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new discussion - how effective are masks?

By Edward Nardell, MD Moderator | 20 Oct, 2010 Last edited by Sophie Beauvais on 01 Dec 2010

S. Mehtar asks
"Dear Ed
This information is very useful to us in countries where there is little ventilation in clinical areas. It is indeed very helpful.
I have another question which I hope the group can help me with.
Do we know how much spread can be contained by the person who is infected wearing a face mask? In other words can we reduce the number of bacilli by covering our mouth when coughing. And if so, by how much?
I can't find any literature on this.
Regards
Shaheen

Dear Shaheen,

The reason that you have not found the answer is that there have been no studies. The use of surgical masks to reduce airborne spread goes back to the 1918 flu pandemic, and they were thought to be effective, but no hard evidence was obtained.

It is not published yet, so I cannot publish too much here, but we have asked our MDR patients at the AIR Facility in Witbank to wear surgical masks every other day. On the days they wore them, exhaust air from the ward went to one guinea pig exposure, and on the days that they did not wear them, it went to a second guinea pig exposure chamber. We did this over a 3 month period. Nurses reminded patients to wear their masks 7 AM to 7 PM, and we only sampled air during that period, feeling that asking patients to wear masks when they slept was not reasonable.

The result was a consistent 50% reduction in the number of guinea pigs infected when patients wore surgical masks. This compares to an 80% reduction when upper room UV was used every other day. This is about what I would have expected, and this is under study conditions, that is, nurses reminding patients to wear the mask. Under routine clinical conditions I would predict even less efficacy, especially if patients are asked to wear them more or less all the time - except when eating, etc. Ideally, the surgical mask is most useful short-term in untreated, symptomatic patients when they can be reasonably expected to comply. For example, while in the waiting room, or in radiology waiting for a chest x-ray, etc, etc. Long term it is hard to wear a surgical mask continuously, and the results, despite reminders show that.

I do not think that better masks are the answer, since no mask or respirator can contain the force and volume of a cough - there will be leakage. We are asking the surgical mask to act as a barrier, like a tissue or hand, stopping large particles before they can evaporate into smaller particles and pose a threat. Surgical masks do that, but the result is a maximum of 50% reduction based on our studies - which will be published.

Ed

Edward A. Nardell, MD
Associate Professor
Harvard Medical School (Medicine; Global Health and Social Medicine)
Harvard School of Public Health (Environmental Health; Immunology and Infectious Diseases)

Brigham and Women's Hospital
Division of Global Health Equity
FXB Building, 709c
651 Huntington Ave.
Boston, MA 02115

Replies

 

S. Mehtar Replied at 2:50 PM, 20 Oct 2010

Thanks Ed
I am looking forward to the publication- sounds exciting!
S

Prof Shaheen Mehtar

Ifedayo Adetifa Replied at 9:59 AM, 27 Oct 2010

Same here. Do you expect the findings to be different is N95 respirators were used instead of the usual surgical masks?

Edward Nardell, MD Moderator Replied at 11:04 AM, 27 Oct 2010

No, I do not expect that N95 respirators would be more effective when worn by patients since it is not the filtration properties that are believed to be important. A surgical mask serves as a barrier, like a hand or tissue, impacting relatively large particles before they evaporate into airborne droplet nuclei. No mask or respirator can contain the force of a cough, so face seal leakage would also be expected from an N95 respirator. The face seal feature of an N95 is important for protecting the wearer from inhaling droplet nuclei, however. The problem with the surgical mask or any barrier is that it can only contain a fraction of a cough and can only be worn part of the day simply because of discomfort, heat, need to eat, and communicate.

Effective treatment (early precise diagnosis including drug susceptibility) and fully supervised treatment is by far the optimal means of TB infection control.

Ed

Junior Bazile Replied at 12:06 PM, 27 Oct 2010

Dear Ed,
Thanks a lot for those great information.
We understand that a mask can only be used as a barrier and cannot contain the strength of a cough when it is worn by a patient. However can you tell us about how effective that you think a mask can be when worn by health professional accessing specific areas of medical facilities like TB isolation ward, MDR-TB ward? I would guess that in this case it would be more effective in protecting the health professional than if it is worn by the patient and not by the health professional. Please let us know your thoughts on this.
Thanks again.
Best,

Junior Bazile.
Village Health Works.

Edward Nardell, MD Moderator Replied at 11:37 AM, 29 Oct 2010

The use of respirators for HCWs (as opposed to surgical masks on patients) has been much discussed, so just a few comments here. The main limitation of respirators are that they cannot be worn all the time and are unlikely to be in use when unsuspected (untreated) TB patients are being seen. That problem aside, the main limitation of respirators is fit. Respirators should be fit tested to make sure it adequately fits the face of the HCW. This is because there is also face-seal leak of at least 15-20% even with good fit testing. So, the answer to your question in my opinion is that respirators on HCWs are at best about 80% effective when in use, but less effective when they do not fit properly, and not effective at all when not work for unsuspected, untreated patients.

