Continuous exposure control

It has to be stated, however, that substances which are covered by the continuous measurement system must be controlled separately according to the control measurement plan. 

Measurement devices for continuous monitoring of workplace concentrations have to fulfill the following requirements  

An installed counter has to record the times when the hmit value is exceeded. 

The system must give an alarm when the set levels for the pre-warning and the main alarm have been reached. 

In the case of a pre-warning or main alarm, clear measures have to be taken, which are to be defined in the standard operation procedures. 

---

The radiological hazard of tritium to operating personnel and the general population is controlled by limiting the rates of exposure and release of material. Maximum permissible concentrations (MPC) of radionucHdes were specified in 1959 by the International Commission on Radiological Protection (79). For purposes of control all tritium is assumed to be tritiated water, the most readily assimilated form. The MPC of tritium ia breathing air (continuous exposure for 40 h/wk) is specified as 185 kBq/mL (5 p.Ci/mL) and the MPC for tritium in drinking water is set at 3.7 GBq/mL (0.1 Ci/mL) (79). The maximum permitted body burden is 37 MBq (one millicurie). Whenever bioassay indicates this value has been exceeded, the individual is withdrawn from further work with tritium until the level of tritium is reduced. 

J. D. Everard and M. C. Drew, Mechanisms controlling changes in water movement through the roots of Helianihus anniais during continuous exposure to oxygen deficiency, Journal of Experimental Botany 40 (1989). 

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. 

Survival of young of year as in controls after continuous exposure for 6 months. After 4 months, viscera contained 0.6 mg Ag/kg ash weight, gills 0.38, and carcass 0.016 mg Ag/kg ash weight 28... 

Continuous exposure of mature females for 144 days before spawning resulted in 50% reduction in number of eggs produced and 15% reduction in egg viability however, 90 days after hatch, trout were 18% heavier and 10% longer than controls 13... 

Continuous exposure to diets containing PCBs from Saginaw Bay carp induced cytochrome P-450 activity in a dose-dependent manner. Cytochrome P-450 activity in animals switched to the control diet from the PCB-containing diet was the same as controls. EROD hepatic activity is a potential biomarker for current exposure to PCBs and other similar cytochrome P-450 inducers... 

Administrative actions to control exposure to toxic materials are clearly vital, and no program of exposure control can succeed without them. They are, however, indirect, in that they do not directly affect the specific causative conditions. For direct action, we need to consider an engineering approach. This will be presented as a continuation of Step Five in the next chapter. 

Similarly, Chow et al. observed an increase in the lysozyme activity of a soluble lung fraction and of plasma after continuous exposure of rats to ozone at 0.8 ppm for 8 days. However, no difference in lung or plasma lysozyme activity from control values was present in rats continuously exposed to 0.2 or 0.5 ppm or intermittently exposed (0.2-0.8 ppm, 8 h/day for 7 days). Histochemical evidence of an increase in lung acid phosphatase, a lysosomal enzyme, has also been reported. ... 

Chlorine is the oldest and most widespread method of water disinfection. In reverse osmosis systems, chlorine may be added to feedwater for control of micro-organisms and, in addition, to prevent membrane fouling by microbiological growth. According to Vos et al. [i,2], chlorine will attack cellulose diacetate membranes at concentrations above 50 ppm. Membranes were found to show a sharp increase in salt permeability and a decrease in strength after one week of continuous exposure. Under milder conditions (10 ppm chlorine for 15 days) no detectable change in performance was observed. Spatz and Friedlander [3] have also found cellulose acetate membranes to be resistant to chlorine when exposed to 1.5 ppm for three weeks. 

Extensive destruction of the olfactory epithelium was observed in male Fischer 344 rats exposed to 200 ppm [780 mg/m ] methyl bromide for 6 h per day for five days. By day 3, despite continued exposure, there was replacement of the olfactory epithelium by a squamous-cell layer, followed by progressive reorganization toward the normal architecture, and by week 10,75-80% of the epithelium appeared histologically normal. Olfactory epithelial-cell replication was maximal on day 3 of exposure, with a labelling index of 14.7% compared with 0.7% in the controls (Hurtt et al., 1988). Degeneration and subsequent regeneration were also observed in an inhalation experiment w ith Fischer 344 rats exposed to 175 ppm [680 mg/m ] 6 h twice, separated by a 28-day interval (Bolon et al., 1991). 

This is a safe, low cost, and more convenient approach as it does not involve special instrumentation, poisonous intermediates, and the growing rate can also be easily controlled. The utility of this process is underlined by the fact that even after continuous exposure to the toxic metal ions, the fungus readily grows and transforms the toxic conditions to nontoxic by reducing Cd to CdS without the use of any external source of sulfur. Another important, potential benefit of the process described is the fact that the semiconductor CdS nanoparticles, which are quite stable in solution, are synthesized extracellularly in large quantities. This is therefore, a very important advantage over other biosynthetic methods where the nanoparticles are entrapped within the cell matrix in limited quantity whereby an additional processing is required to release them from the matrix. 

---