Simulation of temperature and humidity in mattrasses to evaluate risks on house dust mite allergy

Faeces of house dust mites constitute a major source of allergens. In modern, thermally well-insulated houses mattresses and carpets in bedrooms are the most important habitats of house dust mites. There, the human skin provides nutrition, water vapour and heat to create conditions that are conducive to mite growth. The temperature and the time span during which the relative humidity exceeds the critical equilibrium humidity (CEH) mainly determine growth of house dust mites. This paper presents a simulation model that describes transient and spatial distributions of temperature and humidity levels in a mattress influenced by the human body and ambient conditions. The aim of this model is to formulate strategies to reduce risks on house dust mite allergy from mattresses and carpets. Preliminary results of the computations showed that the transmission of water vapour occurred much faster than transmission of heat. This was due to the fact that the diffusion coefficient of water vapour is larger that the thermal diffusivity (on average: Dvap = 6x10-6 and a = 7x10-8 m2s-1). As soon as the human body occupied the mattress, the temperature of the surface of the mattress increased up to 30 to 36 °C. At some depth in the mattress the absolute humidity in the air containing voids increased faster than the temperature. This resulted in an (temporary) increase of relative humidity, often to levels above the CEH.