Changes in Volatile Organic Compound Composition from an Oxidation-Based Air Cleaner

Air cleaning devices, or "air cleaners", have the potential to improve indoor air by decreasing levels of air pollutants, including volatile organic compounds (VOCs), in indoor environments. Many commercial air cleaners aimed at removing VOCs adopt chemically active technologies, such as oxidation-based chemistry, in addition to (or instead of) physical removal. However, these technologies risk forming unwanted oxidation byproducts that may cause adverse health effects, which can offset (or even outweigh) the benefits of decreasing the number of VOCs. Studies characterizing byproduct formation are generally limited; most such studies were restricted to a single or a few model VOC species as challenge compounds. The composition of indoor air, however, can be highly complex, containing a variety of VOC classes that may not be well represented by a few model species. Here, we present a case study in which we challenge an oxidation-based air cleaner (which uses photoelectrochemical oxidation) with a real-world VOC mixture emitted from spraying a commercial air freshener. This mixture contains a complex suite of organic compounds commonly found in indoor environments, including organic solvents (most importantly ethanol), fragrance agents, and other hydrocarbons and oxygenates of various molecular sizes. Experiments were conducted in a controlled environmental chamber with a suite of real-time analytical instruments to measure the identity and concentration of a wide range of VOCs. We find that the VOC composition changes drastically within a few hours due to running the air cleaner, characterized by the decrease in ethanol and large species (those with 4 or more carbon atoms) and the formation of C₁-C₃ oxygenated byproducts; no large oxidation products are observed. A substantial fraction of ethanol (and possibly other VOCs) is converted to acetaldehyde and formaldehyde, whose levels were observed to increase over the course of several hours during the operation of the air cleaner. Our results suggest the importance of ethanol, a ubiquitous VOC in indoor air, in evaluating the benefits and risks of indoor air cleaners, as ethanol can be efficiently oxidized to byproducts known to negatively impact human health.