Hot steam from a hose spray and dew or rains directly threatens firefighter’s health and lives. The thickness of air gap between the protective clothing and the skin has an important and complicated influence on thermal protection. The purpose of this study was to examine the effect of air gap thickness on thermal protection in dry heat exposure (thermal radiation) and wet heat exposure (hot steam and thermal radiation). The results showed that the air gap between the clothing and the skin, while providing better thermal protection for dry heat exposure, also slowed down the steam transfer rate to provide heat protection. There was no critical air gap in dry and wet heat exposures. It was also found that hot steam increased sharply the heat flux absorbed by the skin, leading to the occurrence of steam burns. The increase of air gap thickness could reduce the heat flux during the exposure while no significant correlation between the air gap thickness and the final heat flux was observed during the cooling of wet heat exposure (P>0.05).