FAQ - This Q&A list is a brief summary of important concepts to consider when evaluating a ventilation system and the inhalation hazards that may be present.

Q1. 100% fresh air supply to an enclosed space (room) is sufficient to purge it of harmful (exhaled) particles, True/False?

A1. True. Fresh air would eventually replace old air, taking several minutes to several hours. That’s assuming the sources (people) have been removed. For active zones (w/ people) an equilibrium lever of exhaled vapor would be established and maintained with conventional HVAC practices.

Q2. Can recirculated air be conditioned to remove harmful vapors and particles?

A2. Conditioning equipment such as filters, UV Sources and electrostatic collection can remove harmful vapors and particulate. Conditioned air would be equal to or less clean than the equivalent amount of fresh air. Efficiency of these devices should be quantified through rigorous testing.  Refer to previous explanation for qualification.

Q3. Where do exhaled vapor and vapor nuclei tend to reside in poorly ventilated spaces?

A3. Exhaled vapors and vapor nuclei entrained by air currents tend to migrate to the upper regions of the room. (Similar to smoke particles.)

Q4. Do fans improve air quality and comfort in a room?

A4. Fans can increase comfort in a room when they provide cooling breezes or move warm air to cold areas or vice-versa. Fans do not improve indoor air quality; they promote rapid mixing within the space. Ceiling fans and blowers can move harmful particles back into the breathing zone. Fans that are part of an integrated exhaust system should be beneficial.

Q5. Where do small particles reside in rooms with well mixed flows, such as with RECIRCULATION fans?

A5. Everywhere.

Q6. Where is the best location to bring fresh air into a space?

A6. It is best to admit fresh air into a space horizontally about 4-ft above the floor, into the breathing zone. For example, as open windows do for naturally ventilated spaces.

Q7. How can you visualize air movement in a space?

A7. The best visual method uses smoke released from a device operated by a competent flow specialist,
(See Flow Visualization for a few industrial examples.)

Other physical methods, such as tracer gases can be used to qualitatively and quantitatively characterize air flow patterns. Modeling, either physical or computer, can also be used to establish as-is and modified flow patterns. (See Modeling Tools).

Q8. Where is the best location to remove air from a space?

A8. Above the breathing zone.

Q9. What are the characteristics of a ventilation system that can purge harmful vapors and particulate from an interior space?

A9. 100% fresh air (without recirculation), inlet points based on occupancy, general vertical flow movement, exhaust from upper regions.

Q10. What ARE SOME practices THAT should be eliminated from ventilation systems to improve IAQ?

A10. Recirculation mode without conditioning, devices that promote mixing in the space (ceiling fans and blowers), air inlets in the upper walls and/or ceiling, and room exhaust through entry doors.


Test Your Ventilation IQ

Q - If you enter a room and can smell the coffee brewing, bread baking, fragrances or whatever other aromas or odors are being emitted, what would you conclude about ventilation system effectiveness in that space?

Q - If you were wearing your mask, what would you conclude?

Q - What does a COVID-19 particle smell like?

Q - Do you want to find out?


Note - These questions and answers are not exhaustive, complete nor have they been vetted. They are intended to illuminate and promote discussion about current ventilation practices that impact IAQ relative to airborne hazard.

This effort is not intended to minimize exposure risk to formites and cleaning/sanitation protocols in place.  It is intended to supplement those efforts by addressing one of the main transmission routes - airborne vapors and particles.


A close collaboration between flow control specialists and HVAC professionals would be required to convert conventional practices to the 'New Normal' using the CoVentilation™ method; fresh or conditioned air in horizontal into the breathing zone, up towards the ceiling and vented out of the space. The old air would be exhausted or returned through the conditioning equipment: filters, UV irradiation and/or electrostatic collectors and then readmitted to the occupied space.

Visualization of Airborne Vapors
and Interior Air Quality (IAQ)

The link below is to a PDF file that is an informal summary of recent academic research and Association views on micron-sized particle movement in indoor spaces. This summary provides insight into the airborne movement of a class of organic particles that includes: mold spores, pollen, and aerosols from cooking, smoking, flushing toilets, coughing, sneezing and normal breathing.

The last several entrained particles and vapors can contain infectious biological agents. Once airborne, they can alight on horizontal and vertical surfaces and pose a direct contact hazard. Particles that remain in suspension or that are reintrained can be inhaled.

          Ventilation Letter

                                              DE Ventilation Services


The plot below highlights two controllable parameters that affect IAQ in an enclosed space relative to airborne hazards: 'FLOW PATTERN' and 'AIR CHANGE RATE'. The left side vertical axis shows the range of FLOW PATTERNS in a space: STIRRED to PURGED. The bottom horizontal axis shows the range of AIR CHANGE RATES: LOW to HIGH.

Examples have been added for a few 'PRACTICES'  and '[PLACES]' that fit into a particular ventilation zone (qualitative).  The purpose of the diagram is to help identify current ventilation practice in a particular space and to encourage changes that move those practices into the upper right quadrant.

  1.  Conditioned air (filtering, UV irradiation and/or electrostatic collection) has a fresh air equivalent based on the efficiency of the equipment. Rigorous testing of these devices should be performed  to understand their efficiency. Note that an increase in equivalent fresh air does nothing to address the distribution of air within the space. Refer to Q&A 1& 2 above.               -------->  DE Ventilation Services