Exploring Manganese Fractionation Using a Sequential Extraction Method to Evaluate Welders' Gas Metal Arc Welding Exposures during Heavy Equipment Manufacturing

Abstract

The National Institute for Occupational Safety and Health (NIOSH) has conducted an occupational exposure assessment study of manganese (Mn) in welding fume at three factories where heavy equipment was manufactured. The objective of this study was to evaluate exposures to different Mn fractions using a sequential extraction procedure. One hundred nine worker-days were monitored for either total or respirable Mn during gas metal arc welding. The samples were analyzed using an experimental method to separate different Mn fractions based on selective chemical solubility. The full-shift total particle size Mn time-weighted average (TWA) breathing zone concentrations ranged 0.38-26 for soluble Mn in a mild ammonium acetate solution; 3.2-170 for Mn0,2+ in acetic acid; 3.1-290 for Mn3+,4+ in hydroxylamine-hydrochloride; and non-detectable (ND)-130 µg m-3 for insoluble Mn fractions in hydrochloric and nitric acid. The summation of all the total particulate Mn TWA fractions yielded results that ranged from 6.9 to 610 µg m-3. The range of respirable size Mn TWA concentrations were 0.33-21 for soluble Mn; 15-140 for Mn0,2+; 14-170 for Mn3+,4+; 5.3-230 for insoluble Mn; and 36-530 µg m-3 for Mn (sum of fractions). Total particulate TWA GM concentrations of the Mn (sum) were 53 (GSD = 2.5), 150 (GSD = 1.7), and 120 (GSD = 1.8) µg m-3 for the three separate factories. Although all of the workers' exposures were measured below the OSHA regulatory permissible exposure limit and NIOSH recommended exposure limit for Mn, 70 welders' exposures exceeded the ACGIH Threshold Limit Values® for total Mn (100 µg m-3) and 29 exceeded the recently adopted respirable Mn TLV (20 µg m-3). This study shows that a welding fume exposure control and management program is warranted for Mn, which includes improved exhaust ventilation and may necessitate the use of respiratory protection, especially for welding parts that impede air circulation.

Keywords: GMAW; MIG; Mn; metal inert gas; oxidation state; solubility.

Figure 1.

Total and respirable particulate TWA breathing zone concentrations (µg m⁻³) of manganese fractions from gas metal arc welding at three heavy equipment manufacturing plants. Sol Mn, manganese concentration dissolved in first sequential extraction solution: 0.01 molar ammonium acetate, room temperature (over 90 min); Mn 0, 2+, manganese concentration dissolved in second solution: 25% acetic acid, 75°C (over 90 min); Mn 3+, 4+, manganese concentration dissolved in third solution: 0.5% hydroxylamine-hydrochloride in 25% acetic acid, 75°C (over 90 min); Insol Mn, manganese concentration dissolved in fourth solutions: hydrochloric acid, 95°C (over 15 min) and nitric acid, 95°C (over 15 min); Sum Mn, addition of all four previous extraction manganese concentrations.