The photochemical thermodynamic efficiency factor (PTEF) and the quantum yield (QY) are key parameters for assessing photocatalytic degradation efficiency and photocatalytic reaction pathways in air treatment. QYs and PTEFs can be established in a Photo-CREC-Air reactor unit on the basis of the number of OH center dot consumed, the enthalpy of OH center dot formation and the radiation reaching the photocatalyst. While the PTEF has been considered for photocatalytic conversion of organic pollutants in water, this is the first attempt where a PTEF is considered in air treatment units. Efficiency calculations are performed in the present study, using the data for acetone and acetaldehyde photocatalytic degradation in air with low concentration of water. It is shown that quantum yields both for acetone and acetaldehyde supersede the value of 1 (equivalent to 100%) while PTEFs remain in both cases below the level of 1 as required by thermodynamic constrains. On this basis, a chain reaction mechanism can be supported for photocatalytic conversion of organic pollutants in air. (C) 2010 Elsevier B.V. All rights reserved.