The Heart Is Likely to Stop at What Current Level?

The human heart can be sent into a fatal rhythm at roughly 100 milliamps of alternating current (AC) passing through the chest, a level sometimes called the ventricular fibrillation threshold.¹ That is a strikingly small amount of electricity, only about as much as a 10-watt light bulb draws from a household outlet. But “100 milliamps” is a rough midpoint, not a fixed line. The actual danger depends on the type of current, the path it takes through the body, how long contact lasts, and even what fraction of a heartbeat the shock lands on.

Why 100 Milliamps Is the Common Benchmark

Most safety references list 100 mA (0.1 amp) of 50/60 Hz AC as the threshold for ventricular fibrillation, the chaotic quivering of the heart’s lower chambers that stops effective pumping and leads to death within minutes if not reversed. At around 2 amps, the heart may simply lock in place rather than fibrillate, a condition called cardiac standstill, and internal organs begin to sustain direct thermal and electrical damage.¹ In between those two figures is a range where fibrillation is the primary killer. Below about 30 mA, most people can still let go of a live conductor, though muscle contractions and pain may already be severe. The jump from “painful but survivable” to “potentially fatal” happens over a remarkably narrow band of current.

The Heart’s Extreme Sensitivity to Alternating Current

Household and industrial power supplies deliver alternating current, and the heart is far more sensitive to AC than to direct current (DC). Research has shown that ventricular fibrillation can be triggered by AC at roughly one-eighth the current needed just to pace the heart electrically.² The reason involves how the heart’s electrical cycle works. The heart coordinates each beat through a wave of electrical activity that sweeps through the muscle in a precise sequence. AC oscillates dozens of times per second, and some of those oscillations inevitably land during the window when the heart muscle is resetting for its next beat. That window of vulnerability is when fibrillation is easiest to trigger.

DC shocks can also be lethal, particularly at high voltages like those found in industrial battery banks or rail systems. But at the lower voltages found in most domestic settings, AC is considerably more dangerous per milliamp. This is a practical distinction worth remembering: a 120-volt AC outlet poses a greater cardiac risk than a 120-volt DC source, all else being equal.

The Vulnerable Window in Each Heartbeat

Not every instant during a heartbeat is equally dangerous. On an electrocardiogram, the T wave represents the moment when the ventricles are recovering from one contraction and preparing for the next. A shock that arrives during this brief window is far more likely to trigger fibrillation than one landing at any other point in the cycle. Studies in patients undergoing defibrillator implantation have mapped this “vulnerable period” precisely: it begins on the ascending limb of the T wave and ends at or slightly beyond its peak, occupying only about 12% of the total QT interval.³ In practical terms, this window lasts only a few tens of milliseconds.

With AC, the cyclical nature of the current means that if contact lasts even a second or two, multiple oscillations will pass through during the vulnerable period no matter when the shock starts. That is part of what makes sustained AC contact so dangerous. With a single brief DC pulse, timing matters enormously. Defibrillator research has explored the exact boundaries of this zone, finding that the fibrillation threshold (the weakest shock that can induce fibrillation) occurs at a specific coupling interval relative to the T wave peak, while the upper limit of vulnerability (the strongest shock that can still induce fibrillation) occurs at a slightly shorter coupling interval.⁴ These findings underpin how defibrillators are calibrated to exceed the upper limit, ensuring a shock strong enough to reset the heart rather than throw it into fibrillation.

Why Duration Matters Less Than You Might Think

Intuitively, you might expect that the longer a shock lasts, the more dangerous it becomes. Up to a point that is true, but the relationship plateaus quickly. If the current flowing through the chest is strong enough to cause fibrillation at all, it will generally do so within one to two seconds; longer shocks do not tend to be significantly more dangerous.² The reason ties back to the vulnerable period. Within one or two seconds of AC exposure, the current has cycled through enough heartbeats to catch at least one vulnerable window. After that, additional seconds of contact add thermal injury to skin and tissue but do not dramatically change the probability of fibrillation itself.

This has real implications for rescue situations. Breaking contact within the first second or so can genuinely make the difference between a painful shock and a fatal one. It also means that the “let-go” threshold, the current level at which a person’s muscles contract so forcefully that they cannot release their grip on a conductor, is so critical. If you can let go, you probably survive. If the current locks your hand around the wire, one to two seconds of contact may be all it takes.

Skin Resistance as the Body’s Main Defense

The 100 mA figure describes current actually flowing through the body’s interior. Getting that much current through the chest requires enough voltage to overcome the body’s resistance, and the biggest source of resistance is the skin. Contact resistance of intact, dry skin typically ranges from about 1,000 to 100,000 ohms, depending on moisture, callousness, contact area, and pressure.⁵ Internal body tissues, which are rich in saltwater-based fluids, have far lower resistance, on the order of a few hundred ohms.

This means that the same voltage can produce wildly different internal currents depending on conditions. A 120-volt shock through dry, calloused hands might push only 1 to 2 mA through the chest, enough to be startling but not life-threatening. The same 120 volts through wet skin, an open wound, or a mucous membrane could drive tens of milliamps or more through the torso, entering the danger zone for fibrillation. This is why electricians are taught that wet conditions dramatically raise the risk of electrocution, and why bathrooms and kitchens have ground-fault circuit interrupters rated to trip at just 5 mA of leakage current, a level chosen well below the fibrillation threshold.

Voltage itself is not the direct killer. It is the enabler. A static shock from a doorknob can easily reach 10,000 volts or more, but the current is vanishingly small and the duration is microseconds, so it poses no cardiac risk. Meanwhile, 30 volts of AC can theoretically drive a lethal current through a person standing barefoot in water.¹ The interplay of voltage, resistance, and resulting current is what determines danger, not any single number in isolation.

