OSHA

OSHA does consider exposed heated surfaces, if there is a potential for injury, to be a hazard and will issue citations if employees can come into contact with such surfaces. While there are not any OSHA standards, except those that are applicable only to specific industries, which address exposed heated surfaces, there are several OSHA general standards which address such hazards. Those standards are:

1910.261(k)(11):

Steam and hot-water pipes. All exposed steam and hot-water pipes within 7 feet of the floor or working platform or within 15 inches measured horizontally from stairways, ramps, or fixed ladders shall be covered with an insulating material, or guarded in such manner as to prevent contact.

1910.262(c)(9):

Steam pipes. All pipes carrying steam or hot water for process or servicing machinery, when exposed to contact and located within seven feet of the floor or working platform shall be covered with a heat-insulating material, or otherwise properly guarded.

1910.23(c)(3):

Regardless of height, open-sided floors, walkways, platforms, or runaways above or adjacent to dangerous equipment, pickling or galvanizing tanks, de-greasing units, and similar hazards shall be guarded with a standard railing and toe board.

1910.147:

The control of hazardous energy (lockout/tagout) standard covers hazardous energy, including thermal, during the servicing and maintenance of machines or equipment. Thermal energy may be dissipated or controlled, and it is the result of mechanical work, radiation, or electrical resistance. This standard addresses practices and procedures that are necessary to disable machinery or equipment and to prevent the release of potentially hazardous energy while maintenance and servicing activities are performed.

1910.132(a):

Protective equipment, including personal protective equipment for eyes, face, head, and extremities, protective clothing, respiratory devices, and protective shields and barriers, shall be provided, used, and maintained in a sanitary and reliable condition wherever it is necessary by reason of hazards of processes or environment, chemical hazards, radiological hazards, or mechanical irritants encountered in a manner capable of causing injury or impairment in the function of any part of the body through absorption, inhalation or physical contact.
The personal protective equipment standard would apply to hot surfaces where the hazards have not been eliminated through engineering or administrative controls. This standard requires employers to assess the workplace to determine if hazards that require the use of PPE are present or are likely to be present. The employer must select and have affected employees use properly fitted PPE suitable for protection against these hazards, as well as provide employee training and conduct periodic inspections to assure procedures are being followed. Suitable thermal protection would be necessary to provide employees with thermal insulation from hazardous hot pipe surfaces.

Section 5(a)(1) of the OSHAct:

Each employer shall furnish to each of his employees employment and a place of employment which are free from recognized hazards that are causing or are likely to cause death or serious physical harm to his employees.

Skin Burns

The topic of a safe touch temperature has been investigated thoroughly ever since the first damage function was created in 1947.  Since then, it has been an essential aspect in any design where a heated surface is exposed.  An inquiry into current standards and guidelines was an essential first step in my investigation.  It was also necessary for me to understand the physiological properties of skin and burns in order to investigate the effects of material type and thickness on the safe touch temperature.  I researched the origin of the damage function which yields how much injury occurs for a material at a specific material over time.    

Current Regulations and Guidelines:

There are currently not many general safe touch temperature standards or codes. The US Department of Labor's Occupational Safety & Health Administration (OSHA) does consider exposed heated surfaces, if there is a potential for injury, to be a hazard.  There are not any OSHA standards, except those that are applicable to specific industries, which address exposed heated surfaces.  The American Society for Testing Materials (ASTM) has two standards that address the issue of a safe touch temperature.  ASTM C1055-99 and ASTM C1057-03 establishes a means by which the engineer can determine the acceptable surface temperature of an existing system where skin contact may be made with a heated surface.  It also details how personal injury resulting from contact with heat surfaces can be prevented by proper design of insulation systems or with the usage of other protective measures.  The National Insulation Manufacturers Association (NIMA) has guidelines dealing with the amount of insulation needed to ensure that the acceptable temperature is not exceeded. 

Along with the standards and codes from professional associations, I received information from engineering companies as to their policies regarding the safe touch temperature. The program director of Facilities Services for EMCOR Group Inc. based in Norfolk, CT responded to my email and gave me information about what his company does.  They use common design parameters that are centered around the surface temperature of any material not exceeding 120° F (° 50°C) because this provides a contact exposure limit of eight minutes before 2nd degree burns are probable, and 3rd degree burns can be expected in ten minutes.      

Physiological Properties of Skin & Burns:

The skin is the body's largest organ in both size and weight.  For the average adult the skin makes up 15% of total weight and has a surface area of 1.7 m2.   Diller (1985) has given a good description of the physiology of the skin.  The functions of skin are vital to life; they include: (1) protection of underlying tissues from physical, chemical, and thermal trauma; (2) thermal regulation by sweating, heat conduction (insulation), and control of blood flow to a profuse plexus of minute surface vessels; (3) sensory perception of touch, pain, and temperature.

