Single value risks from individual risks
In this option, a percentage point can be specified for the chosen risk metric (margin of exposure (\(\operatorname{MOE}\)) or hazard index (\(\operatorname{HI}\))). The corresponding percentile is calculated from the distribution of individual risks. The default percentiles are a margin of exposure at 0.1% or a hazard index at 99.9%, but another value can be chosen. It can also be indicated whether the risk metric is calculated using the inverse distribution or not. This option is provided because percentile calculation in small data sets is asymmetric in both tails. When this option is set, the percentile is calculated as the inverse of the complementary percentage of the inverse distribution. E.g., the \(p_{0.1}\) of the \(\operatorname{MOE}\) distribution is calculated as 1/(\(p_{99.9}\) of 1/\(\operatorname{MOE}\) distribution); the \(p_{99.9}\) of the \(\operatorname{HI}\) distribution is calculated as 1/(\(p_{0.1}\) of 1/\(\operatorname{HI}\) distribution).
Adjustment factors and uncertainty specification
Many sources of uncertainty that may affect input data, model assumptions and assessment methodology do not enter the assessment. In EFSA (2020a) and EFSA (2020b), thirty-four sources of uncertainty were identified and the impact of each source on the \(\operatorname{MOE}\) was quantified. Some uncertainties tend to overestimate the \(\operatorname{MOE}\), others tend to underestimate it. Following the guidance of the EFSA Scientific Committee, specific \(\operatorname{MOE}\) and/or \(\operatorname{HI}\) percentiles are adjusted using adjustment factors for exposure and hazard, e.g. from expert elicitation. They may be available as fixed values or as parametric uncertainty distributions. In the nominal run, the percentile is adjusted with the median of the uncertainty distribution. In each uncertainty run, adjustment factors are sampled from the uncertainty distribution. In the MCRA interface, for both exposure and hazard distribution separately, a fixed value or a parametric uncertainty distribution is specified. The available parametric uncertainty distributions are the same as available in the SHELF package that was used by EFSA. The SHeffield ELicitation Framework (SHELF) is a package of documents, templates and software to carry out elicitation of probability distributions for uncertain quantities from a group of experts (http://www.tonyohagan.co.uk/shelf/).
- Options for specifying uncertainty distributions are:
Lognormal(\(\mu\), \(s\)) with offset \(c\). Parameters \(\mu\) and \(s\) specify the mean and standard deviation of the underlying normal.
Log Student t(\(\mu\), \(s\), \(\nu\)) with offset d. Parameters \(\mu\) and \(s\) specify the mean and standard deviation of the underlying normal, \(\nu\) the degrees of freedom, \(\nu\) > 0
Beta(a, b) scaled to the interval [\(c\), d], with shape parameters a and b > 0.
Gamma(a, b) with offset c, with shape and rate parameters a and b > 0.
Figure 93 Scaled lognormal (\(\mu=0.705\), \(s=0.566\), offset=1), table 8, EFSA (2020b).
Figure 94 Scaled logstudents t (\(\mu=-0.593\), \(s=0.367\), \(\nu=3\), offset=0.5), table 9, EFSA (2020b).
Figure 95 Scaled beta (a=2.37, b=4.26, lowerbound=0.5, upperbound=6), table 7, EFSA (2020a).
Figure 96 Scaled gamma (a=3.26, b=3.56, offset=0.9), table 6, EFSA (2020a).
Background-only adjustment factor
When exposures are calculated by combining focal food/substance concentrations with background concentrations, it may be appropriate to have a separate adjustment for the foreground and background. A pragmatic solution agreed with EFSA is to estimate the contribution of the foreground in the tail above the selected percentile. Suppose this contribution is \(c\). Note that \(c\) will vary in uncertainty runs. Then, the adjustment factor should be multiplied by \((1-c)\), i.e. no adjustment for the focal part.
The calculation proceeds as follows:
Note that when the focal substance measurements are converted to active substances using substance conversions or deterministic substance conversions, then \(c\) is the sum of the contributions of the focal food in and all active substances to which the substance translates.
In Figure 97, an example is shown where the margin of exposure is adjusted for the exposure and hazard distribution based on expert elicitation. The median adjustment factors for exposure and hazard are respectively, 1.77 and 3.01. The overall adjustment factor is 5.33.
Figure 97 Margin of exposure (model) and adjusted margin of exposure (model + expert) with uncertainty bounds.