A standard 120-volt household outlet can be lethal, but electrocution is not guaranteed with every shock. The 120 volts of alternating current (AC) used in North American homes possesses enough electrical pressure to push a fatal amount of energy through the human body. Whether this voltage causes a minor jolt or a deadly electrocution depends entirely on a combination of specific conditions. The resulting danger is primarily determined by the amount of electrical current that flows through the body and the path that current takes.
The Critical Factors in Electrical Harm
The potential for harm from an electrical source is not determined by voltage alone; the truly dangerous element is the current, measured in amperes or milliamperes (mA). Current flow is governed by the relationship between voltage and resistance. A fixed voltage of 120V will produce a current proportional to the body’s resistance at the point of contact.
The human body’s external layer, the skin, provides the vast majority of electrical resistance. Dry, intact skin can possess a resistance of tens of thousands of ohms, severely limiting the current that can pass through. Conversely, wet skin, cuts, or punctures drastically reduce this resistance, sometimes to as low as 500 ohms. This sudden drop in resistance allows a much higher, and potentially lethal, current to be driven by the 120V source.
A current of just 100 milliamperes (0.1 ampere) passing through the chest is enough to be fatal. If a person’s body resistance is reduced to 1,200 ohms—a low but achievable number under wet conditions—the 120-volt source can push exactly 100 mA through the body. This demonstrates why environments involving water, sweat, or moisture are significantly more hazardous when dealing with household electricity. Sustained contact also increases the likelihood of physiological damage.
How Electricity Affects the Human Body
Once a dangerous current enters the body, it causes harm through three primary mechanisms: disruption of the heart’s rhythm, interference with the nervous system, and thermal damage. The most immediate and life-threatening effect is the induction of ventricular fibrillation, which prevents the heart from pumping blood effectively. This condition can be triggered by low amounts of 60-Hertz AC current, often in the range of 75 to 100 milliamperes.
The path the electrical current takes through the body is important. A current traveling from one hand to the other, or from a hand to a foot, is particularly dangerous because it crosses the chest cavity and the heart. This transthoracic path maximizes the risk of triggering ventricular fibrillation and subsequent cardiac arrest. Current passing only through a single limb, while still causing severe local injury, is less likely to be immediately fatal.
Electricity also interferes with the nervous system, which relies on weak electrical signals to control muscle function. Alternating current, especially at the 60 Hz frequency common in homes, is highly effective at stimulating and overriding these signals. A current as low as 10 to 20 milliamperes can cause sustained, involuntary muscle contraction, known as tetany, preventing the person from letting go of the energized conductor. This “no-let-go” phenomenon prolongs the duration of exposure, increasing the chance of fatal injury.
The body experiences thermal burns as the electrical energy converts to heat when passing through tissues. These burns are often deep and severe, occurring at both the entry and exit points where the current density is highest. The heat can cause massive tissue destruction, including internal organs, even if external burn marks appear small.
Essential Safety Measures Around Household Outlets
Recognizing the danger posed by 120V requires implementing specific safety practices. The most effective safety device for protecting against household shock is the Ground Fault Circuit Interrupter (GFCI). A GFCI constantly monitors the electrical current flowing into a circuit and compares it to the current returning.
If a difference of as little as 5 milliamperes is detected—indicating current is leaking out, potentially through a person—the GFCI instantly trips and cuts the power. This interruption occurs in a fraction of a second, quickly stopping the flow before a fatal shock can occur. The National Electrical Code requires GFCI protection in locations prone to moisture, including bathrooms, kitchens, garages, basements, and all outdoor outlets.
Never attempt to defeat the purpose of the three-pronged plug by removing the ground pin, as this eliminates a safety feature designed to divert fault currents. Keep electrical cords and appliances away from sinks, tubs, and wet floors, and use plastic safety caps to cover unused outlets, especially in homes with young children. Always inspect cords for fraying or damage before use, as exposed wiring dramatically lowers the resistance and increases the risk of shock.