Duration of exposure

The severity of health effects from chemical exposure is the direct result of dose, the product of chanical conceutration and duration of exposure. It is impossible to predict responses and outcomes in aU situations due to variation in environmental and exposure conditions. Therefore, generic scenarios are nsed to represent typical conditions for estimating the severity of effects. 

The significance of an effect depends on the severity of resnlting health and operational impacts. While the threshold for lethal effects is defined as the high end of low-level exposures, the lower end can then be defined as the exposure dosage that would not cause significant immediate or delayed adverse health consequence. 

Under circumstances where the physiologic damage is reversible, the resulting adverse health consequences will also reverse at some time after the exposure. This leads to observations of immediate, transient adverse health effects and/or delayed, transient adverse health effects. If sufficient transient damage accumulates to cause irreversible deterioration or if the cellular pathologic effects of exposure are irreversible, temporary or short-term exposure may result in permanent chronic illness. Therefore, adverse health effects following a chanical exposure may be a mixture of immediate, delayed, transient, or chronic symptoms. Each symptom may be characteristically associated with a different exposure level. 

The time period that personnel may be exposed to CWAs or other hazards cannot be precisely estimated. Duration periods listed in Table 3.2 describe generalized 

Temporary exposure duration Short-term exposure duration 

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Carbon tetrachloride is toxic by inhalation of its vapor and oral intake of the Hquid. Inhalation of the vapor constitutes the principal ha2ard. Exposure to excessive levels of vapor is characterized by two types of response an anesthetic effect similar to that caused by compounds such as diethyl ether and chloroform and organic injury to the tissues of certain organs, in particular the Hver and kidneys. This type of injury may not become evident until 1—10 days after exposure. The nature of the effect is deterrnined largely by the vapor concentration but the extent or severity of the effect is deterrnined principaHy by the duration of exposure (38). 

Human Health Effects. Any assessment of adverse human health effects from PCBs should consider the route(s) of and duration of exposure the composition of the commercial PCB products, ie, degree of chlorination and the levels of potentially toxic PCDF contaminants. As a result of these variables, it would not be surprising to observe significant differences in the effects of PCBs on different groups of occupationally-exposed workers. 

Toxicology. An excellent review of the toxicity and health assessment of ethylene oxide has been compiled (233). Ethylene oxide (EO) can be relatively toxic as both a Hquid and gas. Inhalation of ethylene oxide ia high concentrations may be fatal. Estimates of lethal ethylene oxide inhalation levels in animals depend on the duration of exposure. The reported 4-h LC q values for rats, mice, and dogs are 1460, 835, and 960 ppm, respectively (234). More recent information (235) indicates that the 1-h LC q in rats is approximately 5000 ppm. 

None of the foregoing methods will tell the frequency or duration of exposure of any receptor to irritant or odorous gases when each such exposure may exceed the irritation or odor response threshold for only minutes or seconds. The only way that such an exposure can be measured instrumentally is by an essentially continuous monitoring instrument, the record from which will yield not only this kind of information but also all the information required to assess hourly, daUy, monthly, and annual phenomena. Continuous monitoring techniques may be used at a particular location or involve remote sensing techniques. 

The important faetors, on exposure to ehemieals that are toxie by absorption via the skin, are the eontaet area and the duration of exposure (refer to Table 13.8). 

Irritants Have a corrosive or a vesicant (blistering) effect on moist or mucous surfaces. Concentration may be more important than duration of exposure. Animals and man react similarly. ... 

In extreme eases irritant ehemieals ean have a eoiTosive aetion. Corrosive substanees ean also attaek living tissue (e.g. to eause skin uleeration and, in severe eases, ehemieal burns with degradation of bioehemieals and ehan ing), kill eells and possibly predispose to seeondary baeterial invasion. Thus whilst aeute irritation is a loeal and reversible response, eorrosion is iiTeversible eell destruetion at the site of the eontaet. The outeome is influeneedby the nature of the eompound, the eoneentration, duration of exposure, the pH (see Figure 5.1) and also, to some extent, by individual suseeptibility ete. Tims dilute mineral aeids may be irritant whereas at higher eoneentrations they may eause eoirosion. 

Exposure is minimized by ehoiee of souree, by duration of exposure, by distanee from souree (at 1 m die radiation level is redueed almost 10-fold), and by shielding. The greater die mass per unit area of shield material die greater die shielding effieieney. Whereas a- and (3-partieles pose few problems (die former ean be absorbed by, e.g., paper and die latter by 1 em Perspex) y- and X-rays are not eompletely absorbed by shield material but attenuated exponentially sueh dial radiation emerging from die shield is given by ... 

