Technik aufs Ohr: Folge 31 Raumluftsysteme vs. Lüften - was hilft gegen Viren? | PODCAST

Summary — scientific concepts, findings and practical recommendations

Main scientific concepts and findings

• Two fundamentally different problems:

  • Virus concentration in room air (directly related to infection risk).
  • CO2 concentration as a proxy measure for stale air and the need to ventilate.

    CO2 indicates when to ventilate but is not directly equivalent to airborne virus load.

• Air cleaning and filtration

  • Recirculating air purifiers fitted with high-efficiency particle filters (HEPA H13/H14) can remove a very high fraction of airborne virus-containing particles (reported efficiencies ≈99.5%+ depending on filter quality).
  • Some devices claim inactivation rather than removal of viruses; mechanisms and performance differ by product.

• Ventilation and CO2

  • Mechanical supply of outside air is the only practical and economical way to reduce CO2 concentration reliably.
  • Window (natural) ventilation can rapidly exchange air—especially in winter when indoor/outdoor temperature differences are large—thereby lowering CO2, odors and airborne viruses, but continuous opening causes thermal discomfort and cold drafts.

• Infection risk and system design

  • Infection risk scales with the dose of viruses inhaled over time; the infectious dose for SARS‑CoV‑2 is not precisely known, so technological measures reduce but do not eliminate risk.
  • Trade-offs and system design depend on the acceptable residual risk chosen; there is currently no universally agreed target for sizing systems precisely.

Key experimental / empirical points

• Classroom tests (≈200 m^3)

  • Several large, commercial air-cleaning units achieved about an order-of-magnitude reduction in virus concentration within ~15 minutes.
  • Whether this reduction and response time are sufficient depends on acceptable risk levels and exposure durations.

• Caution on DIY solutions

  • Homemade air-extraction or venting fixes can pose hazards (e.g., fire risks from flammable materials), reduce thermal comfort, and may violate building-safety norms.

Practical recommendations and methodologies

• CO2 monitors

  • Use CO2 monitors as a behavioral and training aid: they indicate when to ventilate but do not measure virus concentration.

• Ventilation strategy

  • Prefer an intermittent ventilation strategy over leaving windows open continuously.
    • Example guideline referenced: ventilate roughly every 20 minutes for about 5 minutes (adjust for local guidance and conditions) to exchange air without excessive cooling of the room structure.
  • For schools, prioritize mechanical ventilation systems that supply fresh outside air and allow controllability for individual comfort.

• Air-cleaning devices

  • If relying on particle removal, use high-quality HEPA H13/H14 filtration.
  • Choose proven, certified suppliers and documented solutions; avoid improvised DIY fixes.

• Building-level and climate considerations

  • Consider low-exergy (low-energy) cooling strategies and climate-friendly façade design as part of ventilation and comfort planning to ensure year-round performance and resilience.

• Policy and targets

  • Set societal/policy-level targets for acceptable residual infection risk so engineers can design systems to meet those targets; current lack of consensus hinders precise sizing.

Other practical and organizational points

• Existing guidance

  • Technical guidance (for example, VDI recommendations) has long included classroom ventilation practice; earlier implementation would have mitigated many current problems.

• Human factors

  • Technology reduces risk but depends on human behavior (masking, distancing, adherence to rules) to be effective.

Sources, researchers and institutions mentioned

• Professor Dr. Uhr Franzke — Managing Director, Institute for Air and Refrigeration Technology (Dresden); honorary member of the VDI specialist society for building and building technology (interviewee).
• VDI (Verein Deutscher Ingenieure / Association of German Engineers) — VDI guideline on classroom air quality (referred: VDI guideline 6040 / VDI recommendations).
• VDE (Association for Electrical, Electronic & Information Technologies) — referenced in the guideline context.
• Karlsruhe Institute of Technology (KIT) — referenced for releasing an air purifier product/claim.
• Max Planck Institute for Chemistry — referenced for online tools/approaches to risk assessment.
• Freudenberg technology group — episode sponsor; provides filter solutions and services to reduce viral load indoors.
• HFH Hamburger Fern-Hochschule (Hamburg Distance Learning University) — sponsor advertisement featured in the episode.
• Standing Conference of the Ministers of Education and Cultural Affairs (Kultusministerkonferenz) — referenced regarding ventilation timing recommendations.
• The interviewee’s institute/team — authors of the classroom air-cleaning tests described (study of multiple commercial devices in a 200 m^3 classroom).