Vibration, temperature, and humidity influence the perception of electrocutaneous stimulation for occupational warning

Electrocutaneous stimulation can be used to warn workers in hazardous situations. To determine parameters for the operating ranges of such warning systems we investigated the influences of vibrations, temperature, and humidity on the perception, attention, muscle twitch, and intolerance thresholds of healthy volunteers in a multi-center study. In a study on 94 participants, vibrations with amplitudes of 2 mm, 5 mm, and 8 mm and at frequencies of 8 Hz and 9.5 Hz were applied to the right arm. In a second study, 52 participants experienced temperature and humidity variations within a climate chamber in four conditions: dry cold (13.8 ± 1.6 [Formula: see text]C, 33.9 ± 9.2% relative humidity (RH)), dry warm (41.3 ± 1.7 [Formula: see text]C, 24.6 ± 5.8% RH), wet cold (12.0 ± 1.6 [Formula: see text]C, 88.5 ± 8.2% RH), and wet warm (41.3 ± 2.1 [Formula: see text]C, 65.5 ± 9.0% RH). In both studies, the upper right arm was electrically stimulated with electrodes of size 25 mm×40 mm, and thresholds were determined. Perception, attention, and intolerance thresholds increased with vibration amplitude and frequency from median values of 5 mA, 9.9 mA and 19.3 mA for perception, attention and intolerance thresholds during rest to 14.6 mA, 20 mA and 21 mA during vibration amplitude of 8 mm and frequency of 9.5 Hz. Perception thresholds slightly increased with decreasing temperature with median values of 4.1 mA during dry cold vs. 3.8 mA during dry warm condition and 4.0 mA during wet cold vs. 3.8 mA during wet warm condition for an electrode pair at a lateral position. Muscle twitch thresholds slightly increased with increasing temperature at the lateral electrode pair with median values of 17.4 mA during dry cold vs. 18.8 mA during dry warm condition and 17.5 mA during wet cold vs. 18.7 mA during wet warm condition. For both studies, women showed smaller perception and attention thresholds. For illustration, in study 1, the median values of the perception threshold during rest were 4.1 mA for women and 5.7 mA for men, whereas the median values for the attention threshold were 9.1 mA for women and 11.4 mA for men, respectively. Additionally, women experienced less muscle twitching. To exemplify this, 50% of the women experienced muscle twitches compared to 66% of the men at a lateral electrode pair in study 2. We conclude that the operating range of future electrical warning systems needs to be flexibly adjusted to the working conditions. The influence of the climate conditions was minor, suggesting that electrocutaneous warning systems can operate robustly across diverse environments. This supports their practical applicability and motivates future research on optimized dry or textile-based electrodes to enhance usability in real-world settings.