The Path Through the Body

A shock that travels hand-to-hand or hand-to-foot passes directly through the chest and places the heart in the current’s path. A shock that travels across a single hand or between two points on the same leg may cause severe burns and muscle damage but is far less likely to trigger fibrillation, because only a tiny fraction of the current diverts through the heart. The classic worst-case pathway in occupational safety literature is left hand to right foot, which runs current across the chest through the heart, or hand to hand. A foot-to-foot pathway, such as stepping on an energized surface, carries less cardiac risk because the current path mostly stays below the trunk.

Body size plays into this as well. A smaller person, particularly a child, has less tissue between the entry and exit points, meaning the current density through the heart is higher at the same total current. No firm pediatric fibrillation threshold has been established in controlled studies for obvious ethical reasons, but safety guidance generally assumes that children are more vulnerable at lower currents than adults.

Individual Variation and Pre-Existing Conditions

The 100 mA figure is a population-level approximation. In reality, some individuals fibrillate at lower currents and others tolerate higher ones. A large single-center study of 480 patients who survived electrical accidents found that baseline clinical parameters like age and sex were not statistically significant predictors of who developed arrhythmias afterward, though high-voltage injuries showed borderline significance as a risk factor.⁶ In other words, researchers have had trouble pinpointing exactly who is most at risk beyond the obvious variable of how much voltage and current actually made it to the heart.

People with pre-existing heart conditions, particularly those with long QT syndrome, cardiomyopathies, or implanted devices, are generally considered more susceptible. Their hearts already have abnormal electrical conduction patterns that lower the threshold for fibrillation. Medications that prolong the QT interval, a surprisingly long list that includes certain antibiotics, antipsychotics, and antihistamines, could theoretically widen the vulnerable window during each heartbeat, though no controlled study has directly tested this in the context of electrical injury.

Lightning as a Special Case

Lightning delivers an enormous current, often tens of thousands of amps, but for an extremely short duration measured in microseconds. The cardiac effects differ from those of household or industrial electrocution. Lightning can cause both ventricular fibrillation and asystole (a flat-line cardiac arrest), along with a broad range of other cardiovascular complications including abnormal heart rhythms, damage to the heart muscle, injury to the aorta, and problems with the pericardium, the sac surrounding the heart.⁷

The short duration of a lightning strike means the vulnerable-period model used for sustained AC does not straightforwardly apply. Instead, the sheer magnitude of the current overwhelms the heart’s electrical system. Interestingly, many lightning-strike survivors recover normal heart function because the massive current effectively resets the heart’s electrical activity in a way analogous to defibrillation. The primary reason lightning kills is usually respiratory arrest: the current paralyzes the brainstem’s breathing center, and if breathing is not restored, the resulting oxygen deprivation eventually stops the heart. This is why bystander CPR is so effective for lightning victims compared to many other forms of cardiac arrest.

What Happens to Survivors After an Electrical Shock

For people who survive the initial event, the long-term cardiac prognosis is better than you might expect. A large Danish nationwide cohort study tracked immediate survivors of accidental electric shock and found that the five-year cumulative death rate was under 1% for emergency-ward patients and just over 1% for those who required hospital admission. These rates did not differ from matched controls who had never been shocked. Cardiac procedures and new cardiac diagnoses among survivors were rare.⁸ This suggests that if the heart survives the initial insult and returns to a normal rhythm, whether spontaneously or through resuscitation, lasting cardiac damage from a single low-voltage shock is uncommon.

That said, survivors of high-voltage injuries, particularly industrial workers or those struck by lightning, can develop delayed complications including arrhythmias, reduced heart function, and post-traumatic stress. Monitoring protocols typically involve at least several hours of cardiac rhythm observation, with longer monitoring for high-voltage exposures or anyone who lost consciousness.

Forensic Challenges When Electrocution Leaves No Mark

One of the more unsettling aspects of electrical cardiac death is that it does not always leave visible evidence. When a lethal current passes through the body, it sometimes produces characteristic electrical burns at the entry and exit points, but not always. In cases where typical electric marks are absent, confirming that electrocution caused the death becomes a genuine diagnostic challenge for forensic pathologists.⁹ The heart itself may show no gross abnormalities. Researchers have been working on identifying molecular biomarkers in cardiac tissue that could confirm electrical injury at autopsy, a line of investigation that reflects how difficult these cases can be. Low-voltage electrocution deaths in bathtubs or near appliances are among the most frequently missed causes of sudden death in forensic practice.

How Safety Standards Use These Numbers

Electrical safety regulations are built around the current thresholds described above, but with wide margins of safety. Ground-fault circuit interrupters in the United States trip at 5 mA of leakage, twenty times below the fibrillation threshold. Residual-current devices in Europe and elsewhere commonly trip at 30 mA. Industrial lockout-tagout procedures assume that any energized conductor is potentially lethal regardless of voltage, because skin resistance can vary so drastically that even relatively low voltages pose a risk under the wrong conditions.

The 100 mA benchmark also explains the design of medical equipment. Devices that contact the body internally, like cardiac catheters, must limit leakage current to microamps rather than milliamps, because bypassing the skin removes the body’s main protective barrier. A current that would be imperceptible on your fingertip can fibrillate the heart if delivered directly to cardiac tissue. Hospital-grade electrical outlets and isolation transformers exist to keep leakage currents at levels that would be harmless even if conducted through an internal catheter.

For anyone working around electricity, the practical takeaway from all of this research is that the margin between a painful shock and a lethal one is narrower than most people assume. Dry skin, intact insulation, and ground-fault protection are the barriers that keep a 120-volt outlet from being a routine killer, and any one of those barriers failing under the wrong circumstances can be enough.