{"version":1,"type":"rich","provider_name":"Libsyn","provider_url":"https:\/\/www.libsyn.com","height":90,"width":600,"title":"EP246 Beyond MERV: The Truth About Smoke, Sensors, and Standards With Sissi Liu (October 2025)","description":"Episode quotes: \u201cBelow about 0.4 microns, many low-cost PM sensors are basically guessing\u2014right where wildfire smoke and aerosols live.\u201d \u2014 Sissi Liu \u201cElectrostatic filters can look great at first\u2014and then fall off a cliff in smoke. Pressure drop won\u2019t warn you.\u201d \u2014 Sissi Liu \u201cScience is a way of thinking much more than it is a body of knowledge.\u201d \u2014 Carl Sagan   Eric digs into the \u201cfresh air\u201d myth with Sissi Liu, CEO\/co-founder of Metalmark Innovations and active ASHRAE committee member. Sissi explains why \u201coutdoor = fresh\u201d is context-dependent\u2014urban pollution, agricultural activity, and especially wildfire smoke can make outdoor air worse than indoor air. Because air quality is dynamic, she pushes for comparing indoor and outdoor conditions in real time and ventilating intelligently, with attention to the energy cost of conditioning outside air. They then get nerdy on sensors and filters. Many low-cost PM2.5 laser-scattering sensors struggle below ~0.4 \u00b5m and can misread certain particle types (e.g., dark\/black carbon), which matters because smoke and pathogen-carrying aerosols often live in the ~0.1\u20130.3 \u00b5m range. On filtration, Sissi contrasts mechanical vs. electrostatically charged media: electrostatic filters start efficiently with low pressure drop but can lose effectiveness within hours in smoke events. In contrast, mechanical media hold up better (though at higher pressure). She highlights ASHRAE 52.2 Appendix J (loaded efficiency) and argues that standards\u2014along with reporting practices\u2014must evolve for wildfire realities. Key takeaways \u201cFresh air\u201d is conditional: check outdoor AQ (and indoor) before cranking up ventilation. IAQ is dynamic; test and compare locally rather than assuming static conditions. Consumer PM sensors can under-count the tiniest and darkest particles; treat data with caveats. Wildfire smoke clusters in the most-penetrating particle size (~0.1\u20130.3 \u00b5m) for many filters. Electrostatic filters may degrade fast in smoke; pressure drop alone won\u2019t reveal failure. ASHRAE standards (e.g., 52.2 Appendix J, SGPC-44) are evolving\u2014industry needs to catch up. Sissi\u2019s LinkedIn: https:\/\/www.linkedin.com\/in\/liusissi\/ Metalmark website: https:\/\/metalmark.xyz\/     This episode was recorded in October 2025.   ","author_name":"Building HVAC Science","author_url":"https:\/\/www.buildinghvacscience.com","html":"