The future of passive techniques for air change rate measurement

Ventilation is critical in interpreting indoor air quality (IAQ), yet few IAQ assessments report ventilation rates; even when they do, the measurement method is often not fully described. Most ventilation assessments use a tracer gas test (TGT) to measure total air change rate. In a TGT, the indoor air is marked with an easily identifiable gas (tracer) so that the air exchange rate can be inferred by monitoring the tracer’s injection rate and concentration. Passive sampling (adsorptive/absorptive samplers) is mostly preferred to monitor tracer concentration for its simplicity, practicality and affordability. Such samplers are commercialized by a range of companies and are widely used in IAQ studies to assess pollutants levels. Currently used passive TGTs present some limitations: inadequate tracer gas, disconnection from IAQ analysis (providing ventilation rates in a different time-scale than the pollutant concentrations), lack of verifiability/reproducibility. Thus, this paper proposes a new approach on the passive TGT method, using as tracer a suitable gas (non-reactive, insensitive, unique, measurable and safe) which can be co-captured and co-analysed using commercial passive samplers employed in IAQ studies. A literature review was carried out in pursuit of such a gas. Considering that the most relevant compounds in IAQ studies are volatile organic compounds (VOCs), which are sampled separately from inorganic pollutants, the gases considered as possible tracers were the VOCs capable of being captured by the samplers commercialized by Radiello®, 3M and Gradko. They are composed by activated charcoal, which captures all VOCs in the targeted molar mass range by adsorption. The info-sheets for these samplers were consulted. The option currently under consideration is the solvent 2-butoxyethyl acetate (EGBEA), a low-reactivity glycol ether mentioned by Radiello® in their VOC CS2-desorption sampler info-sheet. Although EGBEA is present in various household products, several field studies show that its background indoor concentration is usually very low or negligible. Regarding human health, EGBEA has generally low toxicity and has not been linked to any chronic effects. A preliminary field test was carried out in order to check EGBEA’s measurability: Radiello® samplers were used to measure its concentration in one room before and after the placement of a beaker containing the solvent. Results showed insignificant background EGBEA concentration (EGBEA mass desorbed from the sampler placed before the beaker placement was lower than from a blank sampler). The relatively low volatility of EGBA (0.23 g evaporated in 4 days) did not hinder its measurability by the Radiello® sampler, which measured a 4-days average EGBEA concentration of 14.1 µg m-3. Further test chamber and field tests will be performed in order to determine the sampler’s accuracy in measuring known EGBEA concentrations and the actual applicability of this substance as tracer in TGTs.