An electrical shock is the physiological reaction that occurs when an electric current passes through the human body. This physical effect can range from a barely perceptible tingling sensation to severe injury or death, depending on the level of energy involved. The human body is naturally a conductor of electricity. When the body becomes part of an electrical circuit, the flow of charge disrupts the body’s own delicate electrochemical systems. Understanding the mechanism requires looking at the fundamental electrical concepts that determine current flow and interaction with biological tissue.
The Essential Ingredients: Current, Voltage, and Resistance
The severity of an electrical shock is determined by the amount of electric current, measured in amperes (A) or milliamperes (mA), that flows through the body. While current is the agent of biological damage, it cannot flow without the force provided by voltage, which is the measure of potential difference. Voltage acts as the pressure that drives the current, and the two are linked by resistance.
Resistance is the body’s natural opposition to current flow, varying significantly depending on the point of contact and skin condition. Most of the body’s resistance resides in the skin’s outer layer of dead cells. Dry, calloused skin can have a resistance of over 500,000 ohms, which requires a much higher voltage to push a dangerous current through the body.
Conversely, wet or compromised skin dramatically lowers the resistance, potentially to as low as 1,000 ohms. This drop in resistance means that even relatively low voltages, such as 120 volts from a household outlet, can produce a lethal current. A current of 100 to 200 milliamperes (0.1 to 0.2 A) passing through the body is generally considered lethal, illustrating that current, not necessarily high voltage, is the direct cause of death.
The Path of Least Resistance: How Electricity Enters the Body
For an electrical shock to occur, the body must complete an electrical circuit by providing a pathway for the current to enter and exit. The location of these entry and exit points is a major determinant of the shock’s severity, as the current follows the path of least resistance through the body. A current passing from one hand, across the chest cavity, and out the opposite foot is far more dangerous than one that travels between two fingers. This pathway puts the heart and lungs in the direct line of current flow.
Another factor influencing the danger is the type of current: alternating current (AC) or direct current (DC). AC, which is used in most household power systems, is particularly dangerous at common frequencies (50–60 Hz). Low-frequency AC tends to cause sustained, involuntary muscle contraction, known as tetany, which can “freeze” the victim’s hand to the conductor. This prolonged contact increases the duration of exposure, leading to greater injury.
DC, while also highly dangerous, typically causes a single, convulsive muscle contraction that may throw the person away from the source. However, AC’s alternating nature is more prone to interfering with the heart’s natural electrical timing. The threshold for ventricular fibrillation with AC is often lower than with DC.
The Physiological Mechanism: How Current Disrupts the Body
The body functions through its own complex network of electrical signals, which control the nervous system and muscle movement. An external electric current overrides and scrambles these natural electrochemical signals, leading to the physical manifestations of a shock. One immediate effect is neuromuscular disruption, where the current causes involuntary muscle contractions. This sustained contraction can prevent a person from releasing the source of the current, effectively prolonging the shock.
This disruption can also affect the muscles responsible for breathing, causing respiratory arrest. The current can interfere with the heart’s intrinsic pacemaker cells that regulate the heartbeat. Currents as low as 100 milliamperes passing through the chest can induce ventricular fibrillation, where the heart’s ventricles twitch erratically instead of pumping blood effectively. This uncoordinated fluttering compromises the body’s ability to circulate oxygen, which can quickly lead to death.
Beyond the disruption of electrical signaling, resistance to the current generates heat, causing severe thermal damage. This is based on the principle of Joule heating. The resulting burns can be superficial at the entry and exit points, but the current can also generate extreme heat deep within the body. Internal burns can damage muscles, nerves, and organs along the current’s path, often without significant visible external injury.