Analysing nighttime ventilation rates in bedrooms of Amsterdam residences

Adequate bedroom ventilation is an important determinant of nighttime indoor air quality, yet ventilation performance in the existing housing stock remains difficult to quantify in practice. In this observational study, we analyze three years (2023–2025) of continuous bedroom CO₂ measurements from 93 Amsterdam residences (Peerlings et al., 2024) to assess possible nighttime under-ventilation in bedrooms.

Because humans exhale CO₂ continuously, and indoor CO₂ is primarily removed by ventilation to the outdoors, CO₂ is often treated as a naturally occurring “tracer of convenience” for evaluating ventilation. We evaluate recorded CO₂ concentrations, in parts per million (ppm), against a small set of literature-based ventilation benchmarks. These benchmarks are usually stated as outdoor-air supply rates for a room or occupant. Here, we interpret them as equivalent local purging-flow thresholds (L/s), i.e. the effective flow rate at which contaminants at the sensor location are removed to the outdoors rather than recirculating back. Under plausible home-specific nighttime CO₂ generation rates (L CO₂/s), these flow rates correspond to excess CO₂ thresholds. We postulate that parts of the night may exhibit approximately constant flow conditions and source strengths. Such periods may arise under stable door and window states, slowly varying outdoor conditions, and a fixed number of sleeping occupants. We then distinguish between short-lived exceedance bursts and sustained CO₂ elevation, and report exceedance only in the latter case.

In larger households, CO₂ emitted in other rooms may contribute to the recorded bedroom concentration, and the strength of that contribution can vary from night to night as indoor transport conditions change. If source attribution cannot be resolved, ventilation cannot be uniquely inferred from single-sensor CO₂ data. We therefore restrict the main analysis to homes with household size ≤ 2 people, excluding larger households where we are unsure about how much of the recorded CO₂ originated from occupants in other rooms. Using this approach, we quantify the fraction of nights showing sustained exceedance of thresholds corresponding to a range of literature-based minimum ventilation rates.

Exceedances of these thresholds indicate possible periods of insufficient local pollutant removal capacity and provide a basis for assessing nighttime bedroom ventilation performance across homes and seasons in Amsterdam. Preliminary analyses suggest that some homes exhibit recurring sustained exceedance of lower benchmark values. Finally, we demonstrate how uncertainty in source strength, outdoor reference concentrations, and sensor baselines affects the robustness of this classification, and highlight the limits of interpretating single-sensor CO₂ data as direct night-specific ventilation estimates.