1. How Strong Is the Shock, and How Long Does the Pain Last?
Tables based on research institute data show the relationship between the voltage of electricity stored in the body and how it is felt. At 2000 volts, you might feel something on the outside of your finger, but it is not painful. At 3000 volts, a prickling pain like being stabbed by a needle appears. At 10,000 volts, the whole hand hurts. The sound of the discharge is faintly audible from about 2000 volts, and the light of a spark becomes visible from around 4000 volts [1]. Some company pages explain that as a rough guide, people begin to feel a stimulus at approximately 3000 volts [3]. Since household outlets in Japan are 100 volts, 3000 volts—the guide for a prickling pain—is 30 times higher, and the 10,000 volts that hurts the whole hand is 100 times higher. Even so, the current causing the shock flows for only an instant.
According to the National Institute of Occupational Safety and Health (JNIOSH), an electrostatic shock occurs when a current of a few milliamps flows through the body in a short time of less than one-millionth of a second. If the current is 0.1 milliamps or less, it does not feel like a shock even if it flows for a longer time [2]. The reason for the pain is not just the high voltage, but also the fact that current flows in a very short time. So, when and where did this electricity build up in your body?
2. How Walking Separates Electricity and Stores It in the Body
When a person walks on a floor, the soles of their shoes repeatedly touch and separate from the surface. An introductory document from the EOS/ESD Association, a technical group for electrostatic discharge, explains that static electricity is generated with each contact and separation [4]. While often thought of as something caused by rubbing, it occurs just from touching and separating. Electricity is not created from nothing; rather, positive and negative charges that were already present separate between the shoe and the floor. This electricity accumulates in the walking person's body, though the path it takes varies by conditions. This phenomenon is called "triboelectricity." In Greek, "tribo" means "to rub," and "electros" means "amber" [4]. Amber is a gemstone made from hardened tree resin that attracts light objects when rubbed. Thales in the 6th century BC is said to have noted this. In 1600, William Gilbert created the Latin word "electricus" based on the Greek word for amber, which became the root of the English word "electricity" [5].
The human body is compared to a container that stores electricity. It is like a container that takes in a little electricity with every step [6]. However, in reality, electricity is not a liquid like water; it involves particles like electrons.
3. How Air Stops Insulating and Lets a Spark Jump
Air is normally an "insulator," meaning it does not conduct electricity. But when the voltage becomes very high, air can no longer resist it, turning into a path for electricity and causing a spark. A rough guide for the strength air can withstand is about 3000 volts per 1 millimeter of gap [7]. Simply calculated, 10,000 volts could jump across a gap of about 3 millimeters. However, at pointed areas like fingertips, the calculation may not hold true.
The energy of the flying electricity is surprisingly small. Calculating with the body as a capacitor, 3000 volts is about 0.45 millijoules, and even 10,000 volts is about 5 millijoules [6]. Although the voltage is high, the energy is very small, so it is explained that it is not enough to injure a healthy person. However, in places with gasoline vapors, it can cause a fire. Because fires occurred at self-service gas stations during dry periods when static sparks ignited vapors, the Fire and Disaster Management Agency issued a notice in 2001 urging the removal of static electricity before refueling [8]. The static electricity removal sheets found at many stations today are a result of this. Note that even if the energy is small, it is not always safe.
4. How Humidity Changes Body Voltage by 23 Times
For a person walking on carpet, the body voltage is approximately 35,000 volts when humidity is 10–25%, and about 1,500 volts when humidity is 65–90%. This is a difference of about 23 times [4]. On vinyl floors, there is a difference between 12,000 volts and 250 volts. These are both "typical guides," and these voltages do not always occur. It is important to note that even when humidity is high, static electricity itself is still occurring [4]. What changes is the amount that accumulates. In humid air, water particles or films form on the surfaces of objects. Since water conducts electricity, the charge escapes more easily. Generally, it is a guide that if relative humidity exceeds 65%, static electricity is less likely to generate, and if it does, it escapes naturally [9]. Using the container analogy, humid days are like having a small hole in the bottom, so electricity leaks out immediately. Dry winter days are like having the hole plugged, so it accumulates. In reality, electricity does not flow out of a hole like water; rather, the water film acts as a path for electricity.
