Of the approximately 2.4 million reported burn injuries per year in the United States, ~650,000 are treated by medical professionals, and ~75,000 burn victims are hospitalized. Of those hospitalized, 20,000 have major burns involving at least 25% of total body surface. Approximately one million people sustain substantial or permanent disabilities resulting from their burn injuries, and approximately 10,000 of these injuries are fatal every year. This makes burn injuries second only to motor vehicles as the leading cause of accidental death in the United States. The vast majority of residential burn injuries occur in the kitchen.
Burn injuries are one of the most expensive catastrophic injuries to treat. For example, initial treatment costs for 30% of total body area run in the hundreds of thousands of dollars. For extensive burns, additional costs are incurred for repeat admissions for reconstruction and rehabilitation.
The necrosis of the epidermal layer starts when the temperature of the epidermal cells reaches 44°C; this constitutes the threshold of pain. The human response to pain is rapid as it attempts to distance the part of body experiencing pain from the source of the heat. However, in many instances, the heat transfer rate is fast enough to cause irreversible damage in the short duration of the contact. In other scenarios, it is not possible for the body to distance itself, because the source of heat may have covered the skin or the person’s impairment prevents avoidance. In these cases, the skin temperature reaches and exceeds 44°C long enough to cause second or third degree burns. Thus, the time above a certain skin temperature constitutes the heat dose and determines the degree of the injury.
It is important to note that it is not the temperature of the heat source, but rather that of the skin, that determines the onset of necrosis. Thus, it becomes imperative to determine the heat transfer characteristics from the hot surface to the skin. This heat can be transferred via radiation (e.g., sun burns), convection (burns due to hot gases and fluids) and conduction (direct contact). The analysis of the actual heat transfer can involve complex transient analysis, which factors in the thermal properties of the materials and the geometry of the contact. Exponent’s thermal and biomechanical engineers are experienced in analyzing such scenarios. Exponent also has collected over the years extensive experimental data that are used to assist in the analysis of product safety and accident reconstruction related to dermal injury, as well as to assess warnings and labeling issues that may be associated with products under study.
Burns Due to Conductive Heat Transfer
The thermal conductivity of the surface, the geometry of the contact, and the duration of the contact all determine whether dermal injury occurs in scenarios where conduction is the primary mode of heat transfer to the skin. ASTM C 1055 is the industry standard that establishes criteria for acceptable surface operating conditions for a heated system to prevent injury. ASTM C 1057 provides guidelines for determining skin contact temperature using mathematical models and a Thermesthesiometer. Exponent’s engineers are experienced in applying the analytical models in a wide variety of scenarios. Additionally, Exponent has built calibrated Thermesthesiometers to the ASTM C 1057 standard. Exponent has also developed in-house tools and capabilities to determine the nature of the thermal contact. These techniques allow us to assist clients in recreating accident scenarios and in product recall situations.
Burns Due to Convective Heat Transfer
Hot air can cause external and internal (respiratory tract and lungs) burn injuries. However, the primary convective burn consists of scalding from hot water. Approximately 5000 children are scalded annually in the U.S., with the average bathtub burn covering 12% of the body. The extent of this problem puts a lot of individual users at risk and exposes water-heater manufacturers, plumbers, and property owners to lawsuits. Exponent engineers are experienced in quantifying the actual heat transfer to the skin due to heated fluids in a wide variety of scenarios. We have also collected a broad body of published data on the time/temperature relation and likelihood of survival.
Burns Due to Radiative Heat Transfer
It is common experience that solar radiation can cause skin burns if the exposure is long enough. If the radiation intensity is increased from that of the sun at earth’s surface (about 1 kW/m2), the time to burn the skin decreases exponentially. Exponent engineers use analytical methods, instrumentation, and published data to assist clients in mitigating risks from radiative burns.
Cryogenic and Chemical Burns
Very cold conditions can also cause necrosis of the dermal layer. Additionally, a number of chemicals can react exothermically with human skin and cause burns. Exponent uses the techniques described above to study such injuries.