Suction systems to reduce the formaldehyde load in pathologies

Status:

completed 08/2022

Aims:

The purpose of a pathological investigation is the histological preparation and evaluation of tissue samples obtained through a biopsy. To do so, the samples are usually first fixed in 4% formalin solutions. Preparation starts with the cutting of the tissue samples, followed by the histological examination of each sample. By fixing the samples in formaldehyde solutions, staff are usually exposed to formaldehyde. To reduce the load, cutting is carried out on special cutting tables that are each equipped with an extractor, and the formaldehyde vapours are extracted via the surfaces of the cutting tables. Each table surface consists of a perforated metal sheet, so that the formaldehyde vapour – which is heavier than air and would therefore sink down – is captured by a downward air flow above the table.

Readings taken by the German Social Accident Insurance Institution for Health and Welfare Services (BGW) show that, in numerous cases, the formaldehyde limit for workplaces (0.3 ml/m³) is exceeded in pathological investigations while cutting is performed, even though the tables are equipped with extractors.

Within the scope of this project, two operating parameters were identified on a table for the provision of examples: the air volume flow and the acquisition speed. Both are necessary to ensure safe compliance with the formaldehyde limit.

Activities/Methods:

Some typical cutting jobs were simulated on a pathology lab table for the purpose of providing examples. This involves cutting pieces of pork that had previously been soaked for at least 24 hours in a 4% formaldehyde fixing solution, commonly used in pathological investigations. Next, the formaldehyde solution that occurred in the ambient air was identified through several readings at different measuring points while varying the boundary conditions (especially the extraction volume flow). The measured formaldehyde concentrations were used to derive the air flow / acquisition speed that ensures safe compliance with the formaldehyde limit. However, the extraction volume flow cannot be increased indefinitely, as this would cause the pathologists’ fingers to get cold, making it more difficult for them to carry out such delicate work. Besides cutting operations, the handling of formaldehyde solutions in pathological investigations (e. g. decanting and pouring) was also simulated and the resulting lengths of exposure were measured.

As the pathology lab tables of other manufacturers are similar in design, it can be assumed that the acquisition speeds determined on this table are also applicable to the tables of other manufacturers.

Furthermore, it was shown that the acquisition speed of air on pathology lab tables can be measured with sufficient accuracy using simple measuring devices (e. g. anemometers), so that supervisory staff and other prevention specialists can assess the effectiveness of extraction tables and draw conclusions about formaldehyde exposure in pathological investigations without having to carry out complex workplace measurements.

Results:

At an extraction volume flow of at least 300 m³/h with an extracted table surface of 0.585 m², it can be expected that the cutting of tissue samples will take place within the safe workplace limits for formaldehyde solutions. This value can also be applied to other extraction surfaces, based on air speed as the measured variable. For BGW, this provides a simple way to check the effectiveness of under-table extraction in pathological investigations. With the measuring system used by BGW (Testo air flow hood), a reading of 0.9 m/sec means that extraction has been effective.

Another lesson learned from the project is that there needs to be extraction between the cutting board and the pathologist; otherwise there is thermal buoyancy, which increases the concentration of formaldehyde vapours near the pathologist. All that is needed is a small distance between the cutting board and the front edge of the table. Also, the use of a hood either reduces exposure or it leads to a reduced extraction volume flow while keeping the exposure constant. However, it can obstruct the work.

Furthermore, it was shown that it is impossible to keep within safe limits during waste disposal work and especially while decanting – even if the extraction volume is above 300 m³/h. The reasons are, on the one hand, large surfaces and, on the other hand, the distance from the surface of the table (decanting from a 5-litre canister occurs at a height of 30-40 cm above the table). The use of a hood may not be suitable for such work. To keep safely within the limit, we therefore recommend carrying out any decanting within a fume cabinet.

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