frequently measurements must be taken under normal working conditions. However, if the values are borderline, the decision of whether exposures are below the limit values within the OEA is not clear, and a more comprehensive sampling exercise may be required using, for example, worst-case measurements. This becomes more important the fewer the measurements that have been taken, so in case of doubt the evaluation results should be verified through additional selective measurements. 

There is a shortest reasonable exposure time that is related to the frequency of the heartbeat quartz and the adjustment of the voltage-to-frequency converters. Below the reasonable exposure interval quantization errors become a problem and the measured value will be chosen from a small number of possible steps. 

Halothane (boiling point BP] 50 °C), enfhirane (BP 56 °C), isoflurane (BP 48 °C), and the obsolete methoxyflu-rane (BP 104 °C) have to be vaporized by special devices. Part of the administered halothane is converted into hepatotoxic metabolites (B). Liver damage may result from halothane anesthesia. With a single exposure, the risk involved is unpredictable however, there is a correlation with the frequency of exposure and the shortness of the interval between successive exposures. 

Animals, usually rodents, are exposed to the test substance by an appropriate route, usually by gavage or by intraperitoneal injection. Each treated and control group must include at least five animals per sex. Positive controls should produce micronuclei in vivo at exposure levels expected to give a detectable increase over background. No standard treatment schedule (i.e., 1, 2, or more treatments at 24-h intervals) has been recommended. Three dose levels are generally used these should cover a range from the maximum to little or no toxicity. The erythrocytes are sampled from the bone marrow and/or peripheral blood of the animals. If bone marrow is used, the animals are sacrificed at appropriate times after treatment, the bone marrow extracted, and preparations made and stained. When peripheral blood is used, the blood is collected at appropriate times after treatment and smear preparations are made and stained. Preparations are analyzed for the presence of micronuclei. An increase in the frequency of micronucleated polychromatic erythrocytes in treated animals is an indication of induced chromosome damage. 

Chromosomal aberrations in peripheral lymphocytes were also reported in a study of about 40 workers who had been occupationally exposed to trace quantities of 2-butanone (methyl ethyl ketone), butyl acetate, toluene, cyclohexanone and xylene in addition to dimethylfonnamide. Blood samples were taken at two Ibur-nionth intervals, when exposure was to an average of 180 and 150 mg/nr dimethylformamide, respectively. The frequencies of chromosomal aberrations were 3.82% and 2.74% at these two sampling times. Subsequent sampling at tliree six-month intervals, when average dimethylformamide exposures were to 50, 40 and 35 mg/m- , gave lower aberration frequencies of 1.59%, 1.58% and 1.49%. Aberration frequencies in two control groups were 1.61% and 1.10% (Koudela Spazier, 1981). 

Richer et al. (1993) exposed five male volunteers to 50 ppm [188.5 mg/m ] toluene in a controlled exposure chamber for 7 h per day for three days on three occasions at two-week intervals. Blood samples were taken before and after each three-day exposure. No effects upon sister chromatid exchange frequencies were observed. 

When the EPA considered exposures to insecticide residues in the home they identified at least six possible sources and routes these are given in Table 2.6. Their original approach apportioned the acceptable daily intake (ADI) between the various routes but it soon became clear that this was unrealistic because an individual was unlikely to be exposed via all routes on any one day. The EPA s present strategy is to develop an approach called micro-exposure event modelling. Micro-exposure event modelling is based on statistical data on the frequencies and levels of contamination of food, water, etc. and on behavioural information about the frequency of use of lawn/pet/timber treatments, etc. The combined data are assembled in a probabilistic model called LIFELINE which is able to predict the frequency and level of exposure to a group of hypothetical individuals over their lifetime.12 The model is also able to take account of the relative proportions of different types of accommodation, the incidence of pet ownership or any other data that will affect real levels of exposure. The output from the LIFELINE model allows the exposures of individuals in a population to be modelled over any interval from a single occasion to a lifetime. 

Although it is possible that chronic exposure reduces the mutation frequency, the low mutation frequencies found for chemicals other than ENU have made it difficult to test this question. One notable exception is procarbazine, with which the first demonstration was made of a fractionation effect in mice.108 A pronounced fractionation effect has recently been demonstrated for ENU.380 383 Ten fractions of 10 mg/kg administered at weekly intervals yielded only 13% as many mutations as a single injection of 100 mg/kg. 

Pesticide usage factors include those factors needed to characterize the amount of pesticide an individual is potentially exposed to each day, as well as the duration, frequency and interval of potential exposures. For example, for mixer/loaders/ applicators, this would include hectares typically treated per day, typical application rates, types of equipment used, and whether application is conducted by the farmer or a custom applicator. 

An event that occurs when there is contact at a boundary between a human and the environment with a contaminant of a specific concentration for an interval of time the units of exposure are concentration multiplied by time. The determination or estimation (qualitative or quantitative) of the -magnitude, frequency, duration, and route of exposure. 

In most cases it is impossible to calculate the risk (R) from an exposure level as an exact number (e.g., R = l(b5). It may, however, be possible to agree on an exposure level that is so low that R < l(b5. When performing risk extrapolations we bias intentionally in the worst-case direction to be sure that we do not unintentionally bias in the other (unwanted) direction. Figure 10.1 shows difficulties with extrapolation. We may find a good correlation between aflatoxins in the dose interval 1 to 100 ppb and response as frequency of tumors in mice between 0.05 and 1. However, we would like to know the doses that give a response of 10 5. Is it possible to obtain this information from such data Furthermore, we are not at all interested in mice. If we decide upon a safe aflatoxin exposure for mice, we must further extrapolate to humans. 

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