Exposure duration

One of the most significant laser safety standards is that developed by the Z-136 committee of the American National Standards Institute (ANSI) (55). Although it is voluntary, many organi2ations use the ANSI standard. It contains a number of items including a recommendation for maximum permissible levels of exposure to laser radiation for various wavelengths, exposure durations, and different parts of the body separation of lasers into four different classes according to the level of ha2ard they present and recommendation of safety practices for lasers in each of the classes. 

Data are for comparative purposes only actual resistance depends on many factors including stress, temperature, concentration, and exposure duration. 

Development of subchronic RfDs parallels the development of chronic reference doses in concept the distinction is one of e.xposurc duration. Appropriate studies are evaluated and a subchronic NOAEL is identified. The RfD is derived from the NOAEL by the application of the UFs and MF, as outlined above. When experimental data arc available only for shorter e.xposurc durations than desired, an additional uncertainly factor is applied. This is similar to the application of the uncertainly factor for duration differences when a chronic RfD is estimated from subchronic animal data. On the other hand, if subchronic data are missing and a chronic oral RfD derived from chronic data exists, the chronic oral RfD is adopted as the subchronic oral RfD. Ill this instance, there is no application of an uncertainly factor to account for differences in exposure duration. 

The odor perception threshold for benzene in water is 2 ing/L. The benzene drinking water unit risk is 8.3 x lO L/pg. Calculate the potential benzene intake rate (mg benzene/kg-d) and the cumulative cancer risk from drinking water with benzene concentrations at half of its odor threshold for a 30 year exposure duration. 

There are several limitations to tliis approach that must be acknowledged. As mentioned earlier, tlie level of concern does not increase linearly as the reference dose is approached or exceeded because the RfDs do not luive equal accuracy or precision and are not based on the same severity of effects. Moreover, luizm-d quotients are combined for substances with RfDs based on critical effects of vaiy ing toxicological significance. Also, it will often be the case that RfDs of varying levels of confidence Uiat include different uncertainty adjustments and modifying factors will be combined (c.g., extrapolation from animals to hmnans, from LOAELs to NOAELs, or from one exposure duration to anoUier). 

To assess die overall potential for non carcinogenic effects posed by several exposure pathways, the total haziird index for each exposure duration (i.e., chronic, subchronic, and shorter-term) should be calculated separately. This equation is described below ... 

BR = Breathing rate, ni /d DUR = Exposure duration period, yr ABW = Average body weight, kg LIFE = Lifetime exposure of 70 yr... 

A = absorption factor, i.e.,the fraction of carcinogen absorbed by the human body ED = exposure duration... 

Exposure/ duration/ Species frequency (Strain) (Specific route)... 

Exposure Period Three exposure periods - acute (less than 15 days), intermediate (15-364 days), and chronic (365 days or more) are presented within each relevant route of exposure. In this example, an inhalation study of intermediate exposure duration is reported. For quick reference to health effects occurring from a known length of exposure, locate the applicable exposure period within the LSE table and figure. 

Exposure/ Duration/ Frequency (Specific Route) LOAEL ... 

Species (Strain) Exposure/ Duration/ Frequency (Specific Route) System NOAEL (mg/kg/day) LOAEL Reference Chemical Form... 

The shear forces are mainly in the range of 1 to lONm. This exposure causes cell death between 20 and 80% depending on the exposure duration which is between a few seconds and several hours. Studies performed in a bioreactor have an exposure duration of several days. The results are partly contradictory. Tramper et al. [30] found a critical stress level of 1.5 Nm" for insect cells, whereas Oh et al. [31] could not show an influence on hybridoma cells even at high stirrer speed. This shows that each cell line reacts different and that there is a necessity for defined stress systems if the results is to be comparable. 

Key to figure Species/ (strain) Exposure duration/ frequency System NOAEL (ppm) LOAEL (effect) ... 

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