The skin is made up of three layers: the epidermis, the dermis and the subcutaneous fat layer.  The epidermis is the outermost layer and measures 0.06-0.08 mm in thickness.  Since the epidermis is devoid of blood vessels, lymphatics, and connective tissue, it is dependent on the underlying layer which furnishes its own nourishment.  The epidermis consists of three parts: the stratum corneum, keratinocytes and the basal layer.  The stratum corneum consists of fully mature keratinocytes which contain fibrous proteins. Its main purpose is to prevent the entry of foreign objects as well as loss of fluid from the body.  The second layer contains living keratinocytes which mature to form the stratum corneum.  The basal layer is the deepest layer of the epidermis and it contains basal cells.  Basal cells continuously divide forming new keratinocytes which replace the old ones which shed from the skin's surface.  The dermis is the middle layer of skin which is held together by a protein called collagen.  This layer is 20 to 30 times thicker than the epidermis and contains blood vessels, lymph vessels, hair follicles, sweat glands, collagen bundles, fibroblasts, and nerves.  The deepest layer of the skin is the subcutaneous tissue.  It consists of a network of collagen and fat cells which helps to conserve the body's heat and protects the body from injury by acting as a 'shock absorber.'

There are numerous types of burns: scalds thermal, contact, electrical, chemical, ultraviolet, and inhalation injury.  Scalds are the most common burn and occur when the skin comes in contact with hot liquids.  Thermal burns occur either when the skin comes in contact with flames or when an explosion occurs.  Contact burns are caused when the skin comes in contact with a hot object.  Electrical burns occur from live wires or unprotected electrical outlets. The severity of these burns depends on the electrical current and the time of exposure.  Chemical burns can cause progressive damage but depends on the chemical, length of exposure, and amount of tissue involved.  Ultraviolet burns occurs overexposure to sun or tanning equipment.  Inhalation burns occur when a person has an inflammatory response in their respiratory system from gases produced by a chemical burns.

There are four different levels of severity of burns.  First degree burns are limited to the epidermis and are classified as 'superficial.'  The burn site is red, painful and dry with no blisters.  Second degree burns involve the epidermis and part of the dermis layer of the skin and are classified as 'deep.'  They can cause damage to sweat glands and hair follicles.  The burn site is moist, red, and weepy and is very painful which is often accompanied by intense swelling.  Third degree burns destroy the epidermis and the dermis damage extends well below the hair follicles and sweat glands down to the subcutaneous tissue.  The burn site is bright red, waxy white, tan or brown and with no blisters.  Third degree burns are the least painful because the injury has destroyed the nerve ending.  There are also fourth degree burns which occur when the burn injury is deep enough to involve muscle, bone, tendons and/or ligament.  These burns are life threatening and require amputation or are fatal.

Determination of the Burn Injury:

A thermal burn occurs as a result of a rise in tissue temperature above a threshold value for a finite period of time. Both the temperature of the material and the length of exposure are critical in determining the extent of injury.  In general, the transient tissue temperature integrated over time of exposure must be considered in creating a thermal lesion.  Moritz and Henriques (1947) first demonstrated what was to become the classical inverse relationship between the temperature and time required to produce a graded degree of thermal injury.  They were the first to produce a successful analytical model for thermal injury to skin.  The transient temperature distribution is described in terms of the standard one-dimensional heat conduction equation

                       (1)

Henriques devised a damage equation that has subsequently been used often.  The simulation of a thermal burn in this model has two requisite steps; first, the transient temperature field must be determined for the boundary value problem of interest, and second, the thermal data must be applied to the evaluation of a damage rate function. 

Henriques assumed that the governing biochemical processes could be depicted in terms of an Arrhenius relationship.  The term Ω was used to denote an arbitrary degree of tissue injury, and the rate of production of injury (the damage rate function) was given by

(2)

The total injury at any point in the skin is obtained by integrating the damage rate function over the entire burn period:

                                               

                                                (3)           

 

The Ω function was quantified to identify various injury thresholds.  A value of Ω=0.53 was used to define the minimum conditions to obtain irreversible epidermal injury, and when Ω=1.0, complete transepidermal necrosis.

Henriques' equation provided a theoretical base for future analysis of many scientists.  Since then many researchers of different backgrounds have focused their study on this subject using different applications.  Because I am looking at the interaction between the human finger and hot plates of material, it is necessary for a reexamination of Henriques' semi-empirical equation and empirical constants. 

 After altering Henriques' equation to include the protein break down in thermal injury, Xu and Qian (1995) came up with following reincarnation of the damage function:

                                 (4)

Where A, B, α, β are constants to be determined by curve fits to the experimental data of Henriques and Moritz and where τ = 1-T0, abs/Tabs where T0, abs = 305.65 K.  Looking at a plot of the damage rate dW/dt as a function of temperature made by Subramanian and Chato (1998), the constant a is determined from the slope of the temperature time curve on the semi-log plot.  The constant β can be determined from the slope at lower temperatures at which Be-bt >1 and the denaturation rate is proportional to e (a+b)t.  The constants A and B can be determined by setting the damage functions for reversible epidermal injury at both high and low temperatures to be 1.0.