The question of whether voltage (volts) or current (amperes) is the primary danger in an electrical shock is a common misunderstanding about electricity. The actual lethality of an electrical exposure is not determined by a single factor but is a complex relationship involving the applied electrical pressure (voltage), the body’s opposition to flow (resistance), and the resulting flow of energy through biological tissues (current). Understanding this interaction is necessary to grasp why some low-voltage sources are harmless while others are deadly.
The Direct Killer: Electrical Current (Amps)
The definitive agent of harm in an electrical shock is the current, measured in amperes (A) or milliamperes (mA). This flow of electrical charge disrupts the body’s natural electrical signals, which control the nervous system and muscles. The severity of the injury depends directly on the magnitude of this current.
At extremely low levels, below one milliampere, the current is generally not perceptible. A faint tingling sensation becomes noticeable around one milliampere, and a disturbing but not painful shock is felt at about five milliamperes. As the current increases, it begins to hijack muscle control.
The “let-go” current threshold, typically ranging from 6 to 30 milliamperes, is a particularly dangerous zone. Here, the current causes sustained, involuntary muscle contraction (tetany). If the object is held, this contraction prevents the person from releasing the conductor, drastically increasing exposure duration. Currents from 50 to 150 milliamperes can cause extreme pain, severe muscle reactions, and respiratory arrest.
The most critical current threshold for death is often cited around 100 milliamperes, the approximate level required to induce ventricular fibrillation. Ventricular fibrillation is a chaotic quivering of the heart’s lower chambers, preventing the heart from pumping blood effectively and leading to immediate cardiac arrest. Higher currents, in the range of one to four amperes, can cause the heart to stop beating entirely, leading to severe internal burns and nerve damage.
The Driving Force: Voltage and Resistance
While current is the direct cause of injury, voltage is the driving force that pushes current through the body. The relationship between voltage (\(V\)), current (\(I\)), and the body’s opposition to flow, known as resistance (\(R\)), is described by Ohm’s Law: \(I = V/R\). This formula illustrates that a lethal current can only be achieved if the voltage is high enough to overcome the body’s electrical resistance.
The human body’s resistance varies dramatically, as the skin offers the greatest barrier to current flow. Dry, intact skin can have a resistance over 10,000 ohms, limiting the current that passes through a typical household voltage source. This resistance drops precipitously when the skin is wet, sweaty, or damaged, potentially falling to 1,000 ohms or less.
When skin resistance is low, even a relatively low voltage, such as 120 volts, can drive a current over 100 milliamperes, sufficient to cause ventricular fibrillation. High-voltage sources (generally those above 500 volts) are inherently more dangerous because they can physically break down the skin’s outer, high-resistance layer. Once the skin is compromised, the current flows primarily through internal tissues, which have a much lower resistance of approximately 300 ohms, guaranteeing a massive current flow.
Factors Modifying Electrical Risk
Beyond the magnitude of the current, several situational factors drastically alter the severity of an electrical shock. The pathway the current takes through the body is a major determinant of injury. Current passing from one hand to the other is the most hazardous route, as it directs the current directly through the chest cavity and the heart, significantly increasing the risk of fatal ventricular fibrillation.
The duration of contact is also a critical variable, as the total energy deposited into the body increases with time. Even a small current can become lethal if the exposure is prolonged, increasing the likelihood of tissue damage and interference with the heart’s rhythm. The inability to let go of the conductor, caused by involuntary muscle contraction, makes prolonged duration a high-risk factor.
Furthermore, the type of electricity, Alternating Current (AC) versus Direct Current (DC), has different physiological effects. AC, particularly at common power frequencies like 50 or 60 Hertz, is generally considered three to five times more dangerous than DC at the same voltage level. This is because AC readily induces sustained muscle contraction, preventing release and prolonging exposure. While DC often causes a single, forceful muscle spasm, the tetanic effect of AC increases the overall risk.