Electricity is the flow of electrical charge, which powers modern life. When the human body inadvertently becomes part of an electrical circuit, this flow of charge can disrupt the body’s natural bioelectrical processes. Determining when electricity becomes hazardous depends on specific physical conditions and biological responses. The severity of the resulting injury is related to the amount of current, the duration of exposure, and the pathway the current takes through the body.
Understanding Current as the Primary Danger
The true measure of electrical hazard is the current, measured in amperes (A) or milliamperes (mA). Unlike voltage, which is the electrical potential, current is the factor that directly causes physiological harm. Voltage can be compared to water pressure in a hose, while current is the volume of water flowing out.
High voltage combined with high resistance results in low, relatively safe current. Conversely, low voltage can be dangerous if conditions allow a substantial current to pass through the body. Alternating current (AC), especially at common frequencies like 60 Hz, interferes with the body’s own electrical signaling system.
The nervous system and muscles operate on electrical impulses. An external current overrides these natural signals, causing involuntary muscle contraction and disrupting the heart’s rhythm. This interference dictates the severity of the shock, making current (measured in milliamperes) the focus of safety thresholds. The duration of contact is also important; a brief exposure may only cause pain, while a longer shock can be fatal.
Specific Physiological Thresholds of Injury
The transition from a perceptible tingle to a life-threatening event occurs across a narrow range of current thresholds. The lowest level a person can feel, known as the perception threshold, is around 1 milliampere (mA) for 60 Hz alternating current. This sensation is a faint tingle, marking the point where the body acknowledges the electrical flow.
The next significant threshold is the “let-go” threshold, which occurs when current causes involuntary muscle contraction severe enough to prevent a person from releasing the energized conductor. For adult males, this threshold is typically around 9 to 16 mA, while for women it is slightly lower. Prolonged exposure at this level is dangerous because the inability to let go increases the duration of the shock, significantly raising the risk of severe injury.
Higher currents begin to affect the involuntary systems that sustain life. Currents in the range of 17 to 99 mA can cause extreme pain and respiratory paralysis. This occurs when the current passes through the chest, causing breathing muscles to lock up and leading to suffocation if the current is not interrupted.
The most common mechanism of death from electrical shock is ventricular fibrillation (VF), an uncoordinated twitching of the heart muscle that stops the heart from pumping blood. The threshold for inducing VF is estimated to be in the range of 60 to 120 mA for a one-second shock through the chest, demonstrating that a relatively small amount of current can be lethal.
The Role of Resistance, Voltage, and Path
The amount of current flowing through the body is governed by the relationship between voltage and resistance, described by Ohm’s Law. Voltage is the electrical potential attempting to push the charge, while the body’s resistance acts as a limiting factor. The skin provides the vast majority of this resistance, acting as a natural protective barrier.
When skin is dry and intact, its resistance can be as high as 100,000 ohms (Ω), which significantly limits the current flow. However, moisture, such as sweat or water, drastically lowers this resistance to as little as 1,000 Ω, permitting a much greater current to flow at the same voltage. High voltages can also physically break down the skin’s resistance, further reducing the body’s overall impedance to as low as 500 Ω.
The path that the current takes through the body is a third determining factor in the severity of the injury. Current flowing from one hand to the other, or from a hand to a foot, is particularly hazardous because it passes directly through the chest cavity. This pathway places the heart and lungs directly in the circuit, increasing the probability of respiratory arrest or ventricular fibrillation.
Conversely, a current path that avoids the chest, such as foot-to-foot contact, is less likely to be immediately fatal, though it can still cause severe localized damage. The internal resistance of tissues like blood, muscle, and nerves is much lower than dry skin. The current will always follow the path of least resistance between the points of contact.
Types of Electrical Injury
Once the electrical hazard threshold is crossed, the resulting physical consequences manifest primarily through heat, disruption of function, and tissue destruction. Electrical burns are a common and serious consequence, occurring both externally at the points of entry and exit, and internally along the current’s path. These thermal injuries are caused by the heat generated as the electrical current meets the body’s resistance, leading to massive internal tissue and organ damage that may not be apparent on the surface.
Neurological damage is a frequent outcome, as the current directly affects the central and peripheral nervous systems. This can result in immediate effects like loss of consciousness, seizures, and respiratory arrest, or delayed and long-term issues such as memory disturbances, chronic pain, and peripheral neuropathies. The extent of the damage varies widely, from transient symptoms to permanent disabling conditions.
Beyond the immediate threat of ventricular fibrillation, the heart muscle can sustain direct cardiac damage, including the death of heart tissue and the development of irregular rhythms. Damage to muscle tissue can also lead to the release of cellular contents into the bloodstream, a condition that can overwhelm and injure the kidneys.