Footwear microclimate and its ventilation control

Abstract

The importance of the footwear microclimate has been recognized from the perspectives of physiology and hygiene. An adverse (hot and moist) thermal microenvironment inside footwear results in various problems, including local thermal discomfort, increased probability of skin blistering, overloading of the vascular system of the foot, increased sleepiness (and decreased vigilance), and various hygiene problems caused by bacterial and fungal growth. However, few studies have investigated the characteristics of the footwear microclimate and possible strategies for controlling the microclimate. This work first focused on the investigation of the footwear microclimate, including the in-shoe air temperature and humidity and the ventilation rate. New reliable and portable systems were developed for measurement purposes. The spatial and temporal characteristics of the footwear microclimate were explored for three types of footwear (i.e., casual, running, and perforated shoes). Additionally, a chamber method was developed for evaluating the ventilation rate inside the footwear, whereby water vapor was used as the tracer gas. The ventilation rate increased with the gait speed (4.63–6.20 and 6.45–9.41 L/min at a gait speed of 3 and 6 km/h respectively), and a faster gait resulted in a larger discrepancy (perforated shoes > running shoes > casual shoes) in the ventilation rate among footwear types. We found that bacterial growth on the distal plantar skin had positive linear correlations with the in-shoe temperature and absolute humidity and a negative linear correlation with the ventilation rate, indicating that the increased temperature and humidity inside footwear due to the decreased ventilation rate supported bacterial growth on the foot plantar. The ventilation rate plays a dominant role in microenvironmental control. We thus explored the feasibility of using forced ventilation for footwear microenvironmental control by developing a new mathematical model that considers the forced ventilation of the footwear cavity. Additionally, the model was combined with a thermoregulation model to determine changes in the foot skin temperature in response to changes in the ventilation rate. This integrated model can therefore determine the forced ventilation rate required of a given type of footwear to ensure foot thermal comfort. At an air temperature of 26.4 °C and a foot thermal comfort temperature of 32.2 °C, the required minimum forced ventilation rate was 5.4 to 24.6 L/min, corresponding to static thermal insulation of the footwear of 0.10 to 0.20 m^2∙K∙W^(-1). The maximum reduction in the foot skin temperature ranged from 2.5 to 3.5 °C when the ventilation rate was within 90 L/min at an air temperature of 29 °C. As the ambient air temperature further increased, the reduction in skin temperature due to forced ventilation became negligible, suggesting that other cooling methods will be necessary at higher temperatures. Finally, a portable micro-fan device was designed to provide long-term forced ventilation for a footwear cavity. The device was shown to be effective in reducing the foot skin temperature and inhibiting the growth of bacteria on the distal plantar skin. The forced ventilation has therefore been proved to be an efficient method for the control of footwear thermal microenvironment.