The Impact of Natural Ventilation on Occupant Health and Wellbeing

The impact of natural ventilation on occupant health and wellbeing is directly quantified through indoor CO₂ concentration. The COGfx study from the Harvard T.H. Chan School of Public Health (Allen et al., 2016, Environmental Health Perspectives) exposed 24 professionals to three conditions over 6 days: conventional building (1,400 ppm CO₂), green building (945 ppm), and enhanced green building with improved ventilation (590 ppm). Results showed cognitive scores 61% higher in the green environment and 101% higher in the enhanced green environment compared to conventional, with especially significant improvements in crisis response (97%), strategy (183%), and information usage (299%).

A subsequent study by the same group (Allen et al., 2016, Environmental Health Perspectives, doi: 10.1289/ehp.1510037) confirmed that every 400 ppm increase in CO₂ above 600 ppm reduces cognitive scores by 21%. The WHO and ASHRAE recommend maintaining indoor CO₂ below 1,000 ppm (800 ppm above outdoor levels). With CO₂ generation of 20 l/h per person in an office, maintaining 800 ppm requires a minimum airflow of 8-10 l/s per person, achievable through natural cross-ventilation with openings sized per EN 16798-1 category II.

Sick Building Syndrome (SBS), defined by the WHO in 1986, groups symptoms such as eye and respiratory irritation, headache, fatigue, and difficulty concentrating associated with occupancy of a specific building. The most extensive epidemiological study, the BASE Study by the US EPA (100 office buildings, Apte et al., 2000), demonstrated that each 100 ppm increase in CO₂ above 800 ppm is associated with a 20-30% increase in SBS symptom prevalence.

Naturally ventilated buildings show significantly lower SBS rates than air-conditioned buildings. The longitudinal study by Seppänen and Fisk (2004, Indoor Air), analysing 30,000 workers in 160 buildings, concluded that SBS symptoms are 30-200% more frequent in air-conditioned buildings than in naturally ventilated ones, attributing the difference to duct contamination, artificial humidification, and air recirculation. Absenteeism in naturally ventilated buildings is 35% lower according to Building Use Studies (BUS) data from 600 UK buildings.

VOCs (formaldehyde, benzene, toluene, xylene) are emitted by building materials, furniture, paints, and cleaning products. Typical indoor concentrations are 200-500 μg/m³ total VOC (TVOC), versus the 300 μg/m³ WHO recommendation and 500 μg/m³ EN 16798-1 category II limit. Formaldehyde, classified as a Group 1 carcinogen by the IARC, has a limit of 100 μg/m³ at 30 minutes (WHO, 2010) and 30 μg/m³ annual average in France (decree 2011-1727).

Natural ventilation dilutes VOCs proportionally to outdoor airflow rate. Increasing airflow from 4 to 10 l/s per person reduces indoor TVOC by 50-60% (Wargocki et al., 2000, Indoor Air). Mechanical ventilation systems with recirculation concentrate VOCs if activated carbon filters are not renewed, whereas natural ventilation provides 100% outdoor air without recirculation. WELL v2 certification (IWBI) requires continuous monitoring of TVOC, formaldehyde, PM2.5, and CO₂ with stricter thresholds than general regulation: TVOC < 500 μg/m³ and formaldehyde < 27 μg/m³.

The COVID-19 pandemic confirmed that ventilation is the most effective measure for reducing airborne pathogen transmission indoors. A study by Morawska et al. (2020, Environment International), signed by 239 scientists, demonstrated that SARS-CoV-2 is transmitted predominantly via aerosols (particles < 5 μm) that remain suspended for hours. Infection probability is inversely proportional to ventilation rate: doubling outdoor airflow from 4 to 8 l/s per person reduces relative infection risk by 40-60%, according to the Wells-Riley model.

Natural ventilation with operable openings provides airflow rates of 10-30 l/s per person under favourable conditions, exceeding the 6-10 air changes per hour recommended by the WHO to reduce airborne transmission. EN 16798-1 was updated with a post-COVID informative annex recommending elevation from ventilation category III to II (75% airflow increase) in high-occupancy spaces such as classrooms and waiting rooms.

The adaptive thermal comfort model (EN 15251, updated in EN 16798-1) recognises that occupants of naturally ventilated buildings tolerate a wider temperature range than in mechanically climatised buildings: comfort temperature adapts to the outdoor running mean with a gradient of 0.31 (category II), meaning that with an outdoor mean of 25 °C, occupants accept up to 28.5 °C operative indoor temperature. In air-conditioned buildings, the fixed limit is 24-26 °C.

This greater tolerance is associated with the sense of personal control: opening or closing a window provides environmental autonomy, a factor that Fanger's Predicted Mean Vote (PMV) model does not capture but satisfaction surveys consistently demonstrate. The BUS Survey (Adrian Leaman, Building Use Studies) across 600 UK buildings documented that naturally ventilated buildings achieve overall satisfaction scores 20-25% higher than air-conditioned buildings, with especially marked differences in "air quality" (30% higher) and "sense of control" (40% higher).