A field study of 109 energy-vulnerable older-adult households in Seo-gu, Daejeon, monitored indoor carbon dioxide and particulate levels at ten-minute intervals from 6 August to 11 September 2025, and found that although typical nighttime CO₂ concentrations were low, a substantial minority of dwellings accumulated CO₂ night after night. When researchers tested whether a simple question about ventilation satisfaction could identify those problem homes, it could not: a dissatisfied response missed most of them, and the more satisfied households tended, counterintuitively, to be the more exposed ones.
The monitored homes were recruited through a district energy-vulnerability survey; of the 103 households with recorded income data, most received the Basic Old-Age Pension, and most dwellings were owner-occupied.
The headline numbers look reassuring at first glance. The median household nighttime CO₂ concentration, measured across the 23:00 to 06:59 window, was just 513 ppm, and median PM₂.₅ was 3 µg m⁻³. But central tendency concealed the upper tail: across all nighttime readings, CO₂ exceeded 1000 ppm in 23.2% of cases and 1500 ppm in 14.1%. Exceedance was concentrated in a minority of dwellings. The per-household median nighttime exceedance fraction was 0%, yet 40 of the 106 dwellings with valid nighttime records (38%) exceeded 1000 ppm on at least 10% of readings, 34 (32%) on at least a quarter, and 21 (20%) on at least half. Fifty-nine dwellings, 56%, had no exceedances at all. Retaining the 2,534 readings pinned at the 5000 ppm sensor ceiling, which were concentrated in just 13 households, raised the exceedance fraction only modestly, from 23.2% to 25.6%, leaving the conclusions unchanged.
Valid decay segments were obtained in 60 of the 109 households, from 202 segments with a median fit R² of 0.965. With outdoor CO₂ assumed at a representative 420 ppm, the estimated lower-rate air change, dominated by infiltration rather than window opening, had a median of 0.37 h⁻¹. Thirty of the 48 dwellings with a lower-rate estimate, 62%, fell below 0.5 h⁻¹, the rate specified for mechanical ventilation in Korean multi-family housing. The authors stress that this figure serves only as a descriptive point of reference, not a compliance test, since it is a design requirement for mechanical systems rather than a measured clearance rate in occupied dwellings.
Those absolute values depend heavily on the assumed outdoor baseline, which was not measured. When the outdoor concentration was fitted as a free parameter, the resulting baselines were often implausible as outdoor air, with a median of 610 ppm, because partial decays toward still-elevated indoor levels get absorbed into the baseline term. Three fixed-baseline assumptions, 400, 420 and 450 ppm, agreed closely with one another, and the proportion of homes below 0.5 h⁻¹ ranged only from 57% to 63% across all four treatments. The qualitative finding of frequent low clearance rates therefore does not hinge on the baseline choice, even though the absolute rates do. Because occupancy, source activity and window state were not recorded during the selected decay segments, the authors describe the estimates as screening-level indicators of clearance rate rather than verified whole-dwelling ventilation measurements.
The two CO₂-derived quantities converged as expected: estimated air change was inversely associated with nighttime CO₂ exceedance (all decays r = −0.651, p < 0.001, n = 60), though the authors note this is convergence between two quantities from the same CO₂ series rather than an independent validation. With a freely fitted baseline the association weakened and lost significance, consistent with the baseline term absorbing part of the decay structure. The practical implication is that nighttime CO₂ exceedance, cheap to measure with a low-cost sensor, may serve as a first-stage indicator for prioritising dwellings for closer ventilation assessment.
The study’s most striking result concerns perception. Ventilation satisfaction, rated on a five-point scale, was inversely associated with objective adequacy on the concentration metric: higher satisfaction accompanied higher nighttime CO₂ exceedance (Spearman r = +0.284, p = 0.009, n = 83), with mean exceedance rising monotonically across satisfaction categories. Satisfied households did have cooler indoor temperatures, but adjusting for nighttime temperature, household size, or both left the association essentially unchanged. Adjustment for floor area, tested in the 39 of 83 dwellings where it was recorded, was inconclusive because the subsample was underpowered and the recorded areas were of uncertain validity. The association was specific to the exceedance outcome: it was not significant when the outcome was median nighttime CO₂ (r = +0.186, p = 0.092), meaning it is carried by the frequency of elevated readings rather than typical levels. The authors frame it as a trend within this study population, not a demonstrated mechanism, and note that adaptive or normalised perception, possibly reinforced by age-related decline in chemosensory sensitivity, remains a hypothesis.
