**1. Introduction**

Freedom of design, time efficiency, reduction of labor and machine costs are a few examples of the several advantages mentioned when the subject is metal 3D Printing, also known as metal Additive Manufacturing (AM) [1]. Regardless of its considerable potential, metal AM has been raising some concerns regarding occupational health and safety [2]. Among other occupational risks, it is known that during these processes incidental metal nano-objects are emitted and it is essential to manage the risk of exposure to this airborne matter to reduce possible negative ill-health effects on workers [3].

Different approaches have been used to assess and/or manage the occupational risk of exposure to incidental nanomaterials in AM processes, but the definition of standardized methods still remains an urgent need [4]. Looking at this occupational risk from the point of view of the common industrial hygiene approach, it is possible to monitor and to quantify the airborne matter released during metal 3D printing. Recent publications in this field endorse the use of direct-reading instruments (for example condensation particle counter—CPC, optical particle counter—OPC and scanning mobility particle sizer—SMPS) and/or the collection of samples and subsequent structural and chemical analysis, by using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and/or

**Citation:** Sousa, M.; Arezes, P.; Silva, F. Occupational Exposure to Incidental Nanomaterials in Metal Additive Manufacturing: An Innovative Approach for Risk Management. *Int. J. Environ. Res. Public Health* **2023**, *20*, 2519. https:// doi.org/10.3390/ijerph20032519

Academic Editors: Delfina G. Ramos and Paul B. Tchounwou

Received: 20 October 2022 Revised: 27 January 2023 Accepted: 29 January 2023 Published: 31 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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energy dispersive X-ray analyzers (EDS) [2,3,5–8]. However, this attempt at a more industrial hygiene conservative approach has limitations that cross all these studies: The lack of clearly defined and standardized occupational exposure limits for metal incidental nanomaterials and the lack of standardized sampling strategies. Some of these studies use as comparison reference values for nanomaterials proposed by different competent local entities and institutes, but so far, no specific limits have been proposed for metal incidental nanomaterials. The most common approach is to compare the results to the recommended benchmarks defined by the Nanosafety Research Centre of the Finnish Institute of Occupational Health (FIOH), i.e., 20,000 nanoparticles/cm<sup>3</sup> (with a density higher than 6000 kg/m3) for an 8-h exposure time. This limit was later adopted by the Institute for Occupational Safety and Health of the German Social Accident Insurance (IFA DGUV) and the IVAM Environmental Research UVA BV in the Netherlands [5,9]. Even if this value is assumed to be an appropriate reference for metal AM case studies, the quantitative risk assessment still has limitations, namely the possible lack of access to equipment and laboratory analysis for these monitoring campaigns and also the lack of experts to perform them and interpret the results.

Another possibility to assess this risk during metal AM has been to apply qualitative methods originally designed for engineered nanomaterials (ENM), namely control banding based ones. Sousa et al. [8] and Dugheri et al. [5] applied Control Banding Nanotool v2.0 to assess the risk of exposure to ultrafine particles during metal 3D printing operations. Sousa et al. [8] highlight some difficulties on using this approach for incidental nanoparticles, especially the lack of background information on the particles (such as size, shape, and solubility, among others). These authors suggest the design of new methods for incidental nanomaterials, with different inputs than the ones for ENM, to reduce the uncertainty associated with the assessment. Dugheri et al. [5] also emphasize the importance of searching different strategies to assess this occupational risk.

This article aims to explore different approaches to study the potential exposure to incidental nanomaterials during metal AM, through a case study conducted in an organization using Selective Laser Melting (SLM) technology. The main purpose of this article is to propose a risk management tool, entitled IN Nanotool, designed for incidental metal nanomaterials originated from metal AM processes to overcome the limitations of other existing approaches.
