Initial Exposure

The initial exposure conditions have a marked influence on the subsequent corrosion rate. During the first days of exposure, wet conditions (caused by high relative humidity or rainfall) cause higher corrosion rates than dry conditions. These effects will vary from one material to another. For example, zinc is more sensitive than steel. Differences are explained by the fact that different materials form different corrosion products with different protective properties. A wide variety of structurally related corrosion products can be found on zinc, the nature of which depends on initial exposure conditions. The seasonal dependence on the concentrations of peroxide and ozone in the atmosphere might also be a contributing factor. 

During the third period of exposure, constant corrosion rate is established, thus the terminology stationary period.  

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Initial exposure regime. This involves fluorination and etching of atoms in tire 7 x 7 reconstmction, during which the destmction of tire surface states in tire reconstmction is correlated witli creation of tire species Sip2 and SiF. ... 

SENSITIZER A substance that causes little or no reaction in a person upon initial exposure but which will provoke an allergic response on subsequent exposures. 

Weather conditions at the time of initial exposure of zinc and steel have a large influence on the protective nature of the initial corrosion products This can still be detected some months after initial exposure. Finally, rust on steel contains a proportion of ferrous sulphate which increases with increase in SO2 pollution of the atmosphere. The effect of this on corrosion rate is so strong that mild steel transferred from an industrial atmosphere to a rural one corrodes for some months as though it was still exposed to the industrial environment. ... 

From what has been said already, it is clear that determinations of corrosion rates from small-scale experiments must be treated with great caution. If the metal cannot ptissivate, it will corrode until it becomes immune, at which point the corrosion rate will fall to zero between initial exposure and the attainment of immunity the corrosion rate will be continually changing. If, on the other hand, it is impossible for the metal to come to equilibrium... 

If the diffusivity in the liquid phase is 1.8 x 10 nr/s. at what time after the initial exposure will the resistance attributable to access of gas be equal to about 10 per cent ot the total resistance to transfer. ... 

Mooney viscometer is also used to measure the time it takes, from initial exposure of the compound to a particular temperature, to the time of onset of cure at that temperature [2]. This is known as the scorch time. Scorch time is an important parameter to the rubber processor, as a short time may lead to problems of premature vulcanization. As the test is taken past the onset of cure, the rotor tears the cured rubber, and therefore this device cannot be used to investigate rheological properties after the scorch time. 

Aprotinin. Aprotinin is a naturally occurring serine protease inhibitor, has found widespread applications either by the intravenous route or as a component of biological sealants, because of its ability to decrease blood loss, and, as a consequence, transfusion requirements. Anaphylactic reactions are mediated by IgG and IgE antibodies. The risk of anaphylactic reactions has been estimated between 0.5 and 5.8% when used intravenously during cardiac surgery, and at 5 for 100,000 applications when used as a biologic sealant [25]. Patients previously treated with this drug present an increased risk and any new administration should be avoided for at least 6 months following an initial exposure [25]. 

Temporal Relationships of Adverse Events. The temporal relationship between duration of product exposure and development of an adverse event is important in assessing causality. But how can data on temporal relationships be systematically summarized in a database containing thousands or even hundreds of thousands of subjects Temporal relationships cannot be clearly elicited if only frequencies of adverse events between treatment and control groups are compared. There can be many disparities in the subjects time of exposure or time at risk. Toxic manifestations of drugs may not occur until several months or even years after the initial exposure to the drug. How do we systematically assess delayed toxicity of a previously prescribed drug from the effect of a newly prescribed drug Such a scenario occurred with reported cases of pancreatitis associated with valproic acid therapy, in which some cases appeared several years after therapy [2]. 

In additional studies, Wiles et al. (5) found that rats and rabbits given palytoxin by either the ir or po route appear to be protected against a subsequent iv or im injection. This tolerance, or immunity, appears to be related to the time between initial exposure and subsequent challenge. 

Steady-state molecular beam studies of the reaction of methylacetylene on reduced Ti02 (001) surfaces were undertaken to determine whether this reaction could be performed catalytically under UHV conditions. A representative experiment is presented in Figure 1. Prior to each experiment, the surface was sputtered and annealed to a temperature between 400 K and 550 K surfaces prepared in this manner have the highest fraction of Ti(+2) sites (ca. 30% of all surface cations) of any surface we have been able to create by initial sputtering [3]. Thus these are the surfaces most active for cyclotrimerization in TPD experiments [1]. Steady-state production of trimethylbenzene (as indicated by the m/e 105 signal detected by the mass spectrometer) was characterized by behavior typical of more traditional catalysts a jump in activity upon initial exposure of the crystal to the molecular beam, followed by a decay to a lower, constant level of activity over a longer time scale. Experiments of up to 6 hours in duration showed... 

Initial exposure. Left, Sample I. Right, Sample 2... 

Inexperienced males deprived of the VNO prior to mating do not respond on initial exposure to females — they do not vocalise or show T. surges (Wysocki, 1983). Androgen output in these conditions is not affected by the MOS or by visual cues, suggesting that in naive mice the AOS-mediated chemosignals are processed unmodified, and so... 

