TB Infection Control

I am hoping Paul will comment on Dr. Menthar's question too, which is about masks and respirators. This question comes up all the time and merits discussion here.

Some general comments about surgical masks as protective devices for the wearer. By convention in TB circles, we refer to surgical masks as face and nose covering to protect the operating field - or general environment when worn by a patient. Although they may resemble N95 or equivalent respirators, these devices are not designed to produce a tight face seal, but rather to prevent large respiratory particles expelled during breathing, coughing, or speaking, from falling onto operating fields or evaporating into airborne droplets. Air can usually easily get between the filtering facepiece and the face, often around the bridge of the nose or at the sides of the mouth. Even respirators that are designed to reduce face seal leak still have as much as 20% face seal leak in actual use, even after successful fit testing, surgical masks could have 30-50% face seal leak and are not fit tested.
This is not a matter of moisture so much as a matter of design and materials - no clip around the nose - only one ...

I am hoping Paul will comment on Dr. Menthar's question too, which is about masks and respirators. This question comes up all the time and merits discussion here.

Some general comments about surgical masks as protective devices for the wearer. By convention in TB circles, we refer to surgical masks as face and nose covering to protect the operating field - or general environment when worn by a patient. Although they may resemble N95 or equivalent respirators, these devices are not designed to produce a tight face seal, but rather to prevent large respiratory particles expelled during breathing, coughing, or speaking, from falling onto operating fields or evaporating into airborne droplets. Air can usually easily get between the filtering facepiece and the face, often around the bridge of the nose or at the sides of the mouth. Even respirators that are designed to reduce face seal leak still have as much as 20% face seal leak in actual use, even after successful fit testing, surgical masks could have 30-50% face seal leak and are not fit tested.
This is not a matter of moisture so much as a matter of design and materials - no clip around the nose - only one elastic band, or sides that bulge out in some designs.

As noted above, a good N95 or equivalent respirator can do much better than a surgical mask if properly fit tested, properly worn each time, and not used when they finally loose their integrity or the elastic bands become too weak. Still, even when new, expect at least 10% face seal leak. To get a better seal requires more permanent respirators and for maximum protection, positive air purifying respirators (PAPRs) are used for such hazardous procedures as asbestos removal. These are expensive and require filter changes.

There is not a lot of evidence that fit testing improves respirator performance even though it makes some sense that it should. The selection of a respirator design from several different styles to fit
variations in face shapes and sizes is probably more important. If nothing elsee, however, fit testing demonstrates to the user that there are ways to wear the respirator that are more likely reduce leakage and that should help. I would say that a non-fit tested N95 is still more likely to provide more protection than a surgical mask because of its design features. There are some respirators that perform extremely well on a variety of face shapes without fit testing, but they have other downs sides including high cost. I know Paul and other respirator experts will have comments.

Ed

Dear Ed
Thank you so much. I love a good discussion. The information is invaluable and we are aware of the shortcomings both of surgical masks and ill-fitting respirators. So, the next question that comes to mind is-- how much reduction is adequate? 10%? 50%?. Evidence suggests that one needs approximately 60 infectious particles to cause infection (obviously not disease).Therefore, if one knows the particular load, can one estimate the reduction in particles for the wearer? In other words, what level of leak is acceptable in clinical practice given the distance from the patient?
One of the problems we have with respirators fitted to people with flat noses is that there must, by definition, be more leak than with a "roman" nose! An interesting thought. So, should those with flat noses have an extra bridge support with something like a foam cushion, for example?
Next question. What is the effect of covering the nose and mouth with a hanki or similar?
Any thoughts?
Shaheen

Prof Shaheen Mehtar
MBBS, FRC Path (UK), FCPath (Micro) (SA), MD (Eng)
Head of Academic Unit for Infection Prevention and Control
Tygerberg Hospital & Stellenbosch Uni
PO Box 19063,
Tygerberg 7505, Cape Town

Dr. Mehtar now raises two difficult questions:
1) how much protection from respirators is adequate?
2) what is the infectious dose of TB?

Here is a detailed response to the first of these challenging questions. Others are invited to join in. The other will follow.

How much protection can be generalized beyond respirators - how much protection is adequate from natural ventilation, mechanical ventilation, UVGI, or from a combination of all these when they are available? Does the answer vary if the risk is drug susceptible or MDR/XDR TB? What if those exposed are confined against their will and have no choice about exposure, like prisoners or refugees? What risk can health care workers be expected to assume as part of their work, and what is the hospital or health care systems responsibility versus personal responsibility?

There are no easy answers to these questions as we quickly enter the discipline of risk assessment and avoidance, money spent per life or DALY gained, etc.

However, a paper I published in 1991 on the theoretical limits of protection of building ventilation and another published later with Kevin Fennelly on the additive benefit of respirators and building
ventilation may be useful.

The 1991 ...

Dr. Mehtar now raises two difficult questions:
1) how much protection from respirators is adequate?
2) what is the infectious dose of TB?

Here is a detailed response to the first of these challenging questions. Others are invited to join in. The other will follow.

How much protection can be generalized beyond respirators - how much protection is adequate from natural ventilation, mechanical ventilation, UVGI, or from a combination of all these when they are available? Does the answer vary if the risk is drug susceptible or MDR/XDR TB? What if those exposed are confined against their will and have no choice about exposure, like prisoners or refugees? What risk can health care workers be expected to assume as part of their work, and what is the hospital or health care systems responsibility versus personal responsibility?