I Lesotho PIH has tried using some reusable industrial respirators that cost about 10 times that of a disposable respirator but generally last many months, and many can we wiped clean. The elastic bands are much stronger than on disposable respirators, leading to a better fit in many cases.

I hope these few comments help.

Ed

S. Mehtar Replied at 1:01 PM, 31 Oct 2010

Thanks Ed
Great help!
S
Prof Shaheen Mehtar
MBBS, FRC Path (UK), FCPath (Micro) (SA), MD (Eng)
Chair IPC Africa Network
Head of Academic Unit for Infection Prevention and Control
Tygerberg Hospital & Stellenbosch Uni
PO Box 19063,
Tygerberg 7505, Cape Town

Tel: +27 21 938 5051
Fax: + 27 21 938 5065
Mobile: +27 82 852 3697
Visit our UIPC Website
http://www.sun.ac.za/uipc

Visit the First African IPC Congress website
http://www.ipcan2009.co.za

Junior Bazile Replied at 1:57 PM, 31 Oct 2010

Thanks a lot Ed for your comments.
One last thing though. In the remote resources poor settings where people can be rarely lucky to have access to disposable mask, some HCW are most of the using the disposable respirator more than once, keeping it with them whole day or even week to go in and out of TB isolation ward for example. Do you think that once the respirator is used to access the ward, if it is used again there are some risks?
Thank you so very much
Best,

Bazile

Erin Meier Replied at 9:06 PM, 31 Oct 2010

Thanks Ed - I too look forward to the study.
Erin Meier
Kudjip Nazarene Hospital
Papua New Guinea

Edward Nardell, MD Moderator Replied at 10:37 PM, 3 Nov 2010

Hi again, Dr. Bazile,

This is the most common question we are asked about respirators - how long can we use them? Paul Jensen's answer is, "as long as they are structurally intact". The two main structural problems are the elastic rubber bands and the filter facepiece itself. When the elastic stretches it fails to keep a good face seal and excessive leakage occurs. If the facepiece is crushed and and distorted, it may not seal around the face. All of this indicates that respirator fit testing is important. If a respirator is carefully cared for it might last a month (assuming the elastic is not stretched out) but with frequent use if may last a week. There is no concern with contamination by TB, but other hospital bacteria can spread on respirators. That is another
factor favoring shorter term use. In conclusion, you can reuse respirators but they will gradually become less effective over time.

Ed Nardell



Edward A. Nardell, MD
Associate Professor
Harvard Medical School (Medicine; Global Health and Social Medicine)
Harvard School of Public Health (Environmental Health; Immunology and Infectious Diseases)

Brigham and Women's Hospital
Division of Global Health Equity
FXB Building, 709c
651 Huntington Ave.
Boston, MA 02115

Junior Bazile Replied at 5:11 AM, 4 Nov 2010

Dear Ed,
Thanks a lot for your input and contribution in this topic. Your expertise is hugely useful.
Best,

Bazile

Stefanie Castell Replied at 7:21 AM, 5 Sep 2011

Dear Ed,
This sounds like a very interesting study. Has it already been published?
Thank you,
Stefanie Castell, M.D.

--
Mit freundlichen Grueßen,
Kind regards,

Dr. Stefanie Castell
Ärztin im DZK

Deutsches Zentralkomitee zur Bekämpfung der Tuberkulose
German Central Committee against Tuberculosis
Stralauer Platz 34
D-10243 Berlin

Tel: +49 - (0)30 - 2936 2701
Fax: +49 - (0)30 - 2936 2702

E-Mail:
www.dzk-tuberkulose.de

Edward Nardell, MD Moderator Replied at 8:14 PM, 5 Sep 2011

Nearing the end of the publication process now. By the way, the actual number was 56% protection - just used "about 50%" for sake of discussion.

Ed

S. Mehtar Replied at 1:33 AM, 6 Sep 2011

WE are all looking forward to reading this paper! Much needed and much over due!
Regards
S

Prof Shaheen Mehtar
MBBS, FRC Path (UK), FCPath (Micro) (SA), MD (Lon)
Chair of Infection Prevention and Control Africa Network (IPCAN)
Head of Academic Unit for Infection Prevention and Control
Tygerberg Hospital &Fac of Health Sciences,  Stellenbosch Uni
PO Box 19063,
Tygerberg 7505, Cape Town
Tel: +27 21 938 5051
Fax: + 27 21 938 5065
Mobile: +27 82 852 3697
http://www.sun.ac.za/uipc
Register now for the 3rd IPCAN conference Namibia
1-3rd November, 2011
http://www.ipcan.co.za

karen Gordon-Boyle Replied at 9:12 AM, 6 Sep 2011

Just wanted to say that this sounds truly interesting and I look forward to reading about the study in detail when published. In Guyana we are seeing the emergence of MDR TB and still rely on natural air ventilation and patient education to reduce nosocomial transmission.
Cheers!


Karen Gordon-Boyle, MD. MPH.

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