I feel a mild electric shock if I touch any metal part of the car

Mild electric shock on touching the metal part of the car is due to human Electrostatic Discharge (ESD). Electrostatic discharge is a phenomenon in which a charged body discharges to the ground through the appropriate shortest possible path. Our human body is normally charged to a high voltage level at all times.

A charged human body can discharge to any conductive material by contact or close proximity, and such a discharge happens in the case of touching the metal part of the car.

The charging of the body takes place due to friction, which is called tribo-charging, and the friction of the body with the air or any material can make it charged to high voltages.

In normal cases, a human body ESD is around 2000 volt discharge which has nearly one ampere of current in a very short duration. Since the duration of such discharge is less than 150 nanoseconds, human beings do not feel it often, where the human sensory perception is in milliseconds.

Such tribo-charging and discharging are very much dependent on the relative humidity (RH) in the environment. If the atmosphere is very humid (RH much higher or close to 100 per cent), the tribo-charging will be much less.

For a low RH, the tribo charging will be very high. That means in a dry environment, the possibility of human ESD is very high.

Every frictional movement has an associated static voltage and the relative humidity changes make drastic variations in these voltage levels.

For example, a person walking on a carpet at an environment of 10 per cent RH can generate more than thirty thousand volts where the same action on the same carpet at 80 percent RH will generate only two hundred volts. Very high static voltages can be generated by any human action at locations where relative humidity is very low.

In such an environment, getting out of the car generates high static voltage in the body and that is discharged while touching the metal handle.

The mild shock is the result of this ESD. In places where relative humidity is very low, possibility of such mild shocks associated with ESD are very high while touching metal parts like door handle or car door. ESD is detrimental to electronic devices, especially integrated circuits and a good percentage of new generation electronic device failures are due to ESD.

The field of ESD and its effects are well researched and studies are still continuing to understand its impact in many areas including electronics.