19. Rapid test philosophy and the French field study

The technical information from Caterpillar, MAN and in particular Rockwool partly follows a staged detection and response logic:

1. First, a search is made for visible yellow, yellowish or orange deposits.

2. Where there is such a visible indication, a colorimetric rapid test is used or a laboratory analysis is recommended.

3. Special protective measures are then linked to positive detection or to a specific suspicion.

This approach has a comprehensible purpose as an indicative warning and screening procedure. A positive result is a reliable warning signal and requires protective measures. It is not suitable, however, for concluding from a negative result that Cr(VI) contamination is absent.

In the ProRox instruction, testing for Cr(VI) is linked to conspicuously coloured deposits. In addition, it requires protective measures when such deposits are removed. The visual detection logic, however, does not reliably capture every contamination and must not be understood as exoneration in the absence of colour or in the event of a negative rapid test.

As a release or exoneration logic, this is not technically sound. The material configuration of chromium-alloyed hot component, basic oxides, oxygen and temperature already establishes a specific situation of suspicion. A negative rapid test does not reduce this suspicion to zero, because its detection sensitivity, representative sampling, surface heterogeneity and the location tested are limited. Nor does the subsequent chemical treatment or "neutralisation" of residues already present replace either the prevention of the formation pathway or reliable proof of effectiveness under controlled conditions.

Prevention standard of the German TRGS 561

Section 5.9.3 of the German TRGS 561 (Technical Rule for Hazardous Substances) names alkali/alkaline earth metal oxides from insulation materials or assembly pastes as an influencing factor. Paragraph 5 requires protective measures in accordance with 4.1 and 4.2 as well as supplementary measures where chromium(VI) contamination is present or cannot be ruled out. Basic oxide contents in the thermally stressed material system are therefore an indication of a hazard; the protective approach does not begin only with a positive finding. [19]

The following French field study directly shows the practical consequence of this rapid test philosophy.

The French field study: why negative rapid tests do not permit an all-clear

The French investigation by Passeron et al. (2025) during the maintenance of diesel and marine engines shows a further level of practical significance: chromium(VI) contamination can be overlooked with visual colorimetric rapid tests.

The company used colorimetric tests internally and based preventive decisions on them. The study authors compared these tests at the same sampling points with surface wipe samples analysed for Cr(VI) by ion chromatography. The result was unequivocal:

The values are absolute Cr(VI) quantities per sample, not area-related concentrations.

The negative rapid test on the turbocharger despite a laboratory finding of 869 ng Cr(VI) is a clear false-negative result. The authors also note that several positive surface samples were not detected by visual colorimetric tests. They therefore expressly conclude that these tests are to be used with caution.

For the reliable assessment of surface contamination, representative wipe samples and laboratory analytical Cr(VI) determinations are required. These assess a different part of the hazard than activity-related air measurements. In accordance with section 24, the exposure routes must each be considered independently.