Treated as a screening tool in a post hoc, policy-oriented evaluation, a dissatisfied ventilation response performed poorly. Of the dwellings whose nighttime CO₂ exceeded 1000 ppm on at least half of readings, 14 of 17, or 82%, reported neutral or positive satisfaction and would not have been flagged by a strict dissatisfied-response criterion. Across three exceedance thresholds, the false-negative rate ranged from 0.80 to 0.83 and sensitivity from 0.17 to 0.20, though confidence intervals were wide given the small number of high-exceedance dwellings. A lenient flag that also counted neutral responses cut the false-negative rate to 0.40–0.48 but at such low specificity, 0.24–0.27, that it flagged most dwellings. No simple satisfaction cut-off separated high- from low-exceedance homes. Crucially, both the thresholds and the cut-offs were chosen after inspecting the same data, so the authors warn that these performance values describe this dataset and should not be read as validated screening characteristics.
The findings fit a broader evidence base. Reviews and measured studies report low residential air-change rates in low-income housing, including 0.10–1.98 ACH in low-income US homes and a median CO₂-decay infiltration of 0.34 h⁻¹ in urban Chinese bedrooms. Occupant perception has repeatedly been found decoupled from measured indoor air quality in low-income Spanish dwellings, Japanese homes and a Danish study, with subjective judgement tending to under-rate objectively poor conditions. The present study extends that measured-ventilation and perception evidence to a previously under-represented group and season: energy-vulnerable older-adult dwellings during a hot-humid, cooling-constrained East Asian summer, where cost-constrained cooling can suppress the window opening that would otherwise flush accumulated CO₂.
The limitations are considerable and the authors catalogue them carefully. The design is cross-sectional and observational, so no causal direction can be established. Each dwelling had a single sensor under a single-zone, well-mixed assumption, room type was not systematically coded, and later sensor relocation was not tracked. The CO₂ module’s automatic baseline correction setting was undocumented, and the agreement statistics describe chamber-calibration fit rather than independent validation. Exploratory secondary analyses, including a possible ventilation–particulate trade-off and links to respiratory symptoms and health status, gave no robust signal. Within those bounds, the practical message stands: in this cohort, low population-average concentrations masked a recurrently accumulating subgroup, and a single satisfaction question could not find it. The authors suggest a two-stage workflow for housing-welfare practitioners, first-stage low-cost CO₂ screening to identify dwellings with recurrent nighttime accumulation, followed by targeted inspection of ventilation behaviour and feasible retrofit or operation measures, with longitudinal monitoring and measured outdoor CO₂ needed to test direction and mechanism.
Subject of Research: Nighttime CO₂ accumulation and ventilation screening in energy-vulnerable older-adult dwellings during a Korean summer
Article Title: Nighttime CO₂ Accumulation and the Limits of Satisfaction-based Screening in Energy-Vulnerable Older-Adult Dwellings During Summer
Article References: Lee, J.-W. (2026). Nighttime CO₂ Accumulation and the Limits of Satisfaction-based Screening in Energy-Vulnerable Older-Adult Dwellings During Summer. Architectural Research, 28(1), Article 22. https://doi.org/10.1007/s44515-026-00025-x
Image Credits: AI Generated
DOI: 10.1007/s44515-026-00025-x
Keywords: indoor air quality, carbon dioxide, ventilation, air change rate, energy poverty, older adults, low-cost sensors, occupant perception, housing screening, Daejeon, summer monitoring, CO₂ tracer decay
News Source: Russell Cooper. (October 28, 2026). Nighttime CO₂ Builds Up in Vulnerable Older-Adult Homes—and Satisfaction Misses It. Scienmag.