The reversibility of dA alkylation by QM3 creates both problems and opportunities. For example, an accurate profile of products formed by initial exposure of DNA to QMs may not be easily determined since labile products such as the N1 adduct of dA would not persist through the typical procedures of enzyme digestion and analysis of the subsequent deoxynucleoside products by reverse phase HPLC. Thus, the full extent of DNA alkylation by QMs may be severely underestimated. 

Figure 8.2 Four STM images (175 x 185 A) recorded over the same area of a Cu(l 10) surface during the initial exposure to NO at room temperature. There are present the (2 x 1)0 strings and isolated features which on heating (see Figure 8.1) agglomerate to form the nitrogen chains running in the < 110 > direction. (Reproduced from Ref. 12).

Allergic contact dermatitis is a delayed hypersensitivity reaction.30 Upon initial exposure, a substance penetrates the skin, binds to a protein and develops into sensitizing antigens. Subsequent exposures to that substance will then elicit an allergic reaction.25,29,30 Symptoms of allergic contact dermatitis are similar to those of the irritant type, but may take several hours to several days to develop following re-exposure.25,26... 

In assessing initial exposure, whole body counting techniques measure amounts of americium that have been deposited within organs or tissues and have not been excreted. In cases of accidental ingestion, some of the americium may have been excreted in the urine or through the feces, before exposure is assessed. In... 

The onset of symptoms may be delayed for several minutes after initial exposure (especially with DM) effective exposure may, therefore, occur before the presence of the smoke is suspected. The paranasal sinuses are irritated and fill with secretions and severe frontal headache results. Prolonged exposure may cause retrosternal pain, dyspnoea, and asthma-like symptoms. Symptoms reach their climax after 5-10 min and disappear 1-2 h after cessation of exposure. 

Most chemical agents are essentially cumulative in their effects. The reason is that the human body detoxifies them very slowly or not at all. For example, a 1-h exposure to HD or CG followed within a few hours by another 1-h exposure has about the same effect as a single 2-h exposure. Continued exposure to low concentrations of HD may cause sensitivity to very low concentrations of HD. Other chemical agents also have cumulative effects. For example, an initial exposure to a small (less than lethal) amount of Sarin (GB) would decrease cholinesterase levels a second quantity less than the FDS0... 

In a report comparing community responses to low-level exposure to a mixture of air pollutants from pulp mills, Jaakkola et al. (1990) reported significant differences in respiratory symptoms between polluted and unpolluted communities. The pollutant mixture associated with the pulp mills included particulates, sulfur dioxide, and a series of malodorous sulfur compounds. Major contributors in the latter mixture include hydrogen sulfide, methyl mercaptan, and methyl sulfides. In this study the responses of populations from three communities were compared, a nonpolluted community, a moderately polluted community, and a severely polluted community. Initial exposure estimates were derived from dispersion modeling these estimates were subsequently confirmed with measurements taken from monitoring stations located in the two polluted communities. These measurements indicated that both the mean and the maximum 4-hour concentrations of hydrogen sulfide were higher in the more severely polluted community (4 and 56 g/m3 2.9 and 40 ppb) than in the moderately polluted one (2 and 22 g/m3 1.4 and 16 ppb). Particulate measurements made concurrently, and sulfur dioxide measurements made subsequently, showed a similar difference in the concentrations of these two pollutants between the two polluted communities. 

Figure 1 shows the series of infrared spectra collected during 02 pulse studies into flowing He/propylene at 250 °C. The initial exposure of the catalyst to the He/propylene flow produced bands at 1982 cm 1 and 1810 cm 1. The band at 1982 cm"1, in the range of adsorbed CO, is a result of interaction of propylene with surface OH. This is evidenced by a decrease in the OH intensity which is accompanied by an increase in the intensity of the 1982 cm"1 band. This 1982 cm"1 band can also be produced from adsorption of PO. The band at 1810 cm"1 is due to CH2 wagging of propylene the band at 1590 and 1465 cm"1 can be... 

Adsorption versus Polymerization. It is instructive to examine further the time dependence of the corrosion inhibition. In acid corrosion inhibition tests, steady state is customarily assumed to be reached within 10 to 20 min after initial exposure of the metal specimen. Since the inhibitors function by reducing the available active surface area, we expect an increase in and a corresponding decrease in P. The degree of corrosion protection the inhibitor provides is given by... 

Neurological Signs and Symptoms in Adults. The most severe neurological effect of lead in adults is lead encephalopathy, which is a general term to describe various diseases that affect brain function. Early symptoms that may develop within weeks of initial exposure include dullness, irritability, poor attention span, headache, muscular tremor, loss of memory, and hallucinations. The condition may then worsen, sometimes abruptly, to delirium, convulsions, paralysis, coma, and death (Kumar et al. 1987). Histopathological findings in fatal cases of lead encephalopathy in adults are similar to those in children (see discussion below). 

Depending on the concentration of agent vapor, the effects begin to appear from 30 seconds to 2 minutes after initial exposure. 

Rey and coworkers78 reported methyltin intoxication in six chemical workers exposed to Me2SnCl2 and MesSnCI. After a latent period of 1 -3 days, the first symptoms occurred, including headache, tinnitus, deafness, impair of memory, disorientation, aggressiveness, psychotic behavior, syncope, loss of consciousness and, in the most severe cases, respiratory depression requiring ventilatory assistance. Increased tin excretion was detected in the urine of all patients, particularly those most ill. The patient with the highest tin levels died 12 days after the initial exposure. 

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