Indoor Static Electricity, Semiconductors, And Humans
The factors behind (and the use of) this phenomenon may come as a small shock.
Engineers are well informed about the need for appropriate HVAC systems in rooms that contain IT servers for mission critical facilities. The consequences of interruption of information transfer in these settings can be immediate, expensive, and life threatening. To help prevent circuitboard malfunction and failure, the surrounding indoor temperatures and humidity must be maintained in ranges that dissipate heat and prevent damaging static electricity discharges.
What is static electricity (SE) and how is it associated with indoor conditions? SE is a form of potential energy, which is different from the kinetic energy of flowing electrical currents. Potential energy increases when electrons and their associated negative charge accumulates on certain materials, and are “eager” to jump to a more positively charged material to return to a lower energy state. The “jump” is the little shock we experience when, after walking on a carpet in winter, we pet our friendly dog, or touch a metal doorknob. A lightning bolt is the same phenomenon at a much larger scale.
SE is greater when the air is dry, such as in a heated building in winter, because dry air acts like an insulator that keeps the positive and negative charges on their respective surfaces until the potential energy builds up to a point where the electrons jump across, literally, with a spark. This is less likely to happen when the indoor air contains water vapor (rh 40-60) because the slightly conductive dipole moment of H2O spreads out and neutralizes opposite charges.
To prevent SE discharges from damaging delicate semiconductors in computers, HVAC systems in critical facilities strive to maintain the proper indoor climate at all costs. Unfortunately, these mechanical systems can consume large amounts of energy, clearly an expensive and environmentally unfriendly situation.
To address this energy problem, IT manufacturers have recently developed semiconductor materials that can handle indoor air with higher temperatures and lower humidity so that the allowable IAQ range is broader. Following suit, ASHRAE rewrote their data center standards to allow lower rh and more flexible HVAC designs in an effort to conserve energy.
While semiconductor materials have evolved to be less sensitive to SE, we must ask: Have humans also grown immune to the effects of SE? Research on this topic has shown (so far) that tiny shocks from SE do not damage our internal organs or our nervous system. There is, however, more to this story!
First, studies with human volunteers have shown that the deposition and retention of inhaled aerosols is increased when the individual has higher electrostatic charges. Secondly, there are more particles available for inhalation in the airborne environment when ambient SE charges are higher.
Forces acting on the movement of particles on surfaces
The reason for this is as follows: there are four major resuspension forces on particles with diameters between 0.1~10.0 µm, the size range that can reach the depths of our lungs and then possibly our blood circulation. These are aerodynamic drag, aerodynamic lift, mechanical vibration, and electrostatic forces. Gravity and surface adhesion are the major forces preventing particles leaving the surface. For micron-sized particles, the resuspension forces are comparable to or larger than gravitational force. With even a tiny electrostatic field strength of 1 kv/cm, the electrostatic lifting force can be more than 10 times greater than the gravitational force, allowing for energetic particle resuspension.
Interestingly, the pharmaceutical industry takes advantage of this phenomenon by increasing the electrostatic charge around inhaled medications so that the dose delivered into our body is higher. While that approach might be great for drug delivery, what does this mean for our health when infectious microbes or damaging particles are swirling around us in dry, electrostatically charged air? Since humans cannot be engineered like semiconductors, we have to manage indoor air properly to prevent catastrophic human breakdown.