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Standard Interpretations - Table of Contents
• Standard Number: 1915.4(r); 1915.504(b)

This letter constitutes OSHA's interpretation only of the requirements discussed and may not be applicable to any situation not delineated within the original correspondence.

October 8, 2009

Mr. Malcolm B. Branch
Virginia Ship Repair Association
150 Boush Street, Suite 802
Norfolk, VA 23510

Dear Mr. Branch:

This letter responds to your October 22, 2007, letter to Mr. Leo Edwards, OSHA Norfolk Area Director. You requested an interpretation as to whether grinding on aluminum is considered "hot work" and whether a fire watch is needed in accordance with 29 CFR 1915. You also requested that OSHA address other hazards associated with aluminum grinding, which include possible sparking, large aluminothermic reactions, and combustible dust.

OSHA does not define hot work based on specific types of materials or processes due to the endless possible combinations thereof. Hot work is defined in 29 CFR 1915.4(r) as "riveting, welding, burning, or other fire or spark producing operations." Thus, regardless of the type of material being ground, if fire or sparks are produced under any of the conditions specified in 1915.504(b), then a fire watch is required to be posted. Conversely, if no fire or sparks are produced, then the operation is not considered hot work, and a fire watch is not required.

Your letter states that grinding on aluminum transfers only a small amount of heat and that no sparks are generated. However, the amount of heat generated is dependent upon various factors such as the speed, composition, and surface contact area of the grinder, as well as the duration and amount of pressure applied. Therefore, "a small amount of heat" cannot be quantified given the number of grinding process variables. In addition, sparks may be generated by the aluminum if it is not non-ferrous aluminum. Although pure, non-ferrous aluminum is non-sparking, aluminum alloys are commonly used due to availability, cost, and strength properties. An aluminum alloy, even though generally considered to be a "spark-resistant" material, typically contains 5% of ferric material by weight. If grinding these alloys creates sparks, then it is considered hot work, and a fire watch would be required under any of the circumstances described in 1915.504(b).

Even if pure, non-ferrous aluminum is used, sparks can occur during an aluminothermic reaction, also called a thermic reaction. Such a reaction occurs when an aluminum particle and a metal oxide, such as rust, are ignited by a heat source and chemically burn as a "Class D" fire (i.e., combustible metal). The reaction is similar to a fireworks explosion, can create 4,500ºF sparks, and can occur when a grinder is used on ferric material (e.g., steel) prior to being used on an aluminum material or vice versa. This type of sparking and associated fire hazard may be eliminated by restricting the use of a grinder to only one type of material. In the absence of that safeguard, if grinding under these circumstances creates sparks, then it is considered hot work and a fire watch would be required under any of the circumstances described in 1915.504(b).

Grinding aluminum also can create housekeeping issues. An aluminothermic reaction can occur in situations where there is an accumulation of powder or dust from the grinding operation. A shipyard environment typically contains metal oxide, in the form of rust, and a heat source, in the form of welding, cutting, brazing, or grinding. If enough aluminum powder is introduced, there is a potential for a significant aluminothermic reaction. In an Australian Department of Energy "lessons learned" article, "Fireball from Aluminum Grinding Dust," an individual received first- and second-degree bums on his hands and head from a fireball that developed while grinding a piece of angle iron a few days after another individual ground the heads off approximately twelve aluminum pop rivets. This type of accident may be prevented by employing good housekeeping habits in addition to using material-specific tools.

Aluminum dust also can be combustible or explosive if it becomes suspended in the air at the right concentration. For example, one employee was killed, and three others were severely burned in an Indiana plant that manufactures aluminum automotive wheels after a series of explosions was fueled by aluminum dust. OSHA has recognized the combustible nature of metal dusts in its Combustible Dust National Emphasis Program Directive (CPL 03-00-008, March 11, 2008). In addition, OSHA has issued a fact sheet, "Hazard Alert: Combustible Dust Explosions, 790." Both of these documents describe the hazards and preventive measures associated with combustible dusts, which include aluminum.

Finally, when hot work is performed, 29 CFR 1915 Subpart B - "Confined and Enclosed Spaces and Other Dangerous Atmospheres in Shipyard Employment" and Subpart P - "Fire Protection in Shipyard Employment" standards may apply. Hot work under Subpart B includes fire- or heat-producing operations and has requirements for, among other things, inspection and testing for adjacent spaces that contain a concentration of any flammable or combustible substance greater than 10 percent of the lower explosive limit of that substance (e.g., welding, cutting, brazing, or grinding in the vicinity of fuel tanks).

We hope you find this information helpful. OSHA requirements are set by statute, standards, and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult our website at Should you have further questions about this matter or require additional assistance, please contact the Office of Maritime Enforcement at (202) 693-2399. Your sincere interest in worker safety and health is appreciated.


Richard E. Fairfax
Directorate of Enforcement Programs

Standard Interpretations - Table of Contents

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