Electrical shock occurs when the body becomes part of an electrical circuit, allowing current to flow through tissues. The resulting damage is determined not by voltage (electrical pressure) but by the volume of electrons moving through the body, known as current or amperage. Understanding this relationship reveals why even common household electricity can be deadly under specific conditions. The science of electrical injury focuses on how a small flow of current can disrupt the body’s own electrical systems.
Defining Electrical Current as the Lethal Factor
The fundamental difference between voltage and current is that voltage provides the electrical pressure, while current is the actual flow of charge. Voltage is the force that pushes electrons through a conductor, but it is the resulting current that directly interacts with biological tissue. This relationship is governed by the principles of electrical physics: the amount of current that flows is equal to the voltage divided by the body’s resistance.
When an individual contacts an energized source, the voltage determines how hard the current is pushed, but the tissue damage is caused by the flow of electrons itself. The body’s nervous system and heart rely on micro-level electrical signals to function, and an external current overwhelms these delicate biological processes.
This interruption of normal cellular activity is the mechanism by which current, measured in amperes (A) or milliamperes (mA), causes injury and death. Therefore, the danger of an electrical source is ultimately defined by the amperage it can drive through the human body.
Physiological Effects of Specific Current Levels
The severity of an electrical shock is directly proportional to the amount of current that passes through the body, with effects categorized by milliampere (mA) thresholds. At the lowest level, an alternating current of less than 1 mA typically results in a barely perceptible tingling sensation, known as the perception threshold. As the current increases slightly, reaching between 1 mA and 10 mA, the shock becomes painful, but most individuals can still release the conductor.
A highly dangerous range begins when the current exceeds the “let-go” threshold, which is typically between 6 mA and 30 mA for alternating current, varying based on body mass and gender. Within this range, the electrical energy causes involuntary, sustained muscular contraction, or tetany, preventing the person from releasing the source. This inability to break contact significantly prolongs the exposure time, allowing higher current levels to accumulate damage.
Currents ranging from 30 mA to 75 mA can lead to respiratory paralysis by locking the chest muscles and causing breathing to stop. The most lethal current range for alternating current at household frequency (60 Hz) is between 100 mA and 200 mA. This small amount of current is sufficient to induce ventricular fibrillation, where the heart’s lower chambers quiver chaotically and cannot pump blood effectively. Currents higher than 200 mA can sometimes cause the heart to clamp shut completely, which can prevent fibrillation and improve the chances of survival if the shock is quickly removed.
Variables That Determine Shock Severity
While specific current levels have defined physiological effects, the actual outcome of an electrical shock is modified by several situational variables. One of the most significant factors is the path the current takes through the body. A current traveling from one hand to the other, or from a hand to a foot, is far more likely to be deadly because it passes directly across the chest and the heart. A current confined to a localized area, such as a finger-to-finger path, is less likely to be immediately fatal, though it will still cause severe local injury.
The duration of contact is another variable, as longer exposure means greater total energy delivered and a higher risk of damage. Even a low current can be lethal if the exposure time is long enough to coincide with the heart’s vulnerable repolarization phase. This is relevant when the current exceeds the “let-go” threshold, as sustained muscle contraction forces prolonged exposure.
The body’s resistance largely determines how much current a given voltage will drive through the body. Dry, intact skin has a high resistance, potentially over 100,000 ohms, which limits the current flow. However, resistance drops dramatically to 1,000 ohms or less when the skin is wet or sweaty, meaning a low voltage can suddenly produce a lethal current. High-voltage sources can also physically break down the skin’s resistance, allowing a massive amount of current to flood the internal, low-resistance tissues like blood, nerves, and muscle.
Thermal and Tissue Damage from Electricity
Beyond the disruption of the heart’s rhythm, electrical current causes direct physical damage through its conversion into heat, a process known as Joule heating. This thermal injury is concentrated at the points of entry and exit, often resulting in severe external burns. These contact burns can appear deceptively small on the surface, yet they may conceal extensive destruction of underlying tissue.
Internal damage occurs as the heat generated by the current travels through the body, cooking the deeper structures. Tissues with higher resistance, such as bone, tend to heat up more, leading to thermal injury and necrosis in muscle, blood vessels, and nerves that may not be immediately visible. This internal cooking can lead to severe swelling (compartment syndrome) and the breakdown of muscle tissue, which releases toxins that can damage the kidneys.