Hold Your breathe

ImageWhat started off as a question from the past…

We were looking at comparisons of hospitals in Europe to the U.S. One of the comments that really stuck to me was that health-care facilities (in Europe) let patients open windows to their rooms to bring in fresh air, however in the U.S. they don’t like to do it because of increase contamination and safety reasons depending on the patients case. I asked the professor about it, but he said that the U.S. has tighter rules and that was it. Since now I am re-looking into natural air ventilation, this question popped up in my head again.

There is many open ended questions whether increasing the demand of natural ventilation is the best solution as you can see in Table 2.1 mentioned from Natural Ventilation for Infection Control in Health-Care Settings, each type of system has their own advantages or disadvantages. HVAC filtration is one of the best systems to be used in hospital because depending on the size of the filtration can remove certain size aerosols particles. A quote from Azrimi and Stephens (2013), shows some of the key about implementing this system as the best, “(i) the effectives of particle filtration for controlling airborne infectious aerosols, (ii) the associated risk reductions achievable with HVAC filtration, and (iii) the relative costs of risk reduction by HVAC filtration versus other control mechanisms such as increased outdoor air ventilation rates.”



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One of the ways for the estimating risks dealing with airborne contamination is the Wells-Riley model stated in Azrimi and Stephens (2013):

 Pinfection= (cases/susceptible)=1-e^(-I*q*p*t/Qoa)

Notes for the equation: Pinfection=probability of infection, cases=number of infected cases, susceptible=number of susceptible individuals, I=number of infector individuals, p=pulmonary ventilation rate of a person (m³/hr), q=quanta generation rate (1/hr), t=exposure time (hr), and Qoa=room ventilation rate with pathogen-free air (m³/hr).

When you cough or sneeze, you send little particles or “droplet nuclei” into that air that can carry a small viral bug or something more serious like SARS. The Wells-Riley formula is used to calculate the rate of the virus infecting people in respiratory scenarios, however due to different forms of transportation (long-range or close-contact) of the droplet nuclei. Additional back research is needed to help support the findings for the Wells-Riley model which long-range transportation was a major factor. In newer hospitals, mechanisms removing aerosols with HVAC particles filtrations help reduce the transportation of contaminates, as well as, reducing pollutants from outside that might hurt patients more. However, there is not straight answer on what is the best solution; either system is acceptable for health-care facilities.



Atkinson, J., Chartier, Y., Pessoa-Silva, C.L., Jensen, P., Li, Y., and Seto, Wing-Hong (2009) Natural Ventilation for Infection Control in Health-Care Settings. WHO Publication/Guidelines, 1-133.

Azimi, P., and B. Stephens (2013) HVAC filtration for controlling infectious airborne disease transmission in indoor environments: Predicting risk reductions and operational costs. Building and Environment, 70:150-160.

Chao, Julie (2013) Berkeley Lab Indoor Air Roundup: Natural Ventilation Comes with Health Risks, and More. Berkeley Lab: Lawrence Berkeley National Laboratory. http://newscenter.lbl.gov/feature-stories/2013/09/26/indoor-air-roundup-natural-ventilation-comes-with-health-risks-and-more/

Escombe, Rod Dr., MD, DTM&H, Ph.D., (2008) Natural Ventilation of Health Care Facilities, Engineering Methods for the Control of Airborne Infections: An International Perspective. Harvard School of Public Health, presentation.

Sze To, G.N. and C.Y.H. Chao (2009) Review and comparison between the Wells-Riley and dose-response approaches to risk assessment of infectious respiratory diseases. John Wiley & Sons: Indoor Air, 20:2-16.


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