Volatile Methyl Siloxanes and Other Organosilicon Compounds in Residential Air

Abstract

Volatile methyl siloxanes (VMS) are ubiquitous in indoor environments due to their use in personal care products. This paper builds on previous work identifying sources of VMS by synthesizing time-resolved proton-transfer reaction time-of-flight mass spectrometer VMS concentration measurements from four multiweek indoor air campaigns to elucidate emission sources and removal processes. Temporal patterns of VMS emissions display both continuous and episodic behavior, with the relative importance varying among species. We find that the cyclic siloxane D5 is consistently the most abundant VMS species, mainly attributable to personal care product use. Two other cyclic siloxanes, D3 and D4, are emitted from oven and personal care product use, with continuous sources also apparent. Two linear siloxanes, L4 and L5, are also emitted from personal care product use, with apparent additional continuous sources. We report measurements for three other organosilicon compounds found in personal care products. The primary air removal pathway of the species examined in this paper is ventilation to the outdoors, which has implications for atmospheric chemistry. The net removal rate is slower for linear siloxanes, which persist for days indoors after episodic release events. This work highlights the diversity in sources of organosilicon species and their persistence indoors.

Keywords: cyclic volatile methyl siloxane; emissions; indoor air; linear volatile methyl siloxane; source attribution.

Figure 1

D5 siloxane diurnal plot in the living space during occupied period of the H2W campaign. Top and bottom edges of the blue boxes represent the 75th and 25th percentiles, respectively, and the red lines inside the boxes represent the medians. The black whiskers extend distances of 1.5 times the interquartile range from the box edges. The red symbols denote positive outliers.

Figure 2

Indoor and outdoor cyclic VMS concentration time series measured during a portion of the H2W campaign for (a) D3 (previously reported in Lunderberg et al.), (b) D4, (c) D5, and (d) D6. The shaded region, which represents the vacant period, exhibits no episodic concentration spikes.

Figure 3

Indoor and outdoor concentration time series at H2 for five species: (a) L4, (b) L5, (c) silyl acetate (SA), (d) caprylyl methicone (CM), and (e) C₇H₂₀O₃Si₃ (C7). The shaded region represents the vacant period.

Figure 4

Mean-to-median ratios of ethanol (EtOH), acetic acid (AA), and the nine siloxane species in the H1S, H1W, and H2W campaigns. MMR for H1S and H1W can be found in Liu et al. Colors differentiate species, with white indicating nonorganosilicon species, blue indicating cVMS, orange indicating organosilicon species, and green indicating linear VMS.