5. Why Winter Indoor Air Becomes Dry
Humidity (relative humidity) is the ratio of actual water vapor content to the maximum amount the air at that temperature can hold. The amount of water vapor air can hold changes greatly with temperature. Per 1 cubic meter, it is about 4.8 grams at 0°C, 9.4 grams at 10°C, and 17.3 grams at 20°C [10]. Cold air can hold only a little water vapor. According to the Japan Meteorological Agency's normal values (1991–2020), Tokyo's average temperature in January is 5.4°C with an average relative humidity of 51%. In August, it is 26.9°C and 74% [11]. The outside air in winter is surprisingly humid. However, if this outside air is heated to 20°C without adding water vapor, the humidity drops to about 19% [12]. This falls into the "10–25%" range where the guide showed about 35,000 volts. In actual rooms, humidity does not drop to 19% because water vapor is added by breathing and cooking. Still, this calculation shows that heating alone significantly lowers humidity.
6. Why Some People and Some Days Get More Shocks
Even when touching the same door handle, some people get a shock and others do not, and some days bring a shock while others do not. JNIOSH explains that this difference is not due to body constitution, but the amount of static electricity accumulated in the body. The material of the shoe sole greatly influences this amount [2]. Shoe soles made of natural rubber types resist electricity flow, so accumulated charge does not escape easily, making shocks more likely. Soles made of polyurethane or nitrile rubber types conduct electricity more easily, so charge does not accumulate as much, making shocks less likely. The same person may feel a different strength of shock depending on the shoes they wear. If charge has already accumulated, touching the handle with a leather product allows the electricity to flow gradually, discharging without a shock [2]. The same electricity is painful if it flows instantly, but not painful if it flows over time.
7. How Lightning Is a Giant Version of Static Electricity
The Japan Meteorological Agency describes lightning as a phenomenon where large amounts of positive and negative charges separate in the atmosphere and discharge. It is thought that graupel (small ice pellets) and small ice crystals collide in clouds, separating the electricity [13]. The educational page from NOAA in the United States begins its explanation of lightning with the example of static electricity from touching a door handle [14]. Electricity separates, accumulates, and then jumps when air can no longer resist it. This is the same sequence as the shoe and door handle flow we have looked at. The difference is the size. The separation of electricity in clouds and the delay between light and sound are covered in notes on lightning.
8. What Scientists Still Don't Know About Electricity from Rubbing
We have explained that "electricity transfers from one side to the other." However, the detailed mechanism of what transfers and why is not yet clearly determined. The details of triboelectricity are considered not to be established science as of 2023 [15]. There is a table called the "triboelectric series" that lists materials in order of how easily they become positive. But this is an empirical arrangement, and contradictory combinations have been found, such as A being more positive than B, B more positive than C, but C more positive than A [15]. In 2011, a study by Baytekin et al. published in *Science* showed that surfaces rubbed together do not have one side entirely positive and the other entirely negative. Instead, small areas of positive and negative charge are mixed like a mosaic [16]. Another experiment reported in *Nature* in 2008 found that peeling adhesive tape in a vacuum produced X-rays [17]. This was an experiment in a vacuum, so the conditions differ from peeling tape in a home with air.
9. How to Investigate Winter Humidity in Your Own Region
Does the calculation that heating winter air lowers humidity hold true in your region? You can find January temperature and humidity values from the normal value tables on the Japan Meteorological Agency's website and compare them [11]. The humidity shown in weather forecasts is for outside air. Humidity inside a room is a different number. If you want to look at the materials, there are three options: the abstract of the "mosaic" paper [16], which explains what was measured and found; the paper on adhesive tape and X-rays [17], which explains what was verified in a vacuum; and the Fire and Disaster Management Agency's report on static fires at self-service stations [8], which includes tables showing that they are more common in dry periods and changes in case numbers after the notice. If you see a static electricity removal sheet at a self-service station, read the report to investigate why the sheet is there.