Humidifying Captive Singers Breathing at High Tidal Flow rates >10 L/min

The reaction surprised me. My December CMT post about humidity effects on performance hall acoustics—an exotic topic and one that I addressed with some physics and a spreadsheet—has elicited quite a few emails. The post concerned the timbre and attenuation of sound as a function of distance from the stage. That is, my remarks were from the perspective of the audience and presenters and architects. But so far the comments and emails about the post have been almost entirely from singers, most of them objecting to my “Hypothetically [from an acoustics perspective], it would be wonderful to perform in very low-humidity air” remark. All of the commenters opine that relative humidity (RH) equal to 45% (the nominal target value that HVAC systems in most public buildings aim to maintain) is too low for their vocal health—their throats become dry, their vocal cords become inflamed. The commenters further say that even the higher RH range 55%-65% that I was arguing for (for acoustics reasons, not performer health reasons) in the previous CMT post is lower than they would like. [Absolute humidity (AH) is the amount of water vapor present in a gas. Relative humidity is the ratio of the absolute humidity to the maximum absolute humidity. Relative humidity is perhaps most important, as any deficit then becomes an evaporative ‘challenge’ to the tracheobronchial tree, which has to deal with the humidity gradient—100% RH deep in the alveoli in the lungs, and ambient room RH at the mouth and nostrils.]

I  agree with your comments, about preferring more humid air to sing in! I sing tenor myself! So I think, in this post let’s look at the health rationale for performance hall humidity that’s high. How high is compatible with the comfort and health of all the people in the hall, performers and audience members alike? Can architects do things to make performance halls safer and healthier for singers? Below I suggest some answers to those questions.


First, think about what’s different when you’re singing vs. when you’re not singing. When you’re singing, far less of the liters per minute of air that you breathe in and out goes through your nose. The nose has several functions: it warms, cleanses, and humidifies inhaled air, detects odors, and serves as a resonating chamber to modify the voice. Air enters through the external nares (nostrils) to the nasal cavity. The nasal cavity is divided by the nasal septum into right and left chambers called nasal fossae. The chamber inside the fossa is called the vestibule. The vestibule is lined with stratified squamous epithelium. The air flow over 3 pairs of structures: inferior, superior and middle nasal conchae. The conchae consist of mucous membranes supported by thin, scroll-like turbinate bones. The passage through the nasal cavity warms, moisten and filters the air.


But when you’re singing much of the air you inhale goes through your mouth, and less of the air goes through your nose than when you’re not singing. And the moist surfaces in your mouth and oropharynx have less surface area from which moisture can evaporate, compared to the moist surfaces on the conchae in your nasal passages. What’s more, the velocity of the air going past the moist surfaces in your mouth is greater and the time that the air has to pick up moisture from those surfaces is less than it would be in nose breathing. The net result is that the air reaching your vocal cords is far drier—less humid—when you are singing than it would be if you were not singing and predominantly nose-breathing. The rate of evaporation of moisture from your larynx and your lower respiratory tract is greater when you are singing than it would be at the same air flow rate in L/min by nose-breathing. This situation not only dehydrates the airway lumen, but also increases the effort required to produce sound, as Sivasankar and others have shown.


You can’t bring your shower or a rainforest to the concert hall. Or can you? Could an architect design passive humidifiers (a back-stage waterfall?; an on-stage ‘water feature’?) that would create a microenvironment capable of transferring enough moisture? No. Even if you wanted to do it, it would not be adequate. Passive transfer can’t get enough kg of water into the air, even in a small area, to cope with the losses as the air is mixed and exchanged by the HVAC system. ‘Active’ humidifiers (atomizers/nebulizers, ultrasonic foggers, steam-based systems, etc.) are needed.


On an architectural scale, the situation has a lot in common with the heat and moisture exchanger (HME) elements [for example, Tyco DAR Hygrobac, Pall BB2215, EdithFlex] that are used in intensive care unit (ICU) mechancial ventilators in hospitals. Active heat and moisture exchangers (HMEs) represent a simple and effective way to replace one of the most important upper airway functions: they retain the heat and moisture of expired air and return it to the inspired gases. As the nasal cavities normally play a very active role in this conditioning, bioengineers jokingly refer to HMEs as ‘artificial noses’. And the ability of any nose, natural or artificial, to prevent drying of secretions in the respiratory tract and drying of the respiratory mucosal surfaces depends on the delivered gas temperature and relative humidity. We look at this aspect in more detail below.


