The question of how much electricity can stop a human heart is less about high voltage and more about the flow rate of energy, measured as current in milliamperes (mA). Electrical shock occurs when the body becomes part of an electrical circuit, allowing current to pass through tissues. While voltage determines the potential force driving the current, the magnitude of the current passing through the body determines the severity of the injury. Surprisingly small amounts of current, far less than needed to trip a household circuit breaker, can cause serious harm or death by interfering with the body’s electrical systems.
Critical Current Thresholds for the Human Body
The physiological effects of current depend on its strength, starting with the perception threshold, typically around 1 milliampere (mA), which produces a faint tingling sensation. As current increases, it escalates to causing involuntary muscle contraction.
A critical benchmark is the “let-go” threshold, the maximum current a person can tolerate and still voluntarily release the conductor. For men, this threshold averages around 15.5 mA of 60 Hertz alternating current (AC), and for women, approximately 10.3 mA. Currents exceeding this level cause sustained, tetanic muscle contraction, preventing the person from pulling away and dramatically increasing the shock duration.
Currents from 20 mA to 50 mA can lead to respiratory paralysis if the current passes through the chest. This happens because the electrical energy overrides nervous system signals, causing chest muscles, including the diaphragm, to lock up. Uninterrupted exposure leads to asphyxiation.
The most dangerous range for the heart is between 50 mA and 100 mA of AC, as this level is most likely to induce ventricular fibrillation (V-Fib). V-Fib is a chaotic, unsynchronized quivering of the heart’s pumping chambers, which is the most common cause of death in household electrocution accidents. Higher currents, exceeding 4 Amperes (4,000 mA), can cause the heart to stop completely (asystole), often accompanied by immediate, severe internal burns.
Variables Affecting Electrical Injury Severity
The specific thresholds for harm are not absolute, as injury severity is influenced by several factors. The path the current takes through the body is significant, determining which vital organs are exposed. A current traveling from hand to hand or hand to foot is more dangerous because it places the heart directly in the circuit’s path.
The duration of exposure is also a determinant of injury. A brief, high-level shock may be less damaging than a lower current sustained over several seconds. This time dependence is relevant to the heart’s vulnerability cycle, where a small current can induce V-Fib if it hits the heart during its brief repolarization phase (the T-wave). Longer exposure also increases the risk of thermal burns as electrical energy converts to heat.
The body’s total resistance dictates how much current will pass through for a given voltage. The outer layer of skin provides the majority of this resistance. Dry, intact skin can have a resistance of tens of thousands of Ohms. If the skin is wet, sweaty, or broken, resistance can drop dramatically to a thousand Ohms or less. This reduction allows a much greater current to flow through the body at the same voltage, increasing injury potential.
The type of current also plays a role, with alternating current (AC) generally considered more hazardous than direct current (DC) at similar voltage levels. AC’s oscillating nature is highly effective at stimulating muscles, leading to the “no-let-go” phenomenon and prolonged exposure. AC at common utility frequencies is also more likely to interfere with the heart’s natural pacemaker signals and induce V-Fib at lower current levels compared to DC.
How Electricity Disrupts Heart Rhythm
The heart operates on a finely tuned electrical system, initiated by the sinoatrial (SA) node, which acts as the natural pacemaker. This organized activity ensures the rhythmic, coordinated contraction of muscle fibers, allowing the heart to pump blood effectively. An external electrical current passing through the chest disrupts this delicate internal rhythm by introducing chaotic, uncoordinated signals.
The primary life-threatening consequence is ventricular fibrillation (V-Fib), which short-circuits the heart’s electrical control. During V-Fib, ventricular muscle fibers quiver rapidly and chaotically instead of contracting synchronously. This prevents the heart from pumping blood, leading to rapid loss of consciousness if the normal rhythm is not restored.
V-Fib is typically caused by intermediate current ranges that interfere with signals without causing massive tissue destruction. Extremely high currents, often from high-voltage exposure, can cause the heart to completely seize up in a sustained contraction, known as asystole or cardiac standstill. Both V-Fib and asystole result in the failure of the heart to circulate blood.
Essential Electrical Safety Guidelines
Understanding the low current thresholds underscores the importance of caution around all energized sources.
Safety Measures
Ground Fault Circuit Interrupters (GFCIs) are highly effective safety measures for areas where water is present, such as kitchens and bathrooms. A GFCI constantly monitors current flow and instantly shuts off power if it detects an imbalance of just a few milliamperes. This interrupts the flow before the current reaches dangerous levels.
To mitigate risks associated with electrical exposure, follow these guidelines:
- Always ensure hands and feet are dry when handling electrical cords, appliances, or switches, as wet skin drastically lowers resistance.
- Never operate electrical devices while standing in water or on damp ground, which creates a direct path to the earth.
- Maintain all electrical equipment in good condition, replacing any frayed or damaged cords.
- Never bypass safety mechanisms, such as removing the grounding prong from a plug, as these features provide a safe return path for stray current.