Hexavalent Chromium [Cr(VI)] — Elemental chromium is a metal component in a variety of base materials and alloys, most notably stainless steel. Whilst the metallic and low valence forms of chromium are not toxic, the hexavalent state possesses mutagenic and carcinogenic activity. Hexavalent forms of chromium are reactive and used as an oxidizing agent in organic chemistry. The high chromium content of stainless steel relative to other alloys, and the intense heat generated in welding, combine to provide conditions that lead to the formation of appreciable quantities of hexavalent chromium. Vaporized metals that penetrate the welding shield gas condense upon cooling to metal oxides, including CrO3, a hexavalent form of chromium. The carcinogenic nature of hexavalent chromium, as well as a range of chronic and acute deleterious health effects associated with exposure to hexavalent chromium, has prompted regulation of this substance in the workplace. In 2006, the Occupational Safety and Health Administration (OSHA) lowered the permissible exposure limit (PEL) for hexavalent chromium from 52 to 5 micrograms (μg) of [Cr(VI)] per cubic meter of air. To this end, Sentry Air Systems Series of Welding Fume Extractors equipped with high efficiency particulate absorption (HEPA) filters have been shown to be effective for maintaining safe levels of hexavalent chromium in worker environments.
OSHA enforces strict regulations regarding worker exposure to hexavalent chromium in several industries; and, subsequently established stringent regulations for workers who are potentially exposed to it in the general, construction, shipyard, and marine terminal industries. The health hazards associated with the inhalation or direct contact with hexavalent chromium fumes are well documented and include lung cancer, damage to the respiratory tract and harm or irritation to the eyes and skin, as cited by OSHA.
Also according to this booklet, common processes that can lead to the inhalation of airborne Hexavalent Chromium (dust, fume, or mist) include:
All employers whose employees could potentially be exposed to hexavalent chromium should conduct sampling to determine the level of exposure. The Permissible Exposure Limit (PEL) for hexavalent chromium is 5 micrograms per cubic meter of air (5ug/m3). The “Action Level” is 2.5ug/m3 (calculated as an 8-hour TWA). Specific requirements are mandatory if exposure limits meet or exceed this level.
Recommended Control Measures for reducing exposure to CR(VI) that exceeds the PEL include*:
Work Practice Controls:
At Sentry Air Systems, we have specifically tested our equipment for its effectiveness and efficiency in controlling exposure to hexavalent chromium from welding fume.
Herein, a Sentry Air Systems product study of hexavalent chromium particles, [Cr(VI)], is reported. Due to the popularity of extended life cleanable filter media and health concerns regarding hexavalent chromium exposure in the workplace, Sentry Air Systems conducted a study of the efficacy of cleanable filter systems for the maintenance of safe breathing conditions during welding operations that generate [Cr(VI)]. A Sentry Air Systems 450 Series Welding Fume Extractor was used for the containment of a representative stainless steel welding process. The collected data and analytical results suggest that the use of Sentry Air Systems 450 Series Welding Fume Extractor with MERV 13 Micro Pleat cleanable filter media allows for safe operator breathing conditions, as shown under the conditions outlined in this paper.
To test the efficacy of Sentry Air Systems cleanable filter media for capture of [Cr(VI)], a representative welding application was simulated to produce an appreciable quantity of the desired analyte. Using a Sentry Air Systems Welding Fume extractor to facilitate air filtration, air sampling was conducted to evaluate filter efficiency and air quality at multiple locations. The sampling of air at the inlet and outlet of the fume extractor system, as well as the user breathing zone and ambient room space, allows for meaningful comparisons of relevant [Cr(VI)] levels.
For this test, a continuous metal inert gas (MIG) welding operation was performed using stainless steel “coupons”. A series of parallel seams were welded lengthwise across the secured stainless steel plate, with spent coupons replaced to ensure consistent stainless steel surface interaction. The MIG weld iterations were continued for the duration of a four hour test period to generate a nearly constant fume load at the sampling source.
The sampling method was based on NIOSH Test Method 7605, which recommends a sample volume range between 1 – 400 L and a sampling rate of 1 to 4 L/min. The guidelines were used to determine a target sample volume of 250 L, while sampling at 1.0 L/min, to ensure sufficient analyte loading for subsequent prescribed instrumental analysis.
The evaluation of the filter efficiency was performed using Sentry Air Systems 450 Series Welding Fume Extractor (SS-450-WFE). The SS-450-WFE was configured with a standard carbon pre-filter pad (SS-400-CFP), and a fitted Micro Pleat Series 1 cleanable filter without previous usage or cleaning performed. The stated configuration is representative standard equipment recommended by Sentry Air Systems for most applications where particles from dust or powders are a concern. Similar to the previously reported HEPA filter system, Sentry Air Systems Micro Pleat Series 1 affects particle containment via a combination of mechanisms including interception, impaction, and diffusion.
