The ampere (Amp) measures the flow rate of electrical charge (current) through a conductor. When discussing the danger of electricity to the human body, it is the flow of current, specifically milliamperes (mA), that causes physiological damage. The current flowing through the body is determined by the relationship between voltage (electrical pressure) and resistance (opposition to flow, measured in Ohms). Therefore, the amount of current is a variable resulting from the interaction of all three factors. This article explains the precise current levels that affect the body and the physics that determine whether a shock becomes deadly.
The Critical Current Levels and Physiological Effects
A current is considered hazardous at the milliampere (mA) level, where one Amp equals 1,000 mA. The perception threshold for alternating current (AC) is extremely low, with many people feeling a faint tingle at just 1 mA. As the current increases, it quickly progresses through stages of increasing danger to the neuromuscular system.
The “let-go” threshold is a hazardous range, typically occurring between 6 and 30 mA of alternating current. Within this range, the current causes involuntary muscle contractions, or tetany, in the muscles used to grip the conductor. If the current excites the flexor muscles, the person cannot voluntarily release the electrical source, prolonging the exposure. The maximum current an average man can grasp and still “let go” is approximately 16 mA.
Currents in the range of 20 to 30 mA can begin to affect the respiratory system. At this level, current passing through the chest can cause the respiratory muscles to contract into a tetanic state, leading to respiratory paralysis. If the exposure is sustained, the resulting lack of oxygen can quickly become fatal.
The most common cause of electrical fatality is ventricular fibrillation, a chaotic quivering of the heart muscle that prevents the effective pumping of blood. For an adult, the threshold is generally 50 to 100 mA of alternating current flowing through the chest for longer than one second. This small amount of current is enough to disrupt the heart’s natural electrical rhythm, leading to circulatory arrest.
Why Voltage and Resistance Determine Current Flow
The current flowing through the body is a direct consequence of the electrical pressure (Voltage) divided by the body’s opposition (Resistance). The voltage provides the necessary force to push a lethal amount of current through the body’s resistance. The human body itself is not a simple conductor, and its resistance is highly variable, making the outcome of a shock unpredictable.
The outer layer of the skin, the epidermis, provides the vast majority of the body’s electrical resistance. When the skin is dry and intact, its resistance can be very high, sometimes exceeding 100,000 Ohms. This high resistance acts as a protective barrier, limiting the flow of current, even from a high-voltage source.
However, moisture drastically reduces this natural defense. Wet skin, such as from sweat or rain, may have a resistance of only 1,000 Ohms or less. If the skin’s resistance is low, even 120 volts from a household outlet can push a current over 100 mA through the body, sufficient to cause ventricular fibrillation. Furthermore, at voltages above 500 volts, the electrical pressure can cause the skin’s resistance to break down entirely, allowing a massive current to flow.
Immediate Biological Causes of Electrical Fatality
The immediate cause of death from electric shock is a disruption of normal biological function, primarily affecting the heart and the nervous system. The nervous system operates through electrical impulses, and an external current can completely overwhelm this delicate communication. Ventricular fibrillation is the most frequently cited mechanism of death in electrical accidents involving common household alternating current.
When an external current passes through the heart, it causes the muscle fibers to contract independently and chaotically, eliminating the organized pumping action necessary for circulation. While fibrillation is caused by lower currents (50-100 mA), very high currents, on the order of 4 Amps or more, can cause the heart to stop completely in a sustained contraction, known as cardiac standstill.
Another fatal mechanism involves the respiratory system. Current passing through the chest or the brainstem can interfere with the diaphragm and other muscles responsible for breathing, leading to respiratory arrest. If the current is sustained, the resulting lack of oxygen, or asphyxia, will cause death.
High-amperage current can also cause severe thermal injury. The flow of current through the body’s tissues generates heat, which can cause deep burns to internal organs, muscles, and nerves. These internal injuries may not be immediately apparent but can lead to delayed death from tissue necrosis and organ failure.
External Variables Influencing Shock Severity
The severity of an electrical shock is influenced by several external variables, including the path the current takes through the body. Current that crosses the chest cavity, such as a hand-to-hand or hand-to-foot path, is significantly more dangerous because it directly involves the heart and lungs. A current path that avoids the chest is less likely to be immediately fatal, though it still carries a risk of nerve damage and burns.
The duration of contact also dramatically influences the outcome. Since ventricular fibrillation requires the current to disrupt the heart during its vulnerable cycle, a longer exposure time increases the probability that the current will strike at the precise moment to induce fibrillation. Even a current level that might be survivable for a fraction of a second can become lethal if exposure is prolonged.
The type of current plays a significant role in the body’s response. Alternating current (AC), found in most homes, is generally considered more dangerous than direct current (DC). AC’s constant change in direction is more effective at causing muscle tetany, which can “freeze” the victim to the conductor, leading to longer contact time. AC is also substantially more likely to induce ventricular fibrillation than DC.