In addition, the water resistance of catalysts is another important requirement for removing formaldehyde, due to the relative humidity (RH) of 50 to 70% in indoor environments. If the catalyst is not resistant to water, its activity suffers. According to Pei et al. the RE of formaldehyde on PtRu/Ce0.7Zr0.3O2 is 72% RE at 0% RH, while the RE is reduced by 7% with the introduction of 5 vol.% water vapor. Similarly, the RE of formaldehyde on Fe2O3/TiO2 drops by 31% at 50% RH. However, in an actual air environment, the humidity is obviously higher than at experimental conditions (5 vol.% water vapor), indicating that the RE of formaldehyde on those catalysts may be inferior. Therefore, the ability to achieve efficient catalytic oxidation at room temperature with robust water resistance is key to the removal of formaldehyde.