Air sampling was conducted using SKC-branded personal air samplers (SKC-224-PCXR4), each calibrated with a BIOS International Defender 510 (Defender 510). The flow rate of each sampler was set, measured, and recorded to meet NIOSH and/or OSHA test protocol requirements. The SKC-224-PCXR4 flow rates were recorded both before and after testing and the average of the two values was used (See Table 1). Sample size was determined by using the timer onboard each SKC-224-PCXR4 air sampler along with their average calibrated flow rates.
Test design prescribed that each sampler be setup with a flow rate of about 1.0 L/min for a sample time of about 240 minutes. Following protocols of the NIOSH Method 7602, a sample train consisting of a polyvinyl chloride (PVC) filter in a polystyrene cassette filter holder were used for sample collection.
At the onset of the model welding process, stainless steel coupons (~8 in. x 6 in., 316 gauge) were secured with C-clamps and seam welded using a Lincoln Electric Power MIG 215 electric welder that employed an inert gas mixture of 75% argon and 25% carbon dioxide to blanket the work. Each coupon was exhaustively welded with sequential parallel seams across the length of the plate. Spent coupons were removed and replaced with clean, untreated stainless steel substrates, which were continuously seam-welded in identical fashion. The process was continued for the duration of the 240 minute testing period to generate a consistent fume load at the inlet of the capture hood. Visual inspection of the PVC sample membranes at the conclusion of the simulation indicated noticeable discoloration at the inlet collection location, as compared to the other testing points. The sealed samples were labeled and couriered to a third party analytical lab for testing. The laboratory analyzed the PVC membrane samplers for [Cr(VI)] content and reported the results in micrograms (μg) of [Cr(VI)]. The mass data was converted to a concentration value in μg/m3 using the calibrated air flow sampler data (See Table 2).
Test samples were taken at four locations during the test. Definitions of the test points can be found in the table below:
Table 1. Sampling flow rate data and collected [Cr(VI)] mass.
*Percent Relative Standard Deviation of sample flow rate calculated from pre-test and post-test flow values.
Table 2. Calculated sampling volume and [Cr(VI)] concentration values.
*Parts per billion based on mass. **Calculated from inlet and outlet [Cr(VI)] values.
HIH Laboratory (Webster, TX) prepared the collected samples and measured [Cr(VI)] content using ionic chromatography analysis with UV-Vis detection (IC-UV) monitoring for the dichromate anion, [Cr2O7]2-, a stable, water-soluble form of hexavalent chromium. In accordance with the instrumental technique, the detection limit for [Cr2O7]2- is 0.03 μg, or 0.2 μg/m3. Therefore, for samples containing negligible quantities of [Cr2O7]2-, a concentration less than or equal to the detection limit of IC-UV analysis is assumed.
In evaluating the efficiency of the Sentry Air Systems 450 Series Welding Fume Extractor, the detection limit of the IC-UV technique becomes an important consideration. The minimum efficiency rating, within analytical error, is 97%. Although the actual efficiency may be much higher, the prescribed instrumental technique limits ambient background detection to a minimal concentration of 0.2 μg/m3. Increasing the airborne [Cr(VI)] test load concentration, or improving the sensitivity of the instrumental technique, may result in a significantly improved efficiency rating.
Lab analysis determined 16 μg/m3 of [Cr(VI)] at test point T1A, which corresponds to an enclosure [Cr(VI)] concentration of about 16 μg/m3. The concentration at the filter outlet port, T1B, reflects a total [Cr(VI)] mass of 0.12 μg & 0.47 μg/m3. Based on the measured difference in concentration across the filter stack, the indicated filter efficiency for [Cr(VI)] using Sentry Air Systems MERV 13 Micro Pleat filter media is ≥ 97%. During the course of the test, the operator and room [Cr(VI)] concentration remained unchanged and below the detection limit of the IC-UV analysis.
The study results presented herein suggest that for the test conducted, the use of a Sentry Air Systems 450 Series Welding Fume Extractor configured with cleanable MERV 13 Micro Pleat filter media would prove beneficial in reducing operator exposure to respiratory hazards associated with welding applications. Furthermore, effective application of the aforementioned equipment for welding of chromium containing alloys may reduce operator [Cr(VI)] exposure to an acceptable sub-action level range. Under this investigation, the 97% efficiency rating of the cleanable filter media is consistent with the defined MERV 13 filter rating (95% for 1.0-0.3 µm particles), and under the conditions of this test, the system effectively reduces [Cr(VI)] particulates to a level substantially below the OSHA & NIOSH recommended limits.