In this step, the assessor qiuuitifies tlie magnitude, frequency and duration of exposure for each patliway identified in Step 2. Tliis step is most often conducted in two stages estimation of exposure concentrations and calculation of intakes. The later estimation is considered in Step 4. In tliis part of step 3. the exposure assessor determines the concentration of chemicals tliat will be contacted over the exposure period. E.xposure concentrations are estimated using monitoring data and/or chemical transport and environmental fate models. Modeling may be used to estimate future chemical concentrations in media tliat are currently contaminated or tliat may become contaminated, and current concentrations in media and/or at locations for which tliere are no monitoring data. The bulk of the material in tliis chapter is concerned witli tliis step. 

What are the concentrations and durations of exposure in tlie area of the... 

Sinee 140 in 10 e.xeceds 80 in 10, the eonelusiou is, tliat for identieal durations of exposure to identieal eoueenuations in air, enviromnental tobaeeo smoke poses a greater eaneer risk than gasoline vapors. However, both mixtures exliibit liigh eaneer risks at traec levels. 

Expositions-dauer, /. time or duration of exposure. -zeit, /. time of exposure. 

Gases or vapors that in concentrations of about Vj -1% for durations of exposure of about 5 min are lethal or produce serious injury. Sulfur dioxide... 

Effect of the exposure conditions The absolute values for the rate of rusting given in Table 3.3 would be affected by the mass of the specimen itself and by other factors such as the orientation of the steel, the climatic conditions prevailing at the time of exposure, and the duration of exposure. 

The British Non-Ferrous Metals Research Association carried out two series of tests, the results of which have been given by Gilbert and Gilbert and Porter these are summarised in Table 4.12. In the first series tough pitch copper tubes were exposed at seven sites for periods of up to 10 years. The two most corrosive soils were a wet acid peat (pH 4-2) and a moist acid clay (pH 4-6). In these two soils there was no evidence that the rate of corrosion was decreasing with duration of exposure. In the second series phosphorus-deoxidised copper tube and sheet was exposed at five sites for five years. Severe corrosion occurred only in cinders (pH 7 1). In these tests sulphides were found in the corrosion products on some specimens and the presence of sulphate-reducing bacteria at some sites was proved. It is not clear, however, to what extent the activity of these bacteria is a factor accelerating corrosion of copper. 

Although the positive effects of ERT have been well established, it has been shown that the cell proliferative actions of estrogen can increase the incidence of breast cancer in some patients. In addition, duration of exposure to physiological levels of unopposed estrogens is an established risk factor for breast, uterine, and ovarian cancer. In an effort to attain pharmaceutical agents that oppose the carcinogenic... 

The health effects of different pollutants vary according to the Intensity and duration of exposures and the health status of the persons exposed. A sumnary of these effects Is provided In Table II, together with the WHO (1979) guideline values for the protection of human health. The WHO European office has reviewed these guidelines, WHO (1987), and has recommended additional values for SOj In the presence of particulate matter. 

Estimates of exposure levels posing minimal risk to humans (Minimal Risk Levels or MRLs) have been made for methyl parathion. An MRL is defined as an estimate of daily human exposure to a substance that is likely to be without an appreciable risk of adverse effects (noncarcinogenic) over a specified duration of exposure. MRLs are derived when reliable and sufficient data exist to identify the target organ(s) of effect or the most sensitive health effect(s) for a specific duration within a given route of exposure. MRLs are based on noncancerous health effects only and do not consider carcinogenic effects. MRLs can be derived for acute, intermediate, and chronic duration exposures for inhalation and oral routes. Appropriate methodology does not exist to develop MRLs for dermal exposure. 

In the male offspring whose treatment was continued through 11-12 weeks of age, however, dose-related effects were seen on all the above end points, and these effects were significantly different from controls at all three dose levels (Desi et al. 1998) (see also Section S.2.2.4). The study did not determine the critical period (if any) and duration of exposure for these neurological effects. A limitation of this study is that results specifically for methyl parathion were shown only for the somatosensory electrocortico-gram the other results for this chemical were stated in the text, but not shown. 

Minimal Risk Level (MRL)—An estimate of daily human exposure to a hazardous substance that is likely to be without an appreciable risk of adverse noncancer health effects over a specified route and duration of exposure. 

Where sufficient toxicologic information is available, we have derived minimal risk levels (MRLs) for inhalation and oral routes of entry at each duration of exposure (acute, intermediate, and chronic). These MRLs are not meant to support regulatory action but to acquaint health professionals with exposure levels at which adverse health effects are not expected to occur in humans. They should help physicians and public health officials determine the safety of a community living near a chemical emission, given the concentration of a contaminant in air or the estimated daily dose in water. MRLs are based largely on toxicological studies in animals and on reports of human occupational exposure. 

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