The high efficiency particulate air filter (HEPA) in exhaust system of high-level biosafety laboratory must be disinfected to kill harmful microorganisms before being replaced or tested. In-situ fumigation by vaporized hydrogen peroxide (VHP) is the most commonly used disinfection method for HEPA unit. The disinfection effect of HEPA unit is affected by the internal environment

but the influence of initial internal environment on VHP penetration during the fumigation process is still rarely studied. In this paper

a numerical model was established based on a physical HEPA box unit and field measurement. According to the simulated VHP flow field during in-situ fumigation process

the influence of initial internal temperature and relative humidity on the VHP penetration effect in HEPA was discussed with the indicators of penetration rate and velocity distribution. The results showed that the VHP concentration in the upstream section of HEPA was relatively uniform

while the maximum and minimum concentrations in the downstream section showed great difference and located at the top and bottom

respectively. The distributions of VHP penetration and upstream velocity were both vertically uneven. The VHP penetration rates in the middle of HEPA remained around 73 %. The initial temperature of 32 °C or 36 °C

or the initial relative humidity of 18 % were the suitable initial internal environment for HEPA in-situ fumigation

which could keep the VHP penetration rates of the upper and lower parts of HEPA at about 76 % and 61 %

respectively. In the suitable initial environment

the penetration and disinfection effect could be promoted by appropriately extending the rapid injection duration. This study provides a reference for optimizing the initial environment setting of HEPA in-situ fumigation.