Beyond Ventilation: The Non-linear Impact of Per Capita Spatial Volume on Nosocomial Respiratory Virus Transmission in Hospital Wards — A Real-World Natural Experiment

Technical Summary: Beyond Ventilation – The Non-linear Impact of Per Capita Spatial Volume on Nosocomial Respiratory Virus Transmission

Problem Statement
While mechanical ventilation (specifically Air Change Rate, or ACH) is the primary engineering control for preventing nosocomial respiratory transmission, current guidelines often overlook the absolute volume of air available per occupant. Theoretical frameworks, such as the Wells–Riley model, predict that infectious particle concentration is inversely proportional to room volume, independent of ventilation rates. However, this relationship has not been empirically validated in real-world clinical settings. Previous observational studies have largely relied on binary comparisons (e.g., single vs. multi-bed rooms) or computational fluid dynamics (CFD) simulations, failing to quantify per capita spatial volume as a continuous epidemiological exposure linked to actual transmission outcomes. Consequently, hospital design standards lack empirically derived volumetric thresholds for infection prevention.

Methodology
The authors conducted a retrospective natural experiment at Dankook University Hospital (a 900-bed tertiary center in South Korea) during the January 2022 Omicron (B.1.1.529) surge. The study compared two general medical wards (Building 1 and Building 2) that were structurally identical in every aspect except for floor area per bed:

• Controlled Variables: Both wards featured identical 4-bed room configurations, ceiling heights (2.65 m), centralized mechanical ventilation systems, sealed windows, and standardized infection control protocols. A hospital-wide visitor ban was enforced to minimize external viral introduction.
• Independent Variable: The primary differentiator was the per capita spatial volume. Building 1 offered 72.13 m³/person, while Building 2 offered 34.26 m³/person (a 52.5% reduction).
• Outcome Measures: The index outcome was nosocomial SARS-CoV-2 transmission, defined as a positive test ≥48 hours after admission. The study analyzed two metrics:
  1. Incidence Density: Cases per 1,000 patient-days.
  2. Volumetric Infection Rate: A novel metric calculating cases per cubic meter of total ward air volume (cases/m³).
• Statistical Analysis: Fisher's exact test and Poisson regression were used to compare outcomes, adjusting for patient-days.

Key Results
The study observed a stark divergence in transmission rates between the two wards despite identical ventilation and infection control measures:

• Building 1 (72.13 m³/person): Recorded 2 nosocomial infections (Attack rate: 0.87%; Incidence density: 1.51/1,000 patient-days; Volumetric rate: 0.028 cases/m³).
• Building 2 (34.26 m³/person): Recorded 16 nosocomial infections (Attack rate: 6.45%; Incidence density: 9.49/1,000 patient-days; Volumetric rate: 0.467 cases/m³).
• Statistical Significance: The difference in incidence density was statistically significant (P = 0.001). Poisson regression yielded an Incidence Rate Ratio (IRR) of 6.29 (95% CI 1.45–27.34; P = 0.014), indicating a 6.3-fold higher incidence in the lower-volume ward.
• Volumetric Disparity: The volumetric infection rate in Building 2 was 16.7 times higher than in Building 1.

Key Contributions

1. Empirical Validation: This is the first observational study to directly link per capita spatial volume to nosocomial respiratory virus transmission using clinical measurement rather than theoretical modeling or CFD simulations.
2. Novel Metric: The introduction of the "volumetric infection rate" (cases/m³) provides a tool for normalizing infection burden to the spatial environment, allowing for cross-institutional comparisons independent of ward size.
3. Design Threshold Proposal: Based on the data, the authors propose a candidate design threshold of approximately 70 m³/person for general hospital wards.
4. Decoupling Ventilation and Volume: The study demonstrates that equivalent mechanical ventilation (ACH) does not guarantee equivalent safety if the per capita air volume is insufficient, highlighting volume as an independent and critical risk factor.

Significance and Claims
The paper claims to shift the evidence base for spatial infection risk from theoretical modeling to clinical epidemiology. The authors assert that the physical mechanism of particle dilution as a function of room volume applies broadly to all droplet- and aerosol-transmitted pathogens, not just SARS-CoV-2.

The study concludes that per capita spatial volume warrants "co-equal status" with air change rate in infection prevention guidelines and hospital design standards. The authors suggest that increasing spatial volume is a fundamental, passive defense against respiratory particle accumulation. However, the paper maintains a modest tone regarding its limitations, acknowledging the single-center retrospective design, the small number of outcome events, and the restriction to a single pathogen and surge period. The authors explicitly call for prospective, multicenter validation of the ~70 m³/person threshold across diverse pathogens and healthcare settings before it is adopted as a universal standard.