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By Chrissy Klocker, Engineering Manager, Donaldson Company, Inc.
Hexavalent chromium has become a hot topic in the thermal spray community and among welders. Useful search terms for hexavalent chromium include "chromate" and "chrome six." New OSHA regulations are forcing everyone to take a close look at their processes and implement changes to them or install engineering controls to help reduce hexavalent chromium exposure. This paper addresses some questions regarding the new regulations, including:
Hexavalent chromium shows up in predominantly three forms: 1) Trivalent Chromium, which occurs naturally as chrome ore and is also an essential nutrient for proper metabolism; 2) Metallic or Elemental Chromium, typically found in aerospace alloys; and 3) Hexavalent Chromium, typically from industrial processes like welding and thermal spray.
Hexavalent Chromium is the most toxic form of chromium.
Reading through articles and publications, you will see hexavalent chromium identified in different ways, including: Hex Chrome, Chromium (VI), Cr(VI), and Cr 6+.
The Occupational Safety & Health Administration (OSHA) has defined two levels of exposure for hexavalent chromium: a Permissible Exposure Limit (PEL) and an Action Level (AL). The current PEL for hexavalent chromium is 5 µg/m3 for a single employee over an 8-hr shift. This PEL of 5 µg/m3 was reduced from the previous standard of 52 µg/m3. A factor of 10!
The second exposure limit is called the Action Level (AL) which is currently at 2.5 µg/m3. This is the concentration of airborne Cr(VI) present in a work environment and calculated as an 8-hour time-weighted average (TWA). At either of these levels, employers are required to take specific actions, and failure to take these actions may result in penalties.
There are a variety of industrial processes that can produce hexavalent chromium. Some of these include:
Important Clarification Points: In your facility, you may be producing a dust, fume, or mist that contains hexavalent chromium. It is important to realize hexavalent chromium has its own recommended threshold limit value (TLV) as established by the American Conference of Industrial Hygienists (ACGIH), and that this value may be lower than other components present as airborne contaminants.
Because hexavalent chromium is usually only a percentage of the dust fume and mist generated from a process, it may be worth investigating the percent contribution of Cr(VI) verse other materials in the airstream. Be advised it is entirely possible that one could exceed the TLV for hexavalent chromium while staying under the TLV for other fumes given off.
Typical particle sizes produced of the material containing hexavalent chromium differ by process and application.
|Type of Fume
|Size Range of Fume Particles
|Wet paints with chromates
|0.7 - 34 microns
|0.75 - 6.4 microns
|0.05 - 2.0 micons*
|0.05 - 2.0 microns*
If your company has a process that could be producing hexavalent chromium, workers can be exposed through many sources.
Once in the body, hexavalent chromium typically targets certain organs. Respiratory tract (due to inhalation damage to mucous membranes), perforation of the septum (the tissue between the nostrils of the nose), lungs, eyes, skin, liver, and kidneys are some examples.
A worker exposed to hexavalent chromium may experience symptoms such as sinus irritation, nosebleeds, ulcers (stomach and nose), skin rash, chest tightness, wheezing, and shortness of breath.
If a company has determined that it could be potentially producing hexavalent chromium, it is required to do the following:
The frequency of air sampling a company must do depends on what level of hexavalent chromium was discovered in the facility. If the area is tested above the PEL of 5.0 µg/m3, testing has to be done every three months. If the area is above the action level (AL) of 2.5 µg/m3 but below the PEL, then a company is only required to do air sampling every six months. If the area is below the action level, the company is required to take an initial baseline, and then it is left up to the facility hygienists to decide on a sampling frequency. Sampling yearly is a typical strategy in this case.
Medical monitoring of all employees is required in facilities that test above the AL of 2.5 µg/m3. The standard on hexavalent chromium requires medical surveillance but leaves the selection of the specific tests to the physician or other licensed healthcare professional (PLHCP.) This may be an on-site company nurse or the employee may need to go to his own health care provider for tests.
Some of the things that a company could do include:
Once an exam is completed, a written summary should be provided to the employee within two weeks and kept on file at the company by the industrial hygienist.
On processes producing hexavalent chromium above the PEL of 5 µg/m3, engineering controls must be implemented. Until engineering controls are implemented, respiratory protection should be used mandatory. Please note that rotation of employees to different jobs to achieve compliance is not permitted by OSHA.
Welding and thermal spraying can produce significant hexavalent chromium emissions. The amount of hexavalent chromium in the fume may be impacted by:
When estimating how much hexavalent chromium fume is produced from a process, remember that:
The amount of hexavalent chromium in fume can be estimated using the following formula:
Emission factors are expressed in a number of different ways:
You can find emission factors from many sources* including:
You can also estimate emission factors by taking the fume generation rate and multiplying by the chrome content, then multiplying the result by the hexavalent chromium ratio:
Note: EF has no units = [%] x [%] x [%]
For work area and demarcation of areas that may contain hexavalent chromium, companies are expected to do at least the following:
Companies are expected to make the regulations available for any employee to see.
Employees have responsibilities, too -- to protect themselves. They are required to use proper personal protective equipment, good housekeeping skills, engineering controls once implemented, and good personal hygiene techniques. Good personal hygiene includes not using tobacco, not applying cosmetics, not eating anything, not placing fingers in mouth or nose, and washing hands/face prior to doing any of the above activities, before taking a break, or at the end of their shift.
A well-designed ventilation system, including a properly operated dust collector, can be a factor in reducing general hexavalent chromium exposure levels in a facility.
As shown in the chart above, the sizes of the fume particles that carry hexavalent chromium vary from 5/100 of a micron to 34 microns, and most are in the very small range (sub-micron to 2 microns). It is, therefore, necessary to use filter media in the collection system that can capture a full range of sizes, from sub-micron to large particulate. High-efficiency filters are recommended, such as Ultra-Web® fine fiber filters from Donaldson® Torit®. Each filter should have at least 1.5 inches of water gauge pressure drop across the filters to optimize performance.
A well-designed ventilation system will have sufficient capture velocities at the various hoods to help control any hexavalent chromium-containing emissions. The ACGIH Industrial Ventilation Guide offers several examples including:
Combined with powerful Donaldson Torit Downflo® Evolution dust collectors, Easy-Trunk® or Porta-Trunk® fume collectors, a well-designed and operated ventilation system can reduce employee exposure to dusts and fumes containing hexavalent chromium.
Ambient fume collection (sometimes termed general or ambient ventilation) is not recommended, as it typically only cleans 70% of the air at any given time. The ambient collection is simply not powerful enough to take care of fumes potentially containing hexavalent chromium in the breathing zone of an employee. Capture hoods located as close to the source of generation as possible, and ducted into a well-built dust/fume collector, will ensure better confinement of the fume particles.
Your Donaldson Torit sales representative can help in determining which dust filtration products are best suited to your ventilation system design and operation. While Donaldson Torit engineers are NOT experts on Hexavalent Chromium, they know air filters and can assist you in selecting the filter suitable for your ventilation needs.
In summary, hexavalent chromium is a regulated, toxic material that must be dealt with by implementing proper precautions, including:
(a) Engineering controls where required
(b) Respiratory protection if needed
(c) Good housekeeping practices
(d) Proper personal protective equipment
(e) Good personal hygiene practices.