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What is the risk of airborne droplet transmission from the exhaust of a Monoplace chamber? What elements of risk mitigation (i.e. filters, distance, exhaust location) should be considered?
Published: 05 May 2020

Date:       5/5/2020


Question: What is the risk of airborne droplet transmission from the exhaust of a Monoplace chamber? What elements of risk mitigation (i.e. filters, distance, exhaust location) should be considered?

Answer:  Thank you for your question. The UHMS HBO2 safety committee can provide information to assist you in answering your question, but the ultimate responsibility for these types of questions rests with the medical director and safety director of your facility.

It must first be known that the UHMS Safety Committee cannot definitively answer your specific question of risk without knowing a myriad of factors surrounding the construction of your facility’s exhaust system. Other factors such as the height of the exhaust, wind, temperature, humidity, and variations in ventilation rate may impact the ability of the droplets to travel the length of the exhaust. Even with these factors, we assume there may remain an element of uncertainty, as serial cultures and viral concentration measurements in several locations would be necessary to validate any mitigation measures. In any case, we would encourage you to work with your chamber manufacturer, infection prevention specialists and facilities management teams to determine the best course of action.

We encourage you to review these additional comments from the committee that may assist you in the risk assessment (1) and decision-making process:

  • There is speculation that the droplets will likely salt out on the inside of the exhaust pipe.
  • The exhaust point is typically found on the rooftop of a building, and should not be considered a serious concern, as the location is usually a great distance from the general population.
  • Monoplace chambers have moisture and particulate filtration on the supply side to protect pneumatic gauges, regulators and valves (PVHO-1, 4-4.3). On the exhaust side, PVHO-1 requires a screen to prevent exhaust blockage, and NFPA 99, 14 calls for the installation of a 0.3cm2 screen to protect the exhaust endpoint. Neither of these items would serve as filters in the infection control sense.
  • If the user were to add exhaust-side filters of the same materials used in the N95 respirator, there would be a risk to the chamber’s ability to ventilate and decompress capably. Any attempt to modify the existing exhaust system should be done only with approval of the chamber manufacturer. Alterations like these would also require testing of the decompression capability and documentation describing the testing measures used, along with approval from the manufacturer. The measure is possible, but may be seen as excessive, as this measure is not a current requirement concerning the delivery of Hyperbaric Oxygen to patients with other highly infectious diseases.
  • It is noted that sea-level negative pressure rooms within hospitals require the point of exhaust, without HEPA filtration, to be 25 feet away from potentially contacting the general population (Ventilation of Healthcare Facilities - ASHRAE Standard 170).
  • Other factors include particulate size. The Diver’s Alert Network has investigated this to understand the effectiveness of face masks and the output of compressed breathing air filter systems. The particles range from 4µm down to 0.5µm. From the research found on virus particles (not specifically SARS CoV2), 42% of particles were smaller than 1µm. Here is one source. To quote: “RNA was detected in coughs from 38 (81%) of the 47 subjects who had flu. The RNA was found in 35% of particles larger than 4 microns in diameter, 23% of particles 1 to 4 microns in diameter, and 42% in particles smaller than 1 micron.” Compressed air filters don’t really remove particles smaller than 1µm. However, the N95 respirator can remove up to 95% of particles as small as 0.3 µm. 99.5% for particles down to 0.75µm.
  • Transmission of droplets: there would need to be some studies, but firstly the air/oxygen leaving the chamber is going to be cooler due to the pressure drop from chamber pressure to the 1 psi regulated outlet pressure (adiabatic cooling from the pressure change and some Joule-Thompson cooling from orifices in the flow path). We have actually seen this when decompressing a monoplace chamber rapidly – ice forming at the exhaust screen. This will/should cause droplets to fall out faster and rest on the piping (and associated pressure controls in the exhaust system). Secondly, the longer they travel, the more of them are going to fall out – friction on the piping, temperature, impact at the bends and so on; however, some particles will likely make it to the exhaust outlet. Lastly, this argument is with respect to droplets and does not address airborne transmission. Droplets, which have greater mass, will be more likely to settle at some stage due to the gravitational effects.
  • Exhaust outlet concerns: Physical distancing of at least 6 feet is generally accepted; however this may be insufficient as some claim these droplets can exist in the air for up to 30 minutes before dropping. Additionally, temperature and turbulence will affect these results as will the exit speed of the droplets and wind effects which may carry them further. A higher temperature at the exit will also serve to keep them from coalescing.
  • It is perceived that a safe distance from the exhaust point could be 30 feet. This is based upon comments from infectious disease specialists including Dr. Anthony Fauci, who stated that in extreme cases, sneezing in patients with significant viral loading can project particles into the air up to 27 feet. A typical monoplace chamber exhaust ranges from 85 to 400 lpm, and human exhalation may deliver 6 – 25 lpm (rest to active). In the chamber you’d expect a dilution of 6/85 at the minimum flow rates. Ventilating the chamber at 250 lpm may provide a dilution of approximately 3%. This in combination with the adiabatic and Joule-Thompson cooling, and the length of the exhaust piping (which if it has any metallic section is going to cause the droplet to stick to the wall more easily), would make it highly unlikely for someone to be significantly exposed.
  • It is likely that the exhaust from a monoplace chamber is akin to hospital HVAC exhaust due to its general location (rooftops in most instances). Because of this, there should be minimal exposure to patients, staff or the public at large. If the facility and the safety director have done their due diligence and ensured that NFPA-99 guidelines have been followed for installation of the chamber exhaust, then there should be minimal risk due to this distancing. While the intent of NFPA-99 regarding exhaust requirements was for fire safety, they also serve to help mitigate infection control risks.* Exhaust from all classes of chambers shall be piped outside of the building. Each Class B chamber shall have an independent exhaust line. The point of exhaust shall not create a hazard. The point of exhaust shall not allow reentry of gases into the building.


