Tier 0 screening uncertainty analysis

Tier 0 analysis may be done just once, but not repeated for every individual assessment. Even though Tier 0 assessments are practical to conduct, they are not suited for addressing problems that require a realistic identification of key factors and exposure conditions contributing to assessment outcomes and uncertainties. Higher-tier assessments are often needed to answer such questions. 

A factor of 2-10 is often used as a default estimate of upper-bound uncertainties associated with ambient air quality modelling of exposures to particulates or gaseous pollutants. 

The 1996 Food Quality Protection Act in the United States advocated the use of a 10-fold 

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Tier 0 exposure assessments, which are commonly used for first-tier screening purposes, do not require an analysis of uncertainty on every occasion, provided they include appropriate conservative assumptions or safety factors to take account of uncertainty. 

An uncertainty analysis gives the assessor the opportunity to re-evaluate the scenario, model approaches and parameters of the analysis and to consider their influence in the overall analysis. The practical impact of uncertainty analysis is illustrated within the annexed case-studies, which also clarify how uncertainty analyses follow a systematic methodology, based on a tiered approach, and consider all possible sources of uncertainty. The first step in uncertainty analysis consists of a screening, followed by a qualitative analysis and two levels of quantitative analysis, using deterministic and probabilistic data. The assessor should be aware that an uncertainty analysis cannot answer all the questions, which, moreover, may lead to new questions. 

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