Limited evidence associates inadequate classroom ventilation rates (VRs) with increased illness absence (IA). We investigated relationships between VRs and IA in Californiaelementary schools over two school years in 162 3rd-5th grade classrooms in 28 schools in three school districts: South Coast (SC), Bay Area (BA), and Central Valley (CV). We estimated relationships between daily IA and VR (estimated from real-time carbon dioxide) in zero-inflated negative binomial models. We also compared IA benefits and energy costs of increased VRs. All school districts had median VRs below the 7.1 L/sec-person California standard. For each additional 1 L/sec-person of VR, IA was reduced significantly (p<0.05) in models for combined districts (-1.6%) and for SC (-1.2%), and non-significantly for districts providing less data: BA (-1.5%) and CV (-1.0%). Assuming associations were causal and generalizable, increasing classroom VRs from the California average (4 L/sec-person) to the State standard would decrease IA by 3.4%, increase attendance-linked funding to schools by $33 million annually, and increase costs only $4 million. Further increasing VRs would provide additional benefits. These findings, while requiring confirmation, suggest that increasing classroom VRs above the State
standard would substantially decrease illness absence and produce economic benefits.