Exposure periods

Short-term exposure. This is the maximum concentration to which workers can be exposed for a period of up to 15 minutes continuously without suffering from (a) intolerable irritation, (b) chronic or irreversible tissue change, or (c) narcosis of sufficient degree to increase accident proneness, impair self-rescue, or materially reduce efficiency, provided that no more than four excursions per day are permitted, with at least 60 minutes between exposure periods, and provided the daily time-weighted value is not exceeded. 

Table 3. Minimum Exposure Periods for Sterilization of Hospital Supplies ...

Subchronic Studies. Although short-term repeated exposure studies provide valuable information about toxicity over this time span, they may not be relevant for assessment of ha2ard over a longer time period. For example, the minimum and no-effects levels determined by short-term exposure may be significantly lower if exposure to the test material is extended over several months. Also, certain toxic effects may have a latency which does not allow their expression or detection over a short-term repeated-exposure period for example, kidney dysfunction or disturbances of the blood-forming tissues may not become apparent until subchronic exposure studies are undertaken. 

Higher temperatures result in permanent degradation. The amount of this irreversible loss in mechanical properties depends upon moisture content, heating medium, temperature, exposure period, and, to some extent, species. The effects of these factors on modulus of mpture, modulus of elasticity, and work to maximum load are illustrated in Figures 6—9 (6). The effects may be less severe for commercial lumber than for clear wood heated in air (Fig. 10). The permanent property losses shown are based on tests conducted after specimens were cooled to - 24° C and conditioned to a moisture content of... 

Repeated exposure to elevated temperature has a cumulative effect. For example, at a given set of exposure conditions, the property losses ate about the same after six exposure periods of one month each and after a single 6-month period. 

Duration of Test Although the duration of any test will be determined by the nature and purpose of the test, an excellent procedure for evaluating the effect or time on corrosion of the metal and also on the corrosiveness of the environment in laboratory tests has been presented by Wachter and Treseder [Chem. Eng. Pi og., 315-326 (June 1947)]. This technique is called the planned-interval test. Other procedures that require the removal of sohd corrosion products between exposure periods will not measure accurately the normal changes of corrosion with time. 

When a clean steel coupon is placed in oxygenated water, a rust layer will form quickly. Corrosion rates are initially high and decrease rapidly while the rust layer is forming. Once the oxide forms, rusting slows and the accumulated oxide retards diffusion. Thus, Reaction 5.2 slows. Eventually, nearly steady-state corrosion is achieved (Fig. 5.2). Hence, a minimum exposure period, empirically determined by the following equation, must be satisfied to obtain consistent corrosion-rate data for coupons exposed in cooling water systems (Figs. 5.2 and 5.3) ... 

Equipment Reliability The probability that, when operating under stated environment conditions, process equipment will perform its intended function adequately for a specified exposure period. 

Beeause of the low rates of moleeular diffusion, assessment of workplaee air quality using passive samplers usually entails sampling for a working shift, and exposure periods of one to four weeks tend to be needed to measure eoneentrations in ambient air. 

Sample An equipment population, its exposure period, and stresses—from which a data set is derived. 

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. 

The measure used to describe the potential for noncarcinogenic toxicity to occur in an individual is not expressed as tlie probability of an individual suffering an adverse effect. The EPA does not at tlie present time use a probabilistic approach to estimate tlie potential for noncarcinogenic healtli effects. Instead, tlie potential for non carcinogenic effects is evaluated by comparing an exposure level over a specified time period (e.g., lifetime) witli a reference dose derived for a similar exposure period. Tliis ratio of exposure to toxicity is called a liazard quotient and is described below. (The reader is referred to Chapter 11 for additional details on tlie material tliat follows). The noncancer liazard quotient assumes tliat tliere is a level of exposure (i.e., RfD) below which it is unlikely for even sensitive populations to experience adverse healtli effects. 

To assess tlie overall potential for noncarcinogenic effects posed by more dian one chemical, a liazard index (HI) approach has been developed based on EPA s Guidelines for Healdi Risk Assessment of Chemical Mixtures. This approach assumes that simultaneous subtlu eshold exposures to several chemicals could result in an adverse healtli effect. It also assumes tliat tlie magnitude of the adverse effect will be proportional to tlie sum of the ratios of the subtlireshold exposures to acceptable exposures. The non cancer hazard index is equal to tlie sum of the hazard quotients, as described below, where E and tlie RfD represent the same exposure period (e.g., subclironic, clironic, or shorter-term). 

It should be noted diat E and RfD are expressed in die same units and represent die same exposure period (i.e., clironic. subclironic, or shorter term)... 

It is important to calculate die hazard index separately for clironic, subclironic, and short-temi exposure periods as described below. It is also important to remember to include RfDs for die noncancer effects of carcinogenic substances. 

Note that die total exposure hazard index is calculated separately for clironic, subchronic, and shorter-term exposure periods. 

The potential for noncarcinogcnic health effects is evaluated by comparing iui exposure level over a specified lime period (c.g., lifetime) with a reference dose derived for a similar exposure period. The ratio of exposure to toxicity in called a liazard quotient and, when it is greater tlien unity tlierc is a higher level of concern for potential noncancer effects. 

Odors may be detected for a very short exposure period, perhaps less than one second. It is thus necessary to determine the likely one-second peaks knowing the concentrations derived from the Gaussian equation. This is based on 10-minute average period. The equation to convert the time-averaging period is ... 

Table 2.17 Effect of exposure period on corrosion rate of mild ...

In extended exposure periods of up to 16 years in tropical sea water, Southwell and Alexander obtained an average corrosion rate for steel of 0-18 mm/y in the first year, falling off to a constant rate after 4 years at 0-025 mm/y. They also quote pitting rates as 1 mm/y in the first year falling... 

An interesting example of judicious choice of braze filler is to be found in the selection of silver alloys for the brazing of stainless steels to be subsequently used in a tap-water environment . Although the brazed joint may appear to be quite satisfactory, after a relatively short exposure period failure of the joint occurs by a mechanism which appears to be due to the break-down of the bond between the filler and the base metal. Dezincifica-tion is a prominent feature of the phenomenon and zinc-free braze alloys based on the Ag-Cu system with the addition of nickel and tin have been found to inhibit this form of attack. A similar result is obtained by electroplating 0-007 mm of nickel over the joint area prior to brazing with a more conventional Ag-Cu-Zn-Cd alloy. 

Field and Plant Tests Field exposure of test panels offers the benefit of a high degree of control over surface preparation and application. Moreover, through standardised exposure conditions, broader comparisons between both paint systems and locations are possible. More importantly, since replicates may be removed and laboratory tested periodically, changes in properties can be followed in considerable detail. At least four replicates should be examined for each exposure period to minimise the effects of atypical specimens. 

Tin compound Species Test material Exposure period and dose Effects NOAEL/LOAEL (mg/kg body weight per day) Reference... 

Different values are found in literature. Depending on the degree of exposure, periodic medical examination is suggested. The symptoms of poisoning do not become manifest until days. 

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