Decades of successful air quality policies have significantly reduced fine particulate matter (PM_2.5) concentrations, a major public health achievement. This success, however, presents an atmospheric paradox. The reduction in PM_2.5 mass has lowered the atmospheric condensation sink, which normally scavenges the molecular clusters that initiate new particle formation (NPF). This creates more favourable conditions for the formation of smaller, potentially more hazardous, ultrafine particles (UFPs). Furthermore, technologies central to net-zero strategies, such as amine-based carbon capture, risk creating new, concentrated sources of potent NPF precursors. This confluence of factors exposes a critical blind spot in air quality management. Current regulations and industrial risk assessments are almost exclusively mass-based, overlooking particle number concentrations and the associated health risks of UFPs. This article argues that the pursuit of climate goals must not create unforeseen public health burdens. It calls for the strategic integration of particle number and size distribution measurements into existing air quality networks to build the evidence base needed to validate models, inform future policy, and ensure that climate solutions do not inadvertently establish a new generation of localised air pollution problems.