The question of “how many volts is deadly” is fundamentally focused on the wrong measurement. While voltage, the electrical potential or “push” of an electric source, is a factor, it is not the primary determinant of injury or death. Lethality depends instead on the amount of electrical current that actually flows through the human body and the biological damage it causes. Understanding the distinction between voltage and current is key to comprehending the true hazards of electricity.
The Critical Factor: Current
The flow of electrical energy that interacts with and damages biological tissue is measured as current in amperes (Amps), or more commonly, milliamperes (mA). Voltage provides the force to move electrons, but the resulting current determines the severity of an electric shock. Current flow is directly proportional to the applied voltage and inversely proportional to the resistance it encounters.
The human body’s natural defense against electrical flow is its resistance. Under dry conditions, the outer layer of skin can present resistance as high as 100,000 ohms, significantly limiting current flow at lower voltages. If the voltage is insufficient to overcome this resistance, the resulting current remains low and relatively harmless.
A small voltage, such as 12 volts from a car battery, is not typically dangerous because the body’s high resistance restricts the current flow to a non-lethal level. However, if resistance drops dramatically—for instance, due to wet skin—even standard household voltage, like 120 volts, can push a dangerously high current through the body. Therefore, current, measured in milliamperes, directly correlates with physiological harm, not the voltage alone.
Biological Effects of Electrical Flow
Once a sufficient current enters the body, it causes damage through two primary mechanisms: disrupting nerve and muscle function and generating heat. Very low currents, between 1 and 5 milliamperes, are generally only noticeable as a slight tingling sensation. As the current increases, it begins to interfere with the body’s own electrical signals, which control muscles.
At currents between 6 and 30 milliamperes, the shock can lead to muscle tetany, or sustained, involuntary contraction. If the hands are gripping a live conductor, this level of current can exceed the “let-go” threshold. This effectively freezes the muscles, preventing the person from releasing the source of the shock. This inability to let go dramatically extends the duration of exposure, increasing the chance of a fatal outcome.
The most common cause of death from electric shock is the disruption of the heart’s electrical rhythm, known as ventricular fibrillation. This occurs when current, often between 50 and 150 milliamperes of alternating current (AC), causes heart muscle fibers to twitch chaotically instead of contracting in a coordinated manner. Since the heart can no longer pump blood effectively, this condition is rapidly fatal if not immediately corrected.
Higher currents, often exceeding 1,000 milliamperes, can cause the heart to stop completely (cardiac arrest). These high currents also result in severe thermal burns as the electrical energy is converted to heat.
Variables Determining Lethality
Since current is the killer, any factor that lowers the body’s resistance or directs the current through the heart increases the danger, regardless of the voltage source. The body’s skin resistance is the most important variable, acting as the first line of defense. Dry skin can maintain resistance up to 100,000 ohms, but wet or damaged skin can reduce this to as little as 1,000 ohms, allowing a potentially lethal current to pass through.
High voltages, typically over 450 volts, are especially dangerous because they can physically break down the skin’s resistance. This breakdown forces a high current flow even through dry skin.
The path the current takes through the body is another significant determinant of lethality. A path that crosses the chest, such as from one hand to the other, is far more dangerous because it places the heart directly in the circuit. Shocks traveling across the torso are much more likely to trigger fatal ventricular fibrillation than those traveling, for example, between two fingers on the same hand.
The duration of contact is also a major factor; longer exposure increases the cumulative energy delivered and the likelihood of ventricular fibrillation. Even a seemingly low current can be deadly if sustained over several seconds.
The type of current matters, as low-frequency alternating current (AC), like the standard 60 Hz used in household wiring, is generally considered three to five times more dangerous than direct current (DC) at the same voltage. AC is more likely to induce the sustained muscle contraction that prevents release and is more effective at inducing ventricular fibrillation than DC.