Based on the results of previously reported studies14,15,17,23 that showed typical predominance of micro- and nanoparticles in WFs, the 3D-modeling of clouds was based on the granulometric data, obtained by the ‘Nano’ mode of measurements. It should be noted that depending on the materials that were welded, the median values of particle size distribution (D 50 ) varied from 0.06 µm (electrode EA-395/9-3.0-LD1 E-B20) to 94.71 µm (electrode КК-50N Kiswel). This shows that within a radius of 5 m from the source, the particle size after absorption by water varies over a very wide range. In this case, only a fraction of small particles is capable of forming relatively stable aerosols, whereas large particles are susceptible to rapid precipitation if they do not contain cavities. Regardless of the reasons for the formation of large particles (secondary agglomeration in air and water or the formation of sprays), their presence when absorbed by water indicates the possibility of their absorption by the welder’s body. The minimum particle size potentially absorbed by the particle welder’s body at different points of the working zone was determined with the use of the MR-3 electrode with rutile covering (Ø3 mm) (Fig. 2).

Figure 2 Particle size distribution of WF at ‘Nano’ mode (MR-3 rod with rutile covering). Full size image

Thus, the peculiarities of formation of fume particles of РМ 10 fraction within the entire space of the working zone were examined, using commercial electrodes Cho Sun CR-13, UONI-13/5, Bridge Brand J-421, ESAB OK-46 with various types of covering (Fig. 1 and 3, Tables 1 and 2). Table 2 presents the mean values of measurement results. The differences in values do not exceed 12%. According to other reference data, the presence of РМ 10 particles in the air of workspaces varies within the range 15–80% (depending on the type of the industrial object)25. As a conclusion, maximum levels of pollution with particles of PM 10 fraction occur in the workspace during arc welding operations (Table 2). Figure 3 shows 3D models of РМ 10 particle distribution of within the workspace when the applied amperage is 150 А and use of various types of covered electrodes. 3D models with applied amperage of 100 А were presented in previous studies23,24. These models represent the percentage of particles of РМ 10 fraction of the total amount of WF at different points of the workspace. Therefore, the addition of percentages of each one of the 3 directions (↓S, ← W, → E) corresponds to the total 100% of WFs. Disregarding the types of electrodes used, the 3D models of РМ 10 particle distribution at the floor plane exhibited corrugated morphologies. All 3D models demonstrate high concentrations of the РМ 10 particles at distances 0–3 m and 4–5 m from the emission source (Fig. 3). This peculiarity may be connected with the height of the emission source from the floor line (0.8 m). The fume cloud seems to reach levels of Q (РМ 10 ) > 60% even at distances of 5 m from the emission zone when electrodes with rutile, basic and acidic coverings and applied amperage of 150 А are used (Table 1, Fig. 3b). It should be noted, that this entails the pollution of a space of over 280 m3 during welding operations, able to be caused just by one electrode (~1 min). Therefore, presence of the supporting working staff within this working zone without protective equipment is dangerous for their health (in accordance with Fig. 1).

Figure 3 3D models of particle distribution of РМ 10 fraction of WFs during welding with industrial electrodes Cho Sun CR-13 (a), UONI-13/55 (b), Bridge Brand J-421 (c), ESAB OK-46 (d) (metal plates VSt-3sp, S = 8 mm, I = 150 А). Full size image

Table 2 Granulometric characteristics of WF depending on the amperage of arc welding with covered electrodes of various types (metal plates VSt-3sp, S = 8 mm). Full size table

In Table 3, geometrical types of 3D models (↑H axis) are reported in relevance with the types of covered electrodes and the values of amperage applied23,24. It should be noted that the amplitudes of dispersion of WFs at the floor line (↓S, ← W, → E) are proportional to their dispersion geometry along the height (↑H) (Fig. 3).

Table 3 Geometrical types of 3D models depending on the type of electrode covering. Full size table

In general, when electrodes with rutile and acidic types of covering are used, an increase of amperage from 100 to 150 А causes more even dispersion of the fume cloud in the directions ↓S, ← W, → E. Moreover, use of covered electrodes of acidic type is characterized by minimal difference in values D 50 and Q (PM 10 ) between points of sampling (Fig. 1, Table 2, Fig. 3a,c). In contrast, when electrodes with basic and rutile-cellulose types of coverings are used, the dispersion of particles of the РМ 10 fraction within the space of the work zone is uneven (Fig. 3b,d)23,24. This can be explained by the different intensity of metal vaporization that results from the variableness of the combustible component of the welding vapor that is forming1,16. Therefore, an increase in applied amperage causes a decrease in the burning stability of the welding arc. In electrodes with a basic type of covering, the destabilizing factor of the burning arc is the presence of the fluorine ions F- that play the role of arc deionizers26. An increase in amperage during the welding process when such type of electrodes are being used, leads to a faster size reduction of particles D 50 , in the area of a worker’s breath (↑H), where this parameter decreases by more than two orders of magnitude (Table 2). Samples collected from different points of the space prove the predominance of nano-sized WF components (<100 nm). This corresponds to previously reported results1, showing that the burning of basic type electrodes is less stable in contrast to the rutile ones. The ramp-up of D 50 with increase of applied amperage from 100 to 150 A is typical for welding using electrodes of rutile-cellulose type. Concerning the electrodes with acidic type covering, no significant changes were observed (Table 2). As a result from the experiments, it is found that the maximum hazard is caused when electrodes with basic covering and high values of amperage applied are used, in contrast when the acidic, rutiles and rutile-cellulose types are used, which do not prove to be that dangerous. Moreover, the biological hazard with basic type of covering, in comparison with non-fluoric electrodes, is increased due to the emission of the toxic gases HF and SiF 4 . The peculiarities of the particle morphology and elemental composition of WF that form during welding with this type of electrodes was also investigated (Figs 4 and 5).

