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Natural ventilation has important role to play in TB infection control
Posted by plosmedicine on 31 Mar 2009 at 00:07 GMT
Author: Roderick Escombe
Position: Honorary Research Fellow
Institution: Imperial College London
Additional Authors: DAJ Moore, RH Gilman, JS Friedland, CA Evans
Submitted Date: April 16, 2007
Published Date: April 17, 2007
This comment was originally posted as a “Reader Response” on the publication date indicated above. All Reader Responses are now available as comments.
We would like to thank the correspondents for their thoughtful contributions to this important public health topic. As our abstract and article state, we measured natural and mechanical ventilation and then calculated the effects of these ventilation rates on estimated tuberculosis infection rates using a mathematical model of airborne infection. This appears to be the first published assessment of natural ventilation rates in health care settings and the novel conclusions of our article are that extremely high rates of dilutional ventilation can be achieved through natural ventilation at very little cost by simply opening windows and doors. Indeed, this natural ventilation was far in excess of even the best maintained mechanical ventilation systems used in health care settings. Importantly, this natural ventilation greatly reduced the calculated risk of airborne infection.
Measuring TB transmission itself is difficult, as rates in staff are confounded by exposures outside the workplace, and mechanical air sampling techniques have had limited success. We have established a guinea pig air sampling facility to directly measure TB transmission in a hospital ward in Lima Peru (see next month’s Clinical Infectious Diseases 2007:44;1349-57) and have used this model to evaluate the effects of upper room ultraviolet light and negative air ionisation on TB transmission. We plan to use this facility to further study natural ventilation and its effect on TB transmission.
The results of the current study cannot be generalized to regions too cold to tolerate enhanced natural ventilation and not every room may be as amenable to natural ventilation as the Peruvian rooms that we studied. However, the key conclusions are clear: high rates of natural ventilation were achieved even on days with little wind and even rooms without high ceilings and large windows were well ventilated, such that natural ventilation significantly exceeded mechanical ventilation.
It is therefore clear that natural ventilation has an important role to play in the fight against institutional TB transmission in resource-limited settings. Mechanical ventilation is expensive to install, requires costly ongoing maintenance, may be dangerous if poorly maintained (for example positive instead of negative pressure), and is clearly inappropriate for the great majority of resource-limited settings where the burden of TB is highest. TB infection control is an urgent priority, underscored by the emergence of extreme drug resistant TB strains and the increasing congregation in potentially high risk overcrowded settings of persons living with HIV. When infectious TB patients share rooms with others, opening windows and doors to enhance natural ventilation is a simple and inexpensive weapon in the fight against nosocomial TB transmission.
RE: Natural ventilation has important role to play in TB infection control
hlevin replied to plosmedicine on 26 Apr 2012 at 22:43 GMT
The authors continue to encourage over-generalization in their article and their response to our earlier comment (Levin H (2007) Natural Ventilation for Prevention of Airborne Contagion: Conclusions Overgeneralized. PLoS Med 4(5): e189. doi:10.1371/journal.pmed.0040189).
For example: "The results of the current study cannot be generalized to regions too cold to tolerate enhanced natural ventilation and not every room may be as amenable to natural ventilation as the Peruvian rooms that we studied."
They correctly identify the limitation on the use of natural ventilation in "regions too cold to tolerate enhanced natural ventilation." However, they fail to mention regions too warm to tolerate enhanced natural ventilation. Hot humid climates are common among resource poor contexts where we find the most highly-populated cities and regions with limited resources.
The authors follow the above statement with the following: "However, the key conclusions are clear: high rates of natural ventilation were achieved even on days with little wind and even rooms without high ceilings and large windows were well ventilated, such that natural ventilation significantly exceeded mechanical ventilation."
The ventilation rate measurements themselves are confounded by the fact that most of the rooms were connected to other interior spaces so that it is not possible to determine clearly whether the decrease in carbon dioxide was from air entering from outdoors or from air entering through doors to other spaces or, more likely, some highly variable combination of the two. "In 23 (33%) of the rooms, air passed into the room though the windows and out of the room through the door in over 80% of experiments with windows and doors fully open. These patterns reflected the position of a room and its windows and doors in relation to the prevailing wind in Lima." (Escombe AR, Oeser CC, Gilman RH, Navincopa M, Ticona E, et al. (2007) Natural ventilation for the prevention of airborne contagion. PLoS Med 4: e68. doi: 10.1371/journal.pmed.0040068.)
The authors do not report their wind velocity measurements in the article nor wind direction measurements. They neglect the impact of wind direction on natural ventilation through openings in a building exterior. Their wind velocity measurement methods were made using thermal anemometers, fairly accurate instruments but incapable of determining wind direction. The authorsonly present data on wind velocities below and above 2 km/h (in Table 1); wind <2 km/h represent a very low fraction of the wind velocities reported in Lima.
In fact, the driving force (impact) of wind on natural ventilation is a function of the vector of wind velocity perpendicular to the opening through which outdoor air enters. No mention of wind direction is made other than an over-generalization that prevailing wind is from the Pacific Ocean. "In Lima prevailing winds come from the Pacific Ocean, but wind may be less predictable in other locations."
Wind is also variable in Lima. A review of wind roses and other data readily available on the World Wide Web shows that the wind direction in Lima shifts during the course of the day and year.
Wind speed tends to increase in the afternoon in Lima. Examining historical wind data for four arbitrarily chosen days distributed through the year we found substantial velocity and directional variations in the course of the days and the year. For example, on May 1, 2008 - wind velocities equal to or less than 8 km/h until about 10 am; wind velocities greater than 8 km/h from 1 pm until midnight with peaks reaching 15 km/h. Wind directions fairly steadily shifting from due north to due south from 6 am to 2 pm and remaining due south throughout the afternoon. Diverse wind velocities and similar shifts in wind direction on the other days examined with the exception of one day (September 1, 2008) when the wind direction was fairly consistently from the south with only minor deviations. However, on that day, September 1, 2008, wind velocity varied from <5 km/h to >18 km/h. These variations in wind velocity and direction are not addressed in the paper other than by oversimplification of the wind characteristics and over-generalization of the reported measurements at the window.
When winds are calm, wind does not provide the driving force for natural ventilation. In that case, the driving force for natural ventilation is the difference between indoor and outdoor temperatures (I-O delta-T). Yet no examination of the air exchange under calm conditions was made. "Temperature (8C) and relative humidity (%) were also measured but did not qualify for inclusion in this model (p . 0.15)."
Later, the authors wrote: "In 23 (33%) of the rooms, air passed into the room though the windows and out
of the room through the door in over 80% of experiments with windows and doors fully open. These patterns reflected the position of a room and its windows and doors in relation to the prevailing wind in Lima." No evidence is provided to support these generalizations.