We have spent decades obsessing over PM2.5 —the fine particulate matter small enough to lodge itself in our lungs. We draft laws around it, track it on our phones, and buy home monitors to avoid it. But while our regulatory gaze has been fixed on particle mass, an invisible, almost weightless army of much smaller particles has been slipping through the cracks: Ultrafine Particles (UFPs).
Known technically as PNC0.1, these aerosols have an aerodynamic diameter of 100 nanometers or less. To put that in perspective, they represent a negligible fraction of total ambient mass, yet they often make up over 90% of the total particle number concentration (PNC) in our atmosphere. Because they are so unbelievably small, they don’t just settle in the lungs—they have the unique physical ability to cross into the bloodstream, posing distinct and severe health risks.
Yet, when we step inside our homes, we enter a measurement blind spot. In our current study across two Spanish cities—Granada and Almeria—we exposed a frustrating reality: meeting current PM2.5 standards provides a completely false sense of security when it comes to UFP exposure, and our current measurement tools and guidelines make it nearly impossible to tell if the air we breathe inside is actually safe.
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