Figure 4 Scanning Electron Microscopy images of the morphological types of solid particulates condensed from vapor during welding using the covered electrode UONI-13/55 of the basic type — general view (a), tree-like (coral) (a, insert), solid (b), hollow (c), perforated (d), sharp-edged (e) and ‘nucleus-shell’ structures (e, insert). Full size image

Figure 5 Scanning Electron Microscopy image of WF components (a), as well as their element composition – segment spectrum ‘1’ (b), and ‘2’ (c), accordingly (covered electrode UONI-13/55 of basic type). Full size image

During analysis, the main morphological types of WF were examined and various types of morphologies occurred (solid and hollow spheres, ‘nucleus-shell’ structures27, perforated spheres, sharp-edged plates, aggregates of tree-like (coral) shape (Figs 4b–e and 5a). Formation of WF is a process that involves two stages. At first, vaporization of metal in the arc zone takes place leading to the dispersion of the formed vapors with the subsequent competing mechanisms of growth, such as coagulation and condensation8,9,28. Thus, the melted microparticles seek minimization of the surfaces free energy, reduction of the contact area up to the spheroidizing moment and reaching then isolation (Fig. 4b–d). In case of nanoparticles, high temperatures lead to irreversible changes in the particle morphology (Fig. 5a). The mass heating of particles and the loss of concrete shape results from the significant activation of the diffusion mass-transfer process. This leads to the formation of agglomerates of tree-like (coral) shape and sizes of up to ~100 μm (Fig. 4a, insert; Fig. 5a)29. It should be noted that some microparticles have polycrystalline (ceramic) microstructure (Fig. 4b, insert). The grains of oscillating elemental composition are forming during oxidation of the burning surface of the spherical solid particulates in the atmosphere.

According to data from chemical analysis (Fig. 5b,c), the core of metal composition of the WFs consists of iron Fe, manganese Mn (3rd hazard class), chrome Cr, nickel Ni and copper Cu (2nd hazard class), and calcium Ca, which correlates with the referenced data6,7,30,31. The peculiarity of the fume formation during the arc welding process is the combination of the balanced vaporization and unbalanced (combustible) shift of the molten components into fumes. This explains the bifractional formation of WFs (Fig. 5a ‘Spectrum 1’, Fig. 5b). Therefore, the fraction of smaller agglomerates of the tree-like shape is associated with normal vaporization conditions, when the percentage of WF can be represented as a function that depends on the composition of the electrode molten metal and on the values of vapor pressure of its elements26. The content of the volatile manganese in this fraction is significant (Fig. 5b). At the same time, the explosive character of the melt evaporation prevents rapid increase in the content of the volatile manganese to the equal partial pressure (Fig. 5c, Scanning Electron Microscopy). Since the manganese compounds are found in great concentrations, it can be concluded that almost all the manganese-containing particles have sizes of the PM 10 fraction.

Data on the chemical composition and the morphology of the WF is also important for understanding their biological activity and toxicity to human health. The micron solid particulates may damage tissues of internal organs of a human and the particles of the small fraction and their agglomerates of the tree-like (coral) morphology are highly cytotoxic (Figs 4e and 5). The PM 10 particles (primarily nanoparticles) infiltration of the organism stimulates a protective reaction, which initiates inflammatory processes, including even a development of thrombosis32. With reduction of particle sizes, their infiltration abilities increase, as well as the probability of intravasation into the human blood. Ultrafine particle sizes are able to easily infiltrate in lungs through the membranes of the alveolar ridge10. The microcirculation abnormalities in human organisms in the end leads to the development of diseases of the cardiovascular system and increases the risks of cancer (leucosis, lung cancer), heart attack and apoplectic attack33,34,35,36.

Chronic influence of manganese on the human organism can cause genetic mutations and degeneration of the CNS function. This negative effect is similar to Parkinsonism in nature37,38. The presence of manganese in covered electrodes of the basic type of the volatile fluorine compounds (KCaF 3 -CaF 2 , Na 2 SiF 6 ) and high basicity of the cinder phase promotes an intense flow of alkaline and alki-earthy metal compounds into the WFs (in particular, calcium Ca) (Fig. 5b,c)5. The presence in the WFs of the volatile fluorine compounds can lead to the development of asthma39,40. Moreover, chrome (Cr) and nickel (Ni) compounds, found in welding wires and welded metals, have been proven to have cancerogenic influence on the human organism (Fig. 5b,c)41,42.

Workers of this field need constant biomonitoring of blood and urine for the purposes of evaluation and control of general health risks. Furthermore, warning text and photo messages about the potential risks in the welding zones may help to deliver the information about the hazard levels of ‘industrial sites’ to employees and visitors. In turn, the use of low-fume welding rods and/or elimination of welding fumes by using alternative welding methods, such as friction welding (a solid state process) will make it possible to exclude the negative emissions of welding vapors into the atmosphere.