Substances increasing biological value

The Chemical Substances Threshold Limit Values Committee classifies certain substances found in the occupational environment as either confirmed or suspected human carcinogens. The present listing of substances that have been identified as carcinogens takes two forms those for which a TLV has b n assigned and those for which environmental and exposure conditions have not been sufficiently defined to assign a TLV. Where a TLV has been assigned, it does not necessarily imply the existence of a biological threshold however, if exposures are controlled to this level, we would not expect to see a measurable increase in cancer incidence or mortality. 

To deterniine the risks in the area of human health (workers, consumers, including the absorption of chemicals from the environment) the concentration of a substance is ascertained for so-called end points in the human body (organs or biological systems). The measured or calculated concentration (exposure level) is compared with the NOAEL (No Observed Adverse Effect Level). The NOAEL is the highest concentration at which no more effect is observed (usually in animal testing), i.e. there is no visible or measurable effect. If such a levef cannot be detected, the LOAEL (Lowest Observed Adverse Effect Level) can be applied instead. This is the value at which the effect first becomes visible or measurable with an increasing dose. 

Adjustment of sample pH is another effective measure to minimize biological degradation. Afghan et al. (1974) report that Pseudomonas bacteria, responsible for degradation of phenols, is destroyed by both high and low pH values. High pH values increase the chemical oxidation rate of humic substances, and should be avoided (Stevenson, 1982) low pH values can result in precipitation of humic acid if the solution is sufficiently concentrated. Generally, the concentration of humic acid is low and precipitation is not a problem. 

The value of differs for various substances e.g. -35 to -20 for terrestrial organic matter, —8 to -7 for atmospheric CO2, etc. Figure 5.4 illustrates the use of various values for biological matter. These corrections lead to considerable adjustments of conventional ages e.g. for the period 7 000 to 2 000 years ago, corrections increase ages by up to 1000 y. 

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