There are no easy answers to these questions as we quickly enter the discipline of risk assessment and avoidance, money spent per life or DALY gained, etc.

However, a paper I published in 1991 on the theoretical limits of protection of building ventilation and another published later with Kevin Fennelly on the additive benefit of respirators and building
ventilation may be useful.

The 1991 paper, available here: http://www.ghdonline.org/ic/resource/airborne-infection-theoretical-limits-of..., was based on an actual outbreak investigated by the Massachusetts Health Department when I was TB Control Officer in the mid 1980s. A worker in a welfare office returned from a month-long holiday sick and got progressively sicker while at work for the following month before being diagnosed with cavitary, smear positive TB.

Two rounds of contact TST testing showed that 27 of 67 initially negative co-workers
had converted with strongly positive TSTs (only 1 had active disease at the time), for an infection attack rate of 40% among that cohort.

The building had been the subject of air quality complaints resulting in measurements of the ventilation, and I was asked how much the poor air quality had contributed to the outbreak. To address this question I turned to a well-established mathematical model which estimated that to
produce that rate of infection, given measured building ventilation rates. The paper concludes that had the ventilation been twice as good (possible but not easy to achieve), approximately 13 instead of 27
workers would have been infected. Had the ventilation been 4 times what it really had been (not feasible), still 7 workers would likely have been infected. If it were possible to invest the resources (expensive re-construction and energy costs) to have 2X or 4X the existing ventilation, would that amount of protection be enough? Is it an acceptable risk if 7 of 67 workers exposed for 30 days to an moderately infectious co-worker become infected? Again, we are not talking active disease, but exposure to drug susceptible TB, so they were given INH prevention and only the one secondary case occurred. Had this been Kwazulu Natal, South Africa, however, with XDR TB and many the workers
HIV-infected, suddenly the risk of infection becomes a serious death threat and primary prevention all the more critical.

The reason for these findings is that almost all interventions work by removing some proportion of what organisms remain in the air. In the case of building ventilation, each increase in ventilation removes fewer and fewer of the remaining organisms as they become more dilute.

Unfortunately, there is no lower limit of risk, just a lower probability of infection. This is the subject of the next of Dr. Mehtar's questions.

What about respirators?

In another paper (cannot locate the PDF now – will add as a resource shortly), Fennelly and I used the same approach to show that respirators/masks and ventilation (indeed any interventions) work together to reduce risk. If we reduce risk by half by ventilation by 2X increase, and had poorly fitting respirators with a protection factor of only 50%, the net effect is protection equivalent to 4 times the equivalent ventilation. This may or may not be adequate, depending on the risk (duration, susceptibility, drug resistance, etc).

While it may be difficult to double ventilation night and day in very hot or cold climates by natural or mechanical means, having respirators with protection factors closer to 90% should be possible if they are well designed, properly fit tested, and properly put on each time.

So, my long winded answer to Dr. Mehtar's great question is to strive for at least 90% protection from respirators.

I am out of words for the moment and you are likely out of time so I will come back to Dr. Mehtar's second great question on the infectious dose of TB, above, another time.

Other opinions?

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)

Dr. Mehthar also asks about the effect of covering the nose or mouth with a hand or tissue and I can respond to that one quickly.

There are no quantitative studies yet, but at the AIR facility in South Africa we are planning to study the effects of a surgical mask on patients on reducing transmission for sentinel guinea pigs who serve as surrogates for highly susceptible health care workers. In principal the hand, tissue and surgical mask all work the same - to block large respiratory particles before they are expelled into the air and evaporate into airborne infectious droplets. Dr. Richard Riley who developed the concept of large respiratory droplets evaporating into the dried residua (infectious droplet nuclei) believed very strongly in the effectiveness of simple cough hygiene measures, but there is no objective proof - yet.

The first paper on the use of surgical masks to prevent airborne transmission was for flu during the 1918 pandemic, and some evidence of efficacy was offered, although it would not meet current standards of evidence.

In conclusion, cough hygiene makes sense theoretically, clearly cannot offer 100% protection, but like administrative controls, ventilation, UVGI, and use of respirators, is part of an overall ...

Dr. Mehthar also asks about the effect of covering the nose or mouth with a hand or tissue and I can respond to that one quickly.

There are no quantitative studies yet, but at the AIR facility in South Africa we are planning to study the effects of a surgical mask on patients on reducing transmission for sentinel guinea pigs who serve as surrogates for highly susceptible health care workers. In principal the hand, tissue and surgical mask all work the same - to block large respiratory particles before they are expelled into the air and evaporate into airborne infectious droplets. Dr. Richard Riley who developed the concept of large respiratory droplets evaporating into the dried residua (infectious droplet nuclei) believed very strongly in the effectiveness of simple cough hygiene measures, but there is no objective proof - yet.

The first paper on the use of surgical masks to prevent airborne transmission was for flu during the 1918 pandemic, and some evidence of efficacy was offered, although it would not meet current standards of evidence.

In conclusion, cough hygiene makes sense theoretically, clearly cannot offer 100% protection, but like administrative controls, ventilation, UVGI, and use of respirators, is part of an overall infection control
plan.

Ed

Please refer to this link to view my detailed answer with referenced articles to this discussion: http://www.ghdonline.org/ic/discussion/in-room-recirculating-uvgi-air-disinfe...

In a nutshell, my answer to Dr. Mehtar's great question is to strive for at least 90% protection from respirators and expect 80% or so with a good respiratory protection program.