Methanol and ethanol in indoor environments

Although rarely the subject of focused research, methanol (CH₃OH) and ethanol (C₂H₅OH) are among the most abundant organic compounds in indoor air. We review the state of knowledge about these alcohols as constituents of indoor air, first summarizing their physical, chemical, biological, and toxicological characteristics. Central tendencies of concentrations measured in ordinary indoor environments are 35 ppb for methanol (median; mean = 34 ppb) and 44 ppb for ethanol (median; mean = 163 ppb), much higher than in outdoor air. Concentration variability can be large both among indoor environments and over time within a given environment. Indoor ethanol concentrations above 1 ppm have been reported. Emissions from occupants contribute substantially to indoor concentrations. Other important indoor sources of methanol include wooden building materials and furnishings. Methanol emissions indoors exhibit substantial increases with increasing temperature. Indoor ethanol concentrations are strongly influenced by episodic emission events, including cooking, cleaning activities, and alcoholic beverage consumption. Homogeneous oxidation pathways appear slow as a removal mechanism relative to ventilation. Evidence regarding the importance of sorption to indoor surfaces is not fully resolved, with known sorptive reservoirs indicating little importance of this process, but a variable-ventilation experiment suggesting substantial reversible sorption for ethanol. Photocatalytic oxidation devices, intended to control indoor levels of volatile organic compounds, have a demonstrated tendency to convert ethanol to acetaldehyde, a more toxic indoor air contaminant. Ethanol transported outdoors from indoor sources may contribute to urban and regional photochemical smog.