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Humidity can have an appreciable influence on the photocatalytic degradation of volatile organic compounds (VOCs) in air. It has been proposed that there are multiple layers of water molecules on the surface of the photocatalyst, and a VOC molecule must penetrate this layer and be able to diffuse to the photocatalytic surface for a reaction to occur. An updated analysis of related literature from the past 12 years indicates that this proposed mechanism remains reasonable for explaining observed effects. According to this mechanism the VOC's hydrophilicity should dominate the influence of humidity on the photocatalytic reaction kinetics, and this effect can be seen in a categorization of literature observations based on the octanol‐water partition coefficient. The frequently cited Langmuir‐Hinshelwood kinetic model does not directly account for humidity effects in this way but can be adapted for this purpose. As an example, the rate of photocatalytic decomposition of chlorobenzene in air under humid conditions can be described by a modified Langmuir‐Hinshelwood kinetic model, where the reaction rate coefficient is dependent on the water concentration. Understanding and considering the effect of humidity is useful for optimizing the efficacy of photocatalytic VOC treatment of air.

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