Study of metal-plating processes: characterising the aerosols emitted and biological monitoring of operators

Outline of reasons and objectives
Thermal spraying is a process during which a coating is applied at high temperature and at high speed onto a metal object, for the purposes, for example, of producing a surface that is more resistant to corrosion, or of improving its conductivity. Various processes exist, including wire flame, powder flame, electric arc, HVOF HVOF: High Velocity Oxy-Fuel spraying, also known as “supersonic flame” spraying, and plasma spraying. Depending on their composition, their concentration, and on the duration of exposure, metal-plating fumes might cause adverse effects for health and cause occupational diseases.
The study is based on the observation that metal-plating workers might be exposed differently depending on the processes used. The particle-size distribution, and the number and the quantity of the aerosols produced might vary depending on the {processes/materials} matrix in question. This variability might have consequences on exposure of operators, and thus on the toxicity and the levels of urinary excretion. This cross-cutting study proposed to characterise the emissions and to assess the levels of exposure at the various thermal spraying stations in order to propose a biological monitoring strategy that was suitable for being used by the occupational safety and health specialists. Firstly, and in order to prioritise the prevention actions, an industry survey was taken and examined in order to deepen the knowledge of the sectors that use thermal spraying processes and in order to evaluate the number of potentially exposed employees.

Approach
In the industry study, questionnaires were sent out to over 21,300 sites distributed across the 56 inventoried sectors of activity that included 61,000 sites.
Firstly, the aerosols were characterised on site, in collaboration with a company specialised in developing thermal spraying systems. An inventory of the various thermal spraying techniques was taken in order to determine the base configuration on the basis of which a certain number of experimental parameters were to be tested. The physical and chemical properties and the particle-size distribution of the aerosols of the {processes/materials} pairs that were thus selected were characterised, while specifying the distribution of the ultrafine particles (UFPs) and while, as far as possible, using a single metrology method.
In order to determine the relevance of the metals of interest as biological exposure indicators, sampling campaigns were conducted in the companies, by associating personal and atmospheric sampling with urine sampling.

Main results
The rate of return of the questionnaire was 43% and, after adjustment of the data, it appeared that in the range 1.3% to 1.6% of the companies had at least one thermal spraying facility, mainly located in the Rhône-Alpes Region of France. In general, the sites having integrated workshops were small structures (with fewer than 20 employees), and the number of operators working on or close to the processes was estimated at in the range 4,100 to 4,900. The most widely used process for anticorrosion treatment was electric arc spraying, with zinc-aluminium alloy as the feedstock material. Chromium (Cr) and nickel (Ni) were used in all of the processes.
Characterising the aerosols on site made it possible to describe aerosols in which UFPs were in a majority. The values for concentration in numbers of emitted particles were, in the context of thermal spraying operations, very considerably higher than the concentrations encountered in a “welding fumes” context. Thus, during this study, a number of particles emitted per second of about 1014 was highlighted, for a mean observed concentration in welding fumes of about 2 to 4. 1011 particles.s-1. It is thus highly probable that the population of workers working in the thermal spraying sector are likely to be more exposed to high concentrations of UFPs than the population of welders are.
Prospecting of the companies was focused on the use of Cr and Ni as feedstock metals due to their recognised toxicity. The results of the urinary and atmospheric sampling campaigns, conducted on six sites with 29 volunteers, including 14 metal-plating workers, were processed in descriptive manner only, because of the small number of metal-plating workers monitored. The atmospheric sampling confirmed the majority presence of UFPs in the aerosols. The metal-plating workers spraying Cr oxide using plasma were exposed to Cr(VI). As regards the results of the biomonitoring, the levels of urinary excretion of Ni, for all processes together (11 µg/g creatinine), are relatively high compared to the levels in the controls, whose values were close to those in the general population, i.e. 3.8 ug/g creatinine. The urinary chromium levels measured remained relatively low, lower than 2 ug/g creatinine, remaining very remote from the French guide value applied to welders, that value being 30 µg/g creatinine.
A descriptive statistical analysis, process-by-process, would appear to show that the wire flame process exposes operators to a greater extent, but this result remains to be confirmed with additional data. For the same reason, it was not possible to find a relationship between the atmospheric data and the excreted urinary levels.
Discussion
The industry survey showed that thermal spraying processes are used in numerous sectors of activity but in scattered manner.
Characterisation of the aerosols made it possible to show the existence of a majority of particles of nanometric size (UFPs), and that the total mass concentrations recorded on emission and the mass distributions of the particles varied very considerably depending on the {processes/materials} matrix studied. The multiple information feedback from the experimentations conducted made it possible to assess the potential of certain methods and their limits for future use out in the field.
The Cr and Ni urinary excretion levels observed in the field attest to occupational exposure of metal-plating workers to aerosols emitted during use of thermal spraying processes. Occasional high urinary concentrations show that the exposure risk is not negligible, and recall the importance of wearing personal protective equipment appropriate to the activity.
These preliminary results would encourage us to continue the investigations undertaken in this sector of activity. Additional data should be collected, possibly with other biomarkers of exposure such as intra-erythrocytic Cr, specific to exposure to Cr VI.