Welding
Welding refers to the use of any electric or gas welding or cutting equipment.  Over 500,000 U.S. workers regularly perform welding or torch cutting operations.  Another 2 million perform these operations intermittently.  These employees may be at risk for exposure to hexavalent chromium and many other hazardous substances in the fumes from these operations.

There are numerous ways that welding or cutting can produce fumes containing hexavalent chromium.

Fumes
Welding joins or cuts apart metal using intense heat from either a gas fired flame or electric arc.  This heat can bring a metal to above its boiling point.  This superheating converts small metal particles into metal vapor.  Fumes are formed when the vapor cools and condenses back into particles.  Fumes may be visible or invisible.  Welding smoke is an example of visible fumes.  Regardless if the fumes can be seen particles can still be present.

Particles
Metal particles in welding fumes can come from several sources and can vary in size from 0.00 µm - 20 µm.
These sources include,

• the base metal

• coatings, residues or paint

• electrode material

• flux material

• filler metal

The boiling point for chromium is 2672 ^oC (4841 ^oF).
Since the heat of welding or cutting can exceed 3000 ^oC (5432 ^oF) chromium particles can easily vaporize and then condense into fumes.

Chemical Reaction
Chromium particles generated in welding fumes can undergo a chemical reaction to become hexavalent chromium particles.  There are many variables that can play a part in this chemical reaction.  These variables include,

• electrical current

• voltage

• electrode diameter

• droplet size

• surface temperature

• evaporation rate

• shielding gas

• arc length

• microspatter

• ultraviolet radiation

Detection
Monitoring for exposure from welding fumes is accomplished by drawing air from a worker's breathing zone through a filter.  The filter is then processed in a lab to determine the amount of hexavalent chromium and other metals, such as lead and cadmium, to which the employee was exposed.  This is expressed as an 8-hour time weighted average (TWA).  While important this monitoring alone will not provide information on the origin of the hexavalent chromium. 

The origin of most particles in welding fumes is the metal being cut or welded and/or the paint, coatings or other substances on the surface of the metal.  This also applies to metal that is coated or contaminated with hexavalent chromium compounds.  Yet as explained above chromium particles generated during welding can be changed to hexavalent chromium particles.

Questions?
How can you determine which of the many factors contribute to the chemical reaction that forms hexavalent chromium in a specific welding/torching operation? 

Why would you want to know this?

Explanation
OSHA has established a permissible exposure limit (PEL) of 5 micrograms of hexavalent chromium per cubic meter of air (5 µg/m³) as an 8-hour TWA.  OSHA has also established a PEL of 50 µg/m³  of lead as an 8-hr TWA.  A look at the periodic table of elements shows the atomic weight (rounded up) for chromium is 52 while the atomic weight for lead (Pb) is 207.  So the atomic weight for lead is approximately four times that of chromium.

Now take a scenario where there is both lead and chromium in a metal alloy or a paint or coating.  Torch cutting or welding will generate particles of both metals in the fumes.  According to the atomic weights, at a minimum, there should be four times the amount of lead than chromium in the fumes.  If the chromium is converted to hexavalent chromium the same proportion should apply.

It is then possible to simultaneously have an exposure below the PEL for lead (<50 µg/m3) yet above the PEL for chromium (5 µg/m³) [Example: particulate composition in torch/welding fumes of 40 µg/m3 of lead could also have 1/4 that amount of chromium or 10 µg/m3].  It is also possible to have particulate composition for lead that is below the OSHA action level for that metal (30 µg/m³ ) and still be above the PEL for chromium [Example: particulate composition in torch/welding fumes of 28 µg/m³ of lead could also have 1/4 that amount of chromium or 7 µg/m3]

Answers
We have answered the second question first.

"Why would you want to know this?
It is possible to have exposure levels below the PEL or Action Level for lead yet above the PEL or Action Level for hexavalent chromium.  You could then be faced with needing respirator protection for the hexavalent chromium level yet not for the lead.  Reducing the hexavalent chromium exposure could eliminate the need for respirator use.  Yet to do this it may be necessary to first determine where the hexavalent chromium is coming from.

This brings us to the first question.

"How can you determine which of the many factors contribute to the chemical reaction that forms hexavalent chromium in a specific welding/torching operation?"

HSR Consulting has researched this issue extensively.  We have partnered with a national testing lab to offer a specific type of air monitoring that could provide these answers.  This monitoring is conducted simultaneously with the OSHA required monitoring.

Want to know more?  Contact us.