Burman, F. (2019). Risk Assessment Guide for Installation and Operation of Clinical Hyperbaric Facilities. San Antonio, TX: International ATMO, Inc.

The American Society of Mechanical Engineers. (2019). ASME PVHO-1-2019: Safety Standard for Pressure Vessels for Human Occupancy. Two Park Avenue, New York, NY

National Fire Protection Agency (NFPA). (2018). NFPA 99 2018 Edition: Health Care Facilities Code. Quincy, Massachusetts.

N Engl J Med (2020). “Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1,” April 16, 2020, 382:1564-1567, DOI: 10.1056/NEJMc2004973

Bourouiba, L. (2020), “Turbulent Gas Clouds and Respiratory Pathogen Emissions: Potential Implications for Reducing Transmission of COVID-19,” Published online March 26, 2020, Journal of the American Medical Association, doi:10.1001/jama.2020.4756

Lindsley, W. (2010). “Measurements of Airborne Influenza Virus in Aerosol Particles from Human Coughs,” November 30, 2010, PLOS One,

American Society of Heating, Refrigeration, and Air-Conditioning Engineers (2017), “Ventilation of Healthcare Facilities - ASHRAE Standard 170 -2017,” American Society of Heating, Refrigeration, and Air-Conditioning Engineers,




The UHMS Safety Committee


Neither the Undersea and Hyperbaric Medical Society (UHMS) staff nor its members are able to provide medical diagnosis or recommend equipment over the internet.  If you have medical concerns about hyperbaric medicine you need to be evaluated by a doctor licensed to practice medicine in your locale, which can provide you professional recommendations for hyperbaric medicine based upon your condition. The responsibility of approving the use of equipment resides with the physician and safety director of the facility.  Information provided on this forum is for general educational purposes only.  It is not intended to replace the advice of your own health care practitioner and you should not rely upon it as though it were specific medical advice given to you personally.