Effects of acute inhalation of aerosols generated during resistance spot welding with mild-steel on pulmonary, vascular and immune responses in rats

Abstract

Spot welding is used in the automotive and aircraft industries, where high-speed, repetitive welding is needed to join thin sections of metal. Epoxy adhesives are applied as sealers to the metal seams. Pulmonary function abnormalities and airway irritation have been reported in spot welders, but no animal toxicology studies exist. Therefore, the goal of this study was to investigate vascular, immune and lung toxicity measures after exposure to these metal fumes in an animal model. Male Sprague-Dawley rats were exposed by inhalation to 25 mg/m³ to either mild-steel spot welding aerosols with sparking (high metal, HM) or without sparking (low metal, LM) for 4 h/d for 3, 8 and 13 d. Shams were exposed to filtered air. Bronchoalveolar lavage (BAL), lung gene expression and ex vivo BAL cell challenge were performed to assess lung toxicity. Lung resistance (R(L)) was evaluated before and after challenge with inhaled methacholine (MCh). Functional assessment of the vascular endothelium in isolated rat tail arteries and leukocyte differentiation in the spleen and lymph nodes via flow cytometry was also done. Immediately after exposure, baseline R(L) was significantly elevated in the LM spot welding aerosols, but returned to control level by 24 h postexposure. Airway reactivity to MCh was unaffected. Lung inflammation and cytotoxicity were mild and transient. Lung epithelial permeability was significantly increased after 3 and 8 d, but not after 13 d of exposure to the HM aerosol. HM aerosols also caused vascular endothelial dysfunction and increased CD4+, CD8+ and B cells in the spleen. Only LM aerosols caused increased IL-6 and MCP-1 levels compared with sham after ex vivo LPS stimulation in BAL macrophages. Acute inhalation of mild-steel spot welding fumes at occupationally relevant concentrations may act as an irritant as evidenced by the increased R(L) and result in endothelial dysfunction, but otherwise had minor effects on the lung.

Keywords: Cardiovascular; inhalation studies; particulate matter; volatile organic compounds; welding.

Figure 1

Effect of LM and HM resistance spot welding aerosols on baseline lung resistance (R_L) in rats immediately (0 h) or 24 h after exposure to 25 mg/m³ for 4 h/d for 8 d. Values are mean ± standard error (n = 6–8 per group). Significance from sham 0 h at *p<0.05.

Figure 2

Representative images of cytospin-stained lung BAL cells recovered at 24 h after 3 and 13 d of exposure to 25 mg/m³ for 4 h/d to LM (panels A and C) and HM (panels B and D) spot welding aerosols.

Figure 3

Effect of LM and HM resistance spot welding aerosols on total cell counts (panel A), LDH (panel B) and albumin (panel C) measured in the BAL at 24 h after 3, 8 and 13 d of exposure to 25 mg/m³ for 4 h/d. Data are presented as percent of sham and values are mean ± standard error (n = 4–8 per group). Significance at *p<0.05.

Figure 4

Effect of ex vivo stimulation by LPS on IL-6 and MCP-1 protein levels from lung macrophages harvested by BAL 24 h after 13 d of exposure to 25 mg/m³ for 4 h/d to LM (panels A and B) and HM (panels C and D) resistance spot welding aerosols. Values are mean ± standard error (n = 4 per group). Significance at *p<0.05 between LPS-stimulated and non-stimulated (i.e. baseline) groups. Significance at #p<0.05 between LPS-stimulated sham and LM groups.

Figure 5

Effect of LM and HM resistance spot welding aerosols on total splenocyte number (panel A) and populations of CD4+, CD8+ and B cells (panels B and C) harvested from spleens 24 h after 13 d of exposure to 25 mg/m³ for 4 h/d. Values are mean ± standard error (n = 4 or 8 per group). Significant difference from sham at *p<0.05.

Figure 6

Pre-constricted rat tail arteries after exposure to 25 mg/m³ for 4 h/d of HM (panel A) and LM (panel B) resistance spot welding aerosols to increasing concentrations of ACh. Values are mean ± standard error (n = 4 per group). Significance from sham at *p<0.05.