Cumulative criterion unit

Transfer of the TU principle to the community level results in hazard units or cumulative criterion units (CCUs). The most common types of hazard assessment use this concept. 

Several field gradient studies revealed predictive relationships between chemical stressors and biotic responses. For example, exposure to mixtures of heavy metals has shown concentration-response relationships with macroinvertebrate community indices (Hickey and Clements 1998 (dements et al. 2000). This indicates that response thresholds for ecological indicators (e.g., species diversity and EPT taxa) may be available for use in toxic impacts, providing the nature of the toxicant is known (e.g., metals, organics, and ammonia). The field studies have shown strong concentration-response relationships with stressors, with metal response related to the cumulative criterion unit (CCU) value for the sum of the metals present (Hickey and Clements 1998 Clements et al. 2000). The CCU concept was discussed in greater detail in Chapter 5. 

No more than two units may be below 91% of nominal. In the case a retest is necessary (three or four units failing this criterion), no more than six out of the cumulated 100 units may fail. 

To evaluate maintenance policies, we use the criterion of expected cost of system per time unit C = E (c(t))/t. The cumulative cost of sys-... 

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