Mapping of sectors and processes generating Polycyclic Aromatic Hydrocarbons (PAHs) (

Outline of reasons and objectives
Recent work has highlighted new activities such as carbon densification and repairing facilities covered with pitch-based coatings, and during which occupational exposure to Polycyclic Aromatic Hydrocarbons (PAHs) was unsuspected or little-known since the literature does not address or hardly addresses PAH generation during such activities. This shows how difficult it is for hygienists and companies to identify the potential presence of a PAH risk.
The objective of this study was to develop and to make available a typological tool for identifying the sectors and the processes that might be likely to generate PAHs, to which 111,000 employees in France are potentially exposed according to the SUMER 2010 survey.

Approach
The study consisted firstly in identifying the sectors of activity in which PAHs were emitted and operatives were exposed. Using benzo[a]pyrene (BaP) as a carcinogenic risk tracer, the risk sectors were prioritised. Six sampling campaigns (atmospheric, urinary, surface, and material or residue samples) were then performed in the activities of gas carburising, carbon densification, and coking to supplement that data. These campaigns made it possible to enrich the knowledge on the PAH generation conditions specific to each of the processes. In a third stage, work was done to check the influence of parameters such as temperature, pressure, and gas reagent flow rate on generation of benzene, of PAHs, and of soot in a process such as low-pressure gas carburising.

Main results
Low-pressure carburising, carbon densification, and coking generated BaP at levels significantly greater than the level recommended by the French National Health Insurance Fund, that recommended level being 150 ng/m3 of air inhaled over an eight-hour shift. During a pyrolysis operation, the gas-phase cracking operating conditions are such that PAHs or PAH precursors are generated. On exiting from the furnaces, the PAHs form deposits on coming into contact with cold surfaces, giving rise to exposures during maintenance and cleaning operations. However, if the furnaces are not completely sealed (leaks or doors open), the exposures can take place during production, in particular for coking. Exposure routes are both by inhalation and via the skin. In addition, the theoretical studies conducted using a simplified reaction mechanism for acetylene pyrolysis showed that by using low pressure and by reducing the residence time, it is possible to reduce the concentrations of carcinogenic substances and PAH initiators such as benzene.

Discussion
After having monitored exposure of employees in the activities of low-pressure carburising and of carbon densification, over several years and in various different companies, it was observed that implementing prevention solutions made it possible to reduce the high levels of exposure measured during the initial campaigns. However, in the coking sector, they remained very high, or indeed excessive, for certain jobs/workstations. In that sector, there is room for progress and efforts should be made to limit exposure by inhalation and exposure by contact with the skin. A theoretical study on low-pressure carburising showed that a change in the operating conditions of the pyrolysis made it possible to reduce the PAHs at source. Application to industrial processes is complex and, in any event, will require the parts and the process to be qualified.