Thresholded contaminants

Risk evaluation is an apparently simple task of comparing an estimate of intake with the PTWI. If intakes are below the PTWI then there is no risk whereas if they exceed the PTWI then some risk management action may be required. For non-thresholded contaminants risk is assumed to be proportional to intake and therefore intakes should be as low as practically achievable. In practice risk evaluation is a far less certain science. 

For non-thresholded contaminants some mechanism is required that will allow the benefits in terms of reduced risks and costs associated with control to be taken into account. The costs of control will include enforcement costs as well as costs to producers in reaching ever stricter standards. Ultimately these costs will be borne by consumers in taxes, increased prices or reduced choice. Economic theory dictates that there must be a point where the extra increase in the cost of control is not justified by the corresponding increase in benefit (reduction in risk). This optimal point will differ for each contaminant according to the technology needed to control it, the nature of the hazard, and the relationship between dose and risk. It is in this latter context that quantitative risk assessment (QRA) becomes critical (see section 2.3.4 of this chapter). 

Test the compressor on impure hydrogen streams to determine threshold contamination levels (levels at which compressor performance is affected) for water (H2O), O2, CO, and methane (CH4). 

Repeated exposures of animals to high (near-lethal) concentrations of vapors result in inflammation of the respiratory tract, as weU as degenerative changes in the Hver, kidneys, and heart muscle. These effects arise at concentrations far above those causing irritation. Such effects have not been reported in humans. The low odor threshold and irritating properties of acrylates cause humans to leave a contaminated area rather than tolerate the irritation. 

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