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This concept study presents an approach for resolving the trade-off between energy-efficient building operation and the provision of hygienically harmless indoor air quality. A novel coupling of HVAC-systems (heating, ventilation and air conditioning systems) with DAC-technology (direct air capturing technology) is proposed to separate CO2 in the exhaust air of buildings and recirculate the CO2-depleted air back into the building. In a mainly theoretical approach, the corresponding potentials and limitations of the novel HVAC/DAC-coupling in recirculation mode are evaluated. For that purpose, CO2-loads in the feed and exhaust air of four buildings located in Germany were measured using calibrated non-dispersive infrared (NDIR) sensors with pyroelectric detection principle. Subsequent numerical model simulations resort to typical meteorological data as well as building operation parameters grouped in different scenarios. The measurement and simulation results were assessed with regard to: (i) the unique possibilities of a HVAC/DAC-coupling in recirculation mode for the improvement of indoor air quality, (ii) the energy saving potentials through reduced air conditioning requirements enabled by a HVAC/DAC-coupling in recirculation mode, and (iii) the potential allocation of CO2 separated from building exhaust air for energetic and/or material reutilization in decentralized systems. In conclusion, a HVAC/DAC-coupling in recirculation mode can not only reduce the energy demand of buildings but also facilitates access to unutilized CO2-resources transported in the built environment and additionally offers the potential to improve indoor air quality. However, a suitable DAC module for operation in indoor air is not yet commercially available.
SummaryAirborne pollen allergens—a relevant component of bioaerosols and, therefore, of airborne particulate matter—are considered an important metric in air quality assessments. Although the measurement of airborne pollen allergen concentrations in outdoor environments (namely, in urban areas) has been recognized as a key environmental health indicator, no such obligation exists for indoor environments (dwellings or occupational settings). However, people spend most of their daily time (80–90%) indoors, where the majority of their exposure to air pollution, including pollen allergens, occurs. Nonetheless, the relative importance of airborne pollen allergen exposure indoors differs from outdoors because of differences in pollen loads, sources, dispersion, and degree of penetration from the outdoor surroundings, as well as the differences in the allergenic pollen profiles. In this brief review, we mined the literature over the last 10 years to summarize what existing measurements reveal about the relevance of airborne allergenic pollen in indoor environments. The research priorities on this topic are presented, highlighting the challenges and the motivations for obtaining pollen data in built environments which are key to understand the extent and mechanisms of human exposure to airborne pollen allergens. Thus, we provide a comprehensive assessment of the relevance of airborne allergenic pollen in indoor environments, highlighting knowledge gaps and research needs related to their health effects.
Direct air capturing (DAC) is an energy demanding process for CO2-removal from air. Ongoing research focuses on the potential of indoor air as DAC-feed to profit from currently unused energetic synergies between DAC and the built environment. In this work, we investigated the performance of three different readily available, solid DAC-adsorbers under typical indoor environmental conditions of 16-25°C, 25-60% relative humidity (RH), and CO2-concentrations of less than 800 ppm above atmospheric concentrations. The measured mass-specific CO2-adsorption capacities of K2CO3-impregnated activated carbon, polyethylenimine-snow (PEI-snow), and polyethylenimine (PEI) on silica a amount to 6.5 ± 0.3 mg g−1, 52.9 ± 4.9 mg g−1, and 56.9 ± 4.2 mg g−1, respectively. Among the three investigated adsorber materials, PEI on silica is the most promising candidate for DAC-applications as its synthesis is rather simple, the CO2-desorption is feasible at moderate conditions of about 80°C at 100 mbar, and the competing co-adsorption of water does not strongly affect the CO2-adsorption under the investigated experimental conditions.
Naturally ventilated educational buildings frequently struggle to maintain adequate indoor air quality (IAQ) under variable occupancy and user-driven ventilation behavior. This exploratory case study evaluates IAQ using the CO2-concentration as an IAQ-indicator in 14 naturally ventilated rooms of six different educational facilities during the heating season in Germany, examining the effects of room design, occupancy, and ventilation practices. After evaluation against current German design recommendations, all investigated rooms reveal the conditional necessity of a mechanical ventilation system. However, all investigated rooms are exclusively ventilated naturally. Continuous monitoring of occupied rooms revealed mean indoor CO2-concentrations – grouped into categories (I to IV) according to EN 16798–1 – ranging between 233 ppm and 383 ppm (category I), 654 ppm and 691 ppm (category II), 910 ppm and 1097 ppm (category III), and 1409 ppm and 2553 ppm (category IV) above the outdoor CO2-concentration. The corresponding frequency of the prevailing CO2-concentrations during the room occupancy periods in the categories I to IV range between 6% and 63% (category I), 9% and 38% (category II), 10% and 42% (category III), and 0% and 59% (category IV). Occupant-specific ventilation rates – determined based on the CO2-decay method – varied substantially (4–188 m³ h⁻¹ occupant⁻¹). Seven of the 14 investigated rooms failed to meet the German Federal Environment Agency’s minimum ventilation rate of 25–30 m³ h⁻¹ occupant⁻¹ under typical operating conditions. Corresponding area-specific ventilation energy losses during the German heating period were estimated to range between 1 kWh m−2 a−1 to 40 kWh m−2 a−1. To translate exposure patterns into management guidance, a CO₂-based 4 × 4 risk matrix was developed, integrating exceedance severity (based on EN 16798–1 categories) and occurrence frequency. While most rooms showed only moderate CO₂ elevations for limited occupied periods, two of the 14 investigated rooms (14%) were classified as high IAQ risk in the proposed matrix, indicating severe and frequent exceedances of hygienically acceptable conditions. The proposed risk-matrix framework provides a reproducible decision-support tool for prioritizing ventilation improvements in naturally ventilated educational sector buildings. By linking exposure severity, likelihood, and energy implications, the approach supports IAQ management under operational and structural constraints.