Pre-exposure

Fig. 8.55 The effect of a 0.14Cr addition on the recovery of ductility of Al-6Zn-3Mg alloys during storage in laboratory air at 20° C after pre-exposure to water vapour-saturated air for 5 days at 70°C. The ductility ratio is the ratio of elongation-to-fracture of specimens broken under the cited conditions and under vacuum conditions...

The embrittlement caused to aluminium alloys by pre-exposure to moist atmospheres or stress-corrosion environments is thought to be due to hydrogen in the atomic form. Intergranular bubbles of hydrogen, formed in association with certain precipitates, have been observed by HV and... 

The role of the stress in embrittlement and stress-corrosion processes has been examined in some detail by employing the slow strain-rate technique . Specimens of alloy 7179-T651 tested in air or in vacuum after pre-exposure to water at 70° C or in water at various potentials at ambient temperature exhibited a reversible embrittlement in excess of that arising from testing in moist air . The embrittlement was attributed to hydrogen absorption, and recovery was thought to be due to loss of hydrogen (particularly under vacuum) or to diffusion to traps. Potentiostatic tests revealed... 

In addition to examining pre-exposure effects, the slow strain-rate testing technique has been used increasingly to examine and compare the stress-corrosion susceptibility of aluminium alloys of various compositions, heat treatments and forms. A recent extensive review draws attention to differences in response to the various groups of commonly employed alloys which are summarised in Fig. 8.57. The most effective test environment was found to be 3 Vo NaCl -F 0.3 Vo HjOj. The most useful strain rate depends upon the alloy classification. 

Nature of the metal surface The critical concentration of an anion required to inhibit the corrosion of iron may increase with increasing surface roughness. Thus, Brasher and Mercer" showed that the minimum concentration of benzoate required to protect a grit-blasted steel surface was about 100 times greater than that required to protect an abraded surface. However, surface preparation had little effect on the critical inhibitive concentrations for chromate" or nitrite " The time of exposure of the iron surface to air after preparation and before immersion may also affect the ease of inhibition by anions. There is evidence """ that the inhibition by anions occurs more readily as the time of pre-exposure to air increases. Similarly, if an iron specimen is immersed for some time in a protective solution of an inhibitive anion, it may then be transferred without loss of inhibition to a solution of the anion containing much less than the critical inhibitive concentration . ... 

Pre-exposure immunization of people with greater than usual risk of exposure to rabies virus by reason of occupation (eg, veterinarians laboratory... 

Pre-exposure prophylaxis on days 0, 7, 21 to 28 and then q2-5 years based on antibody titers 1 mL IM (Imovax Rabies Vaccine or Rabies Vaccine Adsorbed) or (continued)... 

Pre-exposure to the organophosphate diazinon at exposures half the LC50 values increased the LC50 value by a factor of about five for guppy (Poecilia reticulata), but had no effect on the value for zebra fish (Brachydanio rerio). This was consistent with the observation that during pre-exposure of guppy there was a marked inhibition in the synthesis of the toxic metabolites diazoxon and pyrimidinol, whereas this did not occur with zebra fish in which the toxicity was mediated primarily by the parent compound (Keizer et al. 1993). 

Experiments using marine sediment slurries have examined the effect of pre-exposure to various aromatic hydrocarbons on the rate of subsequent degradation of the same, or other hydrocarbons. The results clearly illustrated the complexity of the selection process for example, whereas pre-exposure to benzene, naphthalene, anthracene, or phenanthrene... 

It therefore seems premature to draw general conclusions on the influence of pre-exposure on the biodegradability of structurally different substrates. 

Pre-exposure proeessing and preparation of the inner and outer whole-body dosimeters for use in the field should be eonsidered. The analytieal laboratory should determine if the fabric of the dosimeter of choice contains any analytical interference, which may be a problem in subsequent analysis of the fabric. If such analytical interferences are present in the fabric of the dosimeter, they may be reduced by pre-washing the dosimeter material prior to use in the field. The dosimeter is usually pre-washed (sometimes more than once) and rinsed several times prior to thorough drying. The washing detergent of choice should be as free as possible from additive brighteners and other chemicals, which may cause analytical interferences. 

Pre-exposure prophylaxis with IGIM is indicated for individuals at high risk of acquiring the HAV who cannot receive the hepatitis A vaccine (e.g., because of allergy to the components alum or 2-phenoxyethanol). Additionally, travelers who plan to depart for endemic areas within 2 weeks and have not yet received the hepatitis A vaccine should receive IGIM because active vaccine immunity takes several weeks to develop. 

There is no benefit in administering the vaccine after a person has been exposed to the HAV. Two inactivated hepatitis A vaccines, Havrix and VAQTA, are available in the United States and are effective in providing active immunization. The major difference between the two vaccines is that HAVRIX contains 2-phenoxyethanol as a preservative whereas VAQTA is preservative-free.1 Either vaccine is effective in providing active pre-exposure prophylaxis when given in two injections 6 months apart (referred to as months 0 and 6). The two vaccines are considered interchangeable, and doses are dependent on age (Table 21-3). 

Because of the physiological variations of RBC-ACHE and PCHE levels, the sensitivity of these tests to detect low-level inhibitions can be increased by comparison with individual pre-exposure values, adopted as a reference. The World Health Organization (WHO, 1982) recommends calculating individual pre-exposure values as the average of three samples cholinesterase activities after exposure should be expressed as percentage change with... 

In light of their high lipophilic properties, many OC compounds and some of their metabolites are detectable in adipose tissue however, this method cannot be applied to monitor occupationally exposed subjects on a routine basis because of the invasive nature of the sampling procedures. Because intact OC compounds and their metabolites are commonly found in blood and urine of the general population, comparison with appropriate reference groups or with individual pre-exposure values is recommended when surveying occupationally exposed workers. 

Low levels of 3,5,6-TCP were also observed in pre-exposure urine from most of the field workers. These 3,5,6-TCP levels were subtracted from urine field recovery samples and were used to correct levels of 3,5,6-TCP found in the post-exposure urine samples from these same workers. This procedure was necessary to calculate the amount of 3,5,6-TCP in the urine that was attributable to the exposure period. 

Successive 24-hr urine specimens were provided by each volunteer. Collection in the dosimeter studies began 24 hr prior to the chlorpyrifos exposure (study day 0) and continued for 3 days based upon the 27-hr half-life of chlorpyrifos in humans (Nolan et al., 1984). Pre-exposure controls were obtained in all cases. Total urine volume was measured for each of the days, and 20- to 30-mL portions were stored frozen prior to analysis. The Sacramento collections were 48 hr and the Riverside collections were approximately 84 hr after re-entry. 

Harnett, M.M., Deehan, M.R., Williams, D.M. and Harnett, W. (1998a) Induction of signalling anergy via the TCR in Jurkat T cells by pre-exposure to a filarial nematode secreted product. Parasite Immunology 20, 551-564. 

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