Human monitoring analysis calculation
Human monitoring analysis computes substance concentration estimates for a biological matrix (e.g., urine or blood) based on human monitoring data. These estimates are specified at the level of long term average concentrations for individuals in case of chronic assessments, or concentrations for individual-days in case of acute assessments. The concentrations are computed independently for each substance and biological matrix.
The main steps for computing the human monitoring concentration estimates are:
Imputation of censored valuess.
Imputation of missing values.
Calculation of individual concentrations (chronic) or individual day concentrations (acute).
Imputation of censored values
Similar to concentrations measurements in food, human monitoring measurements contain measurements below the limit of reporting and similar to concentrations modelling in foods, human monitoring analysis addresses these censored values and replaces them with imputed concentration values. Two approaches are available:
Replace censored values by zero.
Replace censored values by a factor * LOR, the factor is set between zero and one.
Replace non-detects by a factor * LOD and non-quantifications by LOD + factor * (LOQ - LOD), the factor is set between zero and one.
In Figure 72, the human monitoring concentrations data for three bisphenols are imputed with a factor * LOR and the summary statistics are visualized.
Figure 72 Boxplots for imputed concentration data for bisphenols BPA, BPS and BPF. Lower whiskers indicate the p5 and p10 percentiles, upper whiskers the p90 and p95. The edges of the box indicate the p25 and p75 percentiles with the median in the centre of the box.
Imputation of missing values
For missing concentration measurements, two imputation methods are available:
Replace missing values by zero.
For each substance and biological matrix and sampling type, replace missing values by a random other sample of this substance, biological matrix, and sampling type.
Note that for the second imputation method, more refined methods could be used. E.g., when for a given day multiple samples are available and one is missing, then this sample might be neglected in the computation of an average exposure. Also, when samples have been taken at different time-slots, impute the missing records using samples from the same time-slot.
Calculation of acute human monitoring concentrations
For acute assessments, the monitoring concentrations are computed for each substance and biological matrix as average individual-day concentrations. For a given substance and biological matrix, the acute individual-day concentration \(c_{ij}\) for individual \(i\) on day \(j\) is:
where \(n_{\mathtt{samples}}\) is the number of samples available for individual \(i\) on day \(j\), and \(c_{ijk}\) and \(\mathit{sg}_{ijk}\) denote the concentration and specific gravity correction factor, respectively, of the \(k\)-th sample of the individual day \(j\).
Calculation of chronic human monitoring concentrations
For chronic assessments, the monitoring concentrations are computed as the average monitoring concentrations of multiple individual-days for each substance and biological matrix. The chronic concentration \(c_{i}\) for individual \(i\) is computed as:
where \(n_{\mathtt{days}}\) is the number of days that individual \(i\) was monitored, and \(c_{ij}\) denotes the average monitoring concentration of individual \(i\) on day \(j\).