Intermittent exposure

Intermittant exposure and local conditions need to be considered also. The ballast or storage tanks of ships will experience periods of complete submergence, partial coverage and may at times be empty. Similarly, the... 

Repeated intermittent exposure to stimulants can produce sensitization, where subsequent drag exposures produce increased behavioral and neurochemical responses. The ability of the drag and ultimately of related stimuli to elicit behavior may be increased with repeated administration or intake of the drag. Dopaminergic sensitization within the amygdala has also been found after repeated exposure to amphetamine and this... 

Used to derive an intermediate inhalation Minimal Risk Level (MRL) of 5 x 10 ppm dose adjusted for intermittent exposure and divided by an uncertainty factor of 100 (10 for extrapolation from animal to humans, 10 for human variability). 

Monte Carlo simulation, an iterative technique which derives a range of risk estimates, was incorporated into a trichloroethylene risk assessment using the PBPK model developed by Fisher and Allen (1993). The results of this study (Cronin et al. 1995), which used the kinetics of TCA production and trichloroethylene elimination as the dose metrics relevant to carcinogenic risk, indicated that concentrations of 0.09-1.0 pg/L (men) and 0.29-5.3 pg/L (women) in drinking water correspond to a cancer risk in humans of 1 in 1 million. For inhalation exposure, a similar risk was obtained from intermittent exposure to 0.07-13.3 ppb (men) and 0.16-6.3 ppb (women), or continuous exposure to 0.01-2.6 ppb (men) and 0.03-6.3 ppb (women) (Cronin et al. 1995). 

Continuous exposure for 30-160 days to aerosols of a triaryl phosphate U.S. military hydraulic fluid, triaryl phosphate (see Table 3-1), produced paralysis in rabbits and chickens after exposure to 102 mg/m3 and 23 mg/m3, respectively, but not after exposure to respective concentrations of 34 mg/m3 or 4.4 mg/m3 (Siegel et al. 1965). Continuous exposure to triaryl phosphate at 110, 103, or 4.4 mg/m3, respectively, produced no signs of neurotoxicity in rats after 36 days, dogs after 99 days, or monkeys after 108 days (Siegel et al. 1965). Intermittent exposure (8 hours/day, 5 days/week for 30 exposures) to 50 mg/m3 triaryl phosphate produced no neurotoxic signs in squirrel monkeys (Siegel et al. 1965). Aerosols of Durad... 

An MRL of 0.03 ppm has been derived for intermediate-duration inhalation exposure to hydrogen sulfide. This MRL is based on a NOAEL of 30.5 ppm for respiratory effects in mice based on the CUT 1983a study (Table 2-1 LSE 39). The NOAEL is adjusted for intermittent exposure and the NOAEL m,x- is calculated. An uncertainty factor of 30 is then applied, 3 for... 

Based on a LOAEL of 2 ppm for respiratory effects—bronchial obstruction (30% change in airway resistance) in 2/10 persons with asthma in the Jappinen et al. (1990) study, an acute inhalation MRL of 0.07 ppm was derived. An uncertainty factor of 30 was applied to the LOAEL 10 for the use of a LOAEL and 3 for human variability. Since persons with severe asthma were excluded from the study, an uncertainty factor of 3 is needed to protect all sensitive individuals including children. Further details on the derivation of this MRL can be found in the MRL worksheets in Appendix A of this profile. Based on aNOAEL of 30.5 ppm for respiratory effects in mice observed in the CUT (1983a) study, an intermediated MRL of 0.03 ppm was derived. The NOAEL is adjusted for intermittent exposure and the NOAEL[hec] is calculated. An uncertainty factor of 30 is then applied 3 for extrapolating from animals to humans and 10 for human variability. Further details on this MRL can be found in the MRL worksheets in Appendix A of this profile. 

PbB concentrations reflect the absorbed dose of lead. However, the interpretation of PbB data depends on a knowledge of the past history of exposure to lead. This is because in the body, bone constitutes the major lead sink and this results in lead having a long body half-life. Thus, in the absence of intense exposure to lead for a considerable period up to its body half-life, the PbB concentrations reflect recent lead exposures. However, if intermittent exposure to lead is occurring in several distinct environments, the PbB concentration reflects both recent and past exposures to lead. Thus, biological effects for populations with the same PbB concentrations may not be the same since different exposure times scales may be involved. This is the reason why free erythrocyte protoporphyrin (FEP) and erythrocyte zinc protoporphyrin (ZPP) have been used as additional biological markers since their elevation is more related to chronic lead exposure than acute lead exposure (see Section 2.7). 

No deaths. Whole-body BCF values at 70 days 40 were 400 for intermittent exposure and 4100 for continuous exposure... 

For continuous exposure, no effect on survival or 40 growth BCF of 3000. For intermittent exposure 32% dead, 50% reduction in growth, BCF of 10,700... 

Curtis, L.R., W.K. Seim, and G.A. Chapman. 1985. Toxicity of fenvalerate to developing steelhead trout following continuous or intermittent exposure. Jour. Toxicol. Environ. Health 15 445-457. 

Intermittent exposure has only mild effects on body weight even at high concentrations of u-hexane. In Fischer 344 rats exposed to up to 10,000 ppm for 6 hours a day, 5 days a week, the only effect seen after 13 weeks was a 11% decrease in body weight compared to controls in males, with no effect seen in females (Cavender et al. 1984). No effect on body weight was observed in rats exposed 21 hours a day to 126 ppm or intermittently (6 hours a day, 5 days a week) to 129 ppm -hexane for 26 weeks (Bio/Dynamics 1978). 

The MRL is based on a NOAEL of 0.5 mg/m3 for decreased acetylcholinesterase activity in rats exposed to disulfoton 4 hours/day for 5 days in a study by Thyssen (1978). The NOAEL was adjusted for intermittent exposure, converted to a human equivalent concentration, and divided by an uncertainty factor of 30 (3 for extrapolation from animals to humans and 10 for human variability). Inhibition of erythrocyte cholinesterase activity and unspecified behavioral disorders were observed at 1.8 mg/m, and unspecified signs of cholinergic toxicity were observed at 9.8 mg/m. Similar effects were observed in rats or mice exposed to higher concentrations for shorter duMtions (Doull 1957 Thyssen 1978). The NOAEL value of 0.5 mg/m is supported by another study, in which no significant decrease in the activity of brain, serum, or submaxillary gland cholinesterase was found in rats exposed to 0.14-0.7 mg/m for 1 hour/day for 5-10 days (DuBois and Kinoshita 1971). Mild depression of erythrocyte cholinesterase activity was reported in workers exposed by the inhalation and dermal routes (Wolfe et al. 1978). 

The intermediate MRL is based on a NOAEL of 0.02 mg/m3 for decreased acetylcholinesterase activity in rats exposed to disulfoton 6 hours/day, 5 days/week for 3 weeks in a study by Thyssen (1980). The NOAEL was adjusted for intermittent exposure, converted to a human equivalent concentration, and divided by an uncertainty factor of 30 (3 for extrapolation from animals to humans and 10 for human variability). In the Thyssen (1980) study, 2 separate 3-week experiments... 

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