Rationale

Revision to incorporate P&B data/report and to address member concerns as follows: 1. Low-end performance. The standard only has two test points (10 and 15 µg/m3) in the range where most indoor environments actually operate. Typical well-maintained buildings sit at 3 to 12 µg/m3, WHO annual guideline is 5 µg/m3, and the bulk of the test concentrations are at 50 to 300 µg/m3. That's useful for cooking events or wildfire infiltration, but it doesn't tell us much about whether a sensor can reliably distinguish 5 from 10 from 15, which is where the actual decisions get made. The Phase 2 climate testing is also only done at 10 and 50, so we don't know how humidity affects accuracy right at those decision thresholds. Agreed. The band should be wider, though achieving strong resolution at the lower end may not be within the current capabilities of low-cost sensors. 2. Ultrafines. D8405 acknowledges that optical sensors generally can't detect particles below 0.3 µm, and then basically moves on. Given what we're learning about sub-100 nm particles and health outcomes (deposition efficiency, translocation, cardiovascular, multisystemic effects), this feels like a bit of a blind spot if we're trying to build something forward-looking. Dr. Suresh Dhaniyala from Clarkson University is working on the most interesting and promising sensor innovation to achieve this, that I’m aware of. I would recommend a call with him to learn more. We asked him to educate us on current, conventional low-cost PM sensor size capture capabilities and advances in this area, and what he shared was very informative. He shared this paper of possible interest in this regard: https://www.tandfonline.com/doi/full/10.1080/02786826.2026.2649803 and also indicated his interest in supporting any studies with the low-cost (TelosAir) sensor his lab has developed (or any other sensor-related studies or write-ups; we plan to consult him for an ongoing IAQ metrics review report). 3. Mass vs. number. The standard is entirely mass-based, which makes sense given the mostly outdoor regulatory PM2.5 framework it grew out of. But indoors, sources like cooking, 3D printing, cleaning products, and candles throw off huge numbers of very small particles that barely register as mass. A single 2.5 µm particle has roughly the same mass as millions of 25 nm particles, and those are not equivalent from a health standpoint. Particle number concentration may be a more meaningful metric for indoor air, and there are increasingly affordable sensors that can report it - should we think about this? Not equivalent and definitely worth thinking about – and many are thinking about it, as we know – but the challenge is that the bulk of the health evidence base is weighted toward mass-based metrics. We plan to discuss this dilemma in the metrics review I referred to (and to work with partners to push on the health evidence base so we can collectively overcome this chicken-and-egg problem).