Exposure magnitude

On the other hand, with increasing exposure (more precisely, its intensity), significant irreversible changes in transmissivity are observed (Fig. 7.8). We see that the characteristic at / < 7th and I > Ith, where 7th is the threshold intensity, are quite different. In general, the versus E curve (E denotes exposure magnitude) may be divided into three parts ... 

Human Exposure - Magnitude and Relative Significance of Pathways... 

The role of ozone is also very important in the degradation of rubber. It was found that on exposing a stressed rubber sample to ozone, small cracks formed on the surface. These cracks are perpendicular to the direction of applied stress [450]. The velocity of cracking depends on the chemical structure of the polymer, time of exposure, magnitude of applied stress, content of plasticizers, and so on [13, 14, 111, 186, 239, 360, 361, 553, 554]. Several workers [13, 184] have indicated that the ozonization of diene elastomers is accompanied by an autocatalytic... 

A constant force is applied to an ideal elastomer, assumed to be a perfect network. At an initial temperature Tj the length of the sample is Ij. The temperature is raised to Tf and the final length is If. Which is larger Ij or If (remember F is a constant and Tf > Tj) Suppose a wheel were constructed with spokes of this same elastomer. From the viewpoint of an observer, the spokes are heated near the 3 o clock position-say, by exposure to sunlight-while other spokes are shaded. Assuming the torque produced can overcome any friction at the axle, would the observer see the wheel turn clockwise or counterclockwise How would this experiment contrast, in magnitude and direction, with an experiment using metal spokes ... 

A.irbome Basic Chemical Contamination. A critical, and at-first pu22ling problem, was encountered during early manufacturing trials of CA resists. Sporadically, severely distorted resist profiles would be formed in positive-tone CA resists, displaying what seemed to be a cap on the upper surface of the resist image (Fig. 26). In severe cases this cap or T-top would appear as a kin or cmst over the entire wafer surface that prevented development of the pattern. The magnitude of the effect varied dramatically between laboratories and appeared to grow more severe as the time interval between exposure and post-exposure bake was increased. 

If possible, there should be measurement of the toxic effect in order quantitatively to relate the observations made to the degree of exposure (exposure dose). Ideally, there is a need to determine quantitatively the toxic response to several differing exposure doses, in order to determine the relationship, if any, between exposure dose and the nature and magnitude of any effect. Such dose—response relationship studies are of considerable value in determining whether an effect is causally related to the exposure material, in assessing the possible practical (in-use) relevance of the exposure conditions, and to allow the most reasonable estimates of hazard. 

Human exposure evaluation is used in describing the nature and size of the population exposed to a substance and the magnitude and duration of their exposure. The evaluation could concern past or current exposures, or exposure anticipated in the future. 

Depending on the circumstances of exposure, any given material may produce more than one type of toxic effect. Therefore, when describing toxicity for a particular material, it is necessary to define whether the effect is local, systemic, or mixed the nature of the injury the organs and tissues affected and the conditions of exposure, including route of exposure, number of exposures, and magnitude of exposure. 

Number of Exposures. Some toxic effects are produced in response to a single exposure of sufficient magnitude, while others require multiple exposures for their development (see Table 1). 

Magnitude of Exposure. As discussed in detail later, the magnitude of the exposure will influence both the likelihood of an effect being produced and its severity. 

Route of Exposure. As discussed below, the route of uptake may have a significant influence on the metaboHsm and distribution of a material. Differences in route of exposure may influence the amount of material absorbed and its subsequent fate. These differences may be reflected in variation in the nature and magnitude of the toxic effect. 

Parent substances and metaboHtes may be stored in tissues, such as fat, from which they continue to be released following cessation of exposure to the parent material. In this way, potentially toxic levels of a material or metaboHte may be maintained in the body. However, the relationship between uptake and release, and the quantitative aspects of partitioning, may be complex and vary between different materials. For example, volatile lipophilic materials are generally more rapidly cleared than nonvolatile substances, and the half-Hves may differ by orders of magnitude. This is exemplified by comparing halothane and DDT (see Anesthetics Insectcontholtechnology). 

It is important to appreciate that the magnitude of the absorbed dose, the relative amounts of bio transformation product, and the distribution and elimination of metaboUtes and parent compound seen with a single exposure, may be modified by repeated exposures. For example, repeated exposure may enhance mechanisms responsible for biotransformation of the absorbed material, and thus modify the relative proportions of the metaboUtes and parent molecule, and thus the retention pattern of these materials. Clearly, this could influence the likelihood for target organ toxicity. Additionally, and particularly when there is a slow excretion rate, repeated exposures may increase the possibiUty for progressive loading of tissues and body fluids, and hence the potential for cumulative toxicity. 

The importance of determining a relationship between the magnitude of the exposure and the frequency of occurrence of a toxic effect is considered in detail below. 

In addition to the effect of biological variabihty in group response for a given exposure dose, the magnitude of the dose for any given individual also determines the severity of the toxic injury. In general, the considerations for dose—response relationship with respect to both the proportion of a population responding and the severity of the response are similar for local and systemic effects. However, if metabohc activation is a factor in toxicity, then a saturation level may be reached. 

Exposure should be by the practical route. Other conditions, such as number and magnitude of exposures, should kiclude at least one level representative of the practical situation monitoring should be appropriate to the needs for conducting the study and when practically and economically possible, pharmacokinetic observations should be undertaken ki order to better define the relationship of dose to metaboHc thresholds. 

Ha2ard is the likelihood that the known toxicity of a material will be exhibited under specific conditions of use. It follows that the toxicity of a material, ie, its potential to produce injury, is but one of many considerations to be taken into account in assessment procedures with respect to defining ha2ard. The following are equally important factors that need to be considered physicochemical properties of the material use pattern of the material and characteristics of the environment where the material is handled source of exposure, normal and accidental control measures used to regulate exposure the duration, magnitude, and frequency of exposure route of exposure and physical nature of exposure conditions, eg, gas, aerosol, or Hquid population exposed and variabiUty in exposure conditions and experience with exposed human populations. 

The threshold limit value for ethyl alcohol vapor in air has been set at 1000 ppm for an 8-h time-weighted exposure by the ACGIH (1989 listing). The minimum identifiable odor of ethyl alcohol has been reported as 350 ppm. Exposure to concentrations of 5,000—10,000 ppm result in irritation of the eyes and mucous membranes of the upper respiratory tract and, if continued for an hour or more, may result in stupor or drowsiness. Concentrations of this latter order of magnitude have an intense odor and are almost intolerable to begin with, but most people can become acclimated to the exposure after a short time. Table 7 gives the effects of exposure to even heavier concentrations. 

---