The question of how many volts it takes to stop a heart is common, but it is fundamentally incomplete. Electricity is a complex phenomenon, and assuming voltage alone determines lethality overlooks the physical principles of how electrical energy affects the human body. The danger of an electrical shock is not simply a matter of electrical “pressure,” but rather the resulting flow of energy through the body’s tissues. To understand the true hazard, one must consider the factors that govern the actual current that passes through the body.
The Difference Between Voltage and Amperage
Voltage (V) is the electrical potential difference, or the “pressure” that pushes an electrical charge. It represents the force available to move electrons through a circuit, including the human body. High voltage is not inherently lethal unless it can overcome the body’s resistance to drive a flow of current.
The true measure of harm is the electrical current, known as amperage (A), which is the volume of electrons flowing through the tissue. For biological injury, this flow is measured in milliamperes (mA), or one-thousandth of an amp. Voltage is like water pressure in a hose, while amperage is the volume of water flowing out.
This relationship is governed by Ohm’s Law, where current equals voltage divided by resistance. The body acts as a resistor, meaning a high-voltage source can be harmless if the body’s resistance limits the current flow. Conversely, a low voltage can be dangerous if the body’s resistance is low, allowing a lethal current to flow. Current directly causes physiological damage, while voltage provides the force to drive that current.
How Electrical Current Disrupts the Heart
The heart’s function relies on a precisely timed, internal electrical system that coordinates the contraction of its four chambers. This system, including the sinoatrial (SA) node and the atrioventricular (AV) node, acts as the natural pacemaker. When an external electrical current passes through the chest, it interferes with these signals.
The external current bombards the heart’s electrical pathways with disorganized stimuli, causing muscle fibers to contract randomly instead of rhythmically. This chaotic depolarization often results in ventricular fibrillation (V-fib), where the ventricles quiver uselessly instead of pumping blood. Alternating current (AC) at 50 or 60 hertz is effective at inducing V-fib because its frequency disrupts the heart during its vulnerable repolarization phase.
Another outcome, common with high-voltage or direct current (DC) shocks, is asystole. Asystole is the complete cessation of electrical activity, often called a “flat-line” on an electrocardiogram. The overwhelming external current causes the entire electrical system to shut down. While V-fib is chaotic beating, asystole means the heart has stopped beating entirely.
Factors Influencing Electrical Injury Severity
The amount of current that flows through a person depends on several variables that determine the body’s overall electrical resistance. The primary barrier to current flow is the skin, which has high resistance when dry. Dry, intact skin can offer resistance of 100,000 ohms or more, which limits the current that can pass from a low-voltage source.
This protection is easily compromised, as resistance drops significantly when the skin is wet. Wet skin can lower resistance to 500 to 1,000 ohms, allowing a much higher current to flow, even from a household voltage source. High voltage can also compromise the skin’s dielectric properties, breaking down resistance and allowing a massive surge of current.
The path the current takes through the body is a major determinant of injury severity. A current flowing from hand to hand, or hand to foot, is more dangerous because it passes directly through the chest cavity and across the heart. Conversely, a current that travels a short path, such as between two fingers, is less likely to be lethal because it bypasses the heart.
The duration of contact is the final factor. A longer exposure time increases the likelihood that the current will pass through the heart during its vulnerable cycle, escalating the potential for fatal ventricular fibrillation.
Physiological Thresholds for Cardiac Harm
The direct threat to life is defined by the amount of current (mA) that passes through the body. The threshold of perception, or mild tingling, can be felt with as little as 1 milliampere of alternating current. As current increases, it affects muscle control, with the average adult male losing the ability to voluntarily release a conductor at the “let-go” threshold, typically between 10 and 20 milliamperes.
The most dangerous threshold is the one that causes ventricular fibrillation. For alternating current passing through the chest, a current as low as 30 to 50 milliamperes can induce V-fib, especially if contact is sustained for more than a second. The probability of V-fib increases significantly in the 50 to 100 milliampere range, which is the lethal threshold for most people.
A standard 120-volt household outlet can supply thousands of milliamperes, far more than the 50 mA required to stop a heart. Most shocks from household current are not fatal because the high resistance of dry skin limits the resulting current flow to a non-lethal level. The body’s resistance and the source voltage determine if the current will reach the deadly milliampere range.