This study discusses the reasons for the inconsistency between the glass and the acrylic desiccator methods which have been used in Japan to classify building materials for the emission rate of formaldehyde. Evidences from experiments showed that the theoretical equations used in the glass desiccator method were not applicable to the acrylic desiccator method. Three-dimensional models for the two desiccators were built for numerical calculations to analyze emission, diffusion and absorption associated with the desiccator methods. The results showed that the time to reach the steady state in the acrylic desiccator method is much long which deviates from the conditions of the equations to apply. Smaller exposed area of the testing materials and larger volume of the desiccator in the acrylic desiccator method are included as the major factors for the delay in the formation of steady state.