HMEs are made of cellulose or synthetic materials and have an active surface area that is about 2,000 cm². This provides absolute humidity (AH) levels of 30 to 35 mg/L (H₂O content) in the air going from the endotracheal tube into the patient’s trachea—an AH that is somewhat above the level required by the ISO-8185 and ISO-9360 standards, which specify performance guidelines for active humidifiers in medical mechanical ventilator circuits. Temperature of the air at the HME output into the ventilator breathing circuit is between 27°C and 34°C, depending on the device type and the flow rate. And, of course, the amount of moisture the air can hold is temperature-dependent: air at 20°C can only hold a maximum of 17.3 mg/L, whereas at 37 °C (normal core body temperature, 98.6°F) it can hold 43.9 mg/L. So the air at the output of a well-functioning HME is pretty close to saturated with water—a relative humidity of 80% or higher (in the upper right-hand corner [white cells] of the spreadsheet below). But as the air gets warmer in the respiratory tract of the patient, the relative humidity in that inspired air drops—to 70% or less in the trachea and mainstem bronchi. Click on this spreadsheet screenshot to download the spreadsheet and see the 4th-order statistical regression equations for the quantitative relationships between RH, temperature, and AH. What we’d be aiming for in an architectural / HVAC design is to take room air (about 20 °C) along the olive-shaded humidity “isobar” or “isopleth”(lower right-hand ellipse, arrow toward upper left-hand ellipse in the spreadsheet screenshot). The best we could possibly do is get the air to be about 35% RH when it hits your vocal cords.




In fact the flow-rate/moisture-transfer-effectiveness point made above about velocity and residence-time in the mouth-vs-nose breathing discussion above is also a very real issue in mechanical ventilator management and HME design. Moisture content (absolute humidity, AH) in mid-airway at 2 hours (ISO-9360) is a fuction of ventilator tidal volume, and, with typical ICU ventilator and typical HME-type humidification of ventilator circuits, we get values like these:

• V_t = 250 mL: AH = 34.4 mg H₂O/L @ 32°C
  
• V_t = 500 mL: AH = 33.6 mg H₂O/L @ 32°C
  
• V_t = 750 mL: AH = 33.1 mg H₂O/L @ 32°C
  
• V_t = 1000 mL: AH = 32.9 mg H₂O/L @ 32°C

Other things being equal, the greater the flow-rate through the HME per minute, the less moisture gets transferred to the air going through it—and the lower the AH in the air that gets into the respiratory tract, and the more moisture the respiratory mucosae will lose to the air.

The aim is to lose as little moisture by evaporation from the respiratory mucosal surfaces as possible—to maintain the mucociliary function of the cells in the respiratory tract that get rid of debris and micro-organisms; and to prevent injury to vocal cords, trachea, and oropharyngeal mucosa.


The biophysics and bioengineering principles that have been used for designing mechanical ventilators and humidification devices for very sick ICU patients might seem to be pretty remote from respiratory design considerations pertinent to healthy singers. But they are not. When you’re singing, the higher air velocity, larger tidal volumes, and less convoluted path (smaller average pathlength) and smaller surface area in the larger-caliber airway reduce moisture transfer into the inspired air, compared to breathing through your nose. Let’s say your concert performance is nearly 3 hours long, and your part has you singing nearly continuously. Physiologically speaking, that situation is actually not too different from what the air path and flow rates would be if you were intubated and on a ventilator without an HME, running on only dry room air for 3 hours. If you were on a ventilator for a 3-hour surgery, there’d surely be an HME or similar device maintaining the humidity in the air you breathe and keeping your respiratory tract moist. It takes very little time inspiring dry air to cause damage!

And look at the microscopic picture below! This shows the acute inflammation that comes with just 3 hours of breathing dry air. This is from Hirsch’s 1975 journal article, on the physiology of trachea mucus velocity and humidification in dog trachea. Obviously, you don’t get this sort of histologic data in humans, in autopsy pathology data or otherwise. The only way to get this kind of data is in experimental animal models, as Hirsch did. The amazing thing about this is how fast the onset of inflammation happens—the activation of the mast cells underneath the mucosal surface, the release of cytokines, the infiltration by polymorphonuclear leukocytes and other cells in the inflammatory response—over the course of only several hours exposure to dry air. Your subjective impressions as a singer (about your throat feeling injured or inflamed after performing in dry air) are dramatically confirmed by this microscopic photo!


This evaporative drying and inflammation process is, in fact, why proper humidification of ICU ventilators has been such a prominent topic over the past 30 years in critical care medicine. Keeping the respiratory tract from drying out is essential for preventing ventilator-acquired pneumonia (VAP) and avoiding injury to the respiratory tract. Heating and humidifying the inspired gas has for many years been an established standard of care for patients on mechanical ventilators.

Keeping the vocal cords and surrounding tissues from drying out is also important with regard to the biomechanics of phonation and musical sound production, as Verdolini and colleagues have shown.

A study by Tanner and coworkers examined whether any of several inhaled nebulized ‘mist’ products could help alleviate the effects of trans-oral breathing of dry air. The answer was, basically, ‘No.’ Whatever benefit they provide lasts only a few minutes—far too short a time to be of much help for a 2-hour performance.

So, if personal-use products won’t help, could systematically improving building designs help? Yes. Architects could design or retrofit performance halls to provide proportional zone humidity control. The humidity is sensed in each ‘zone’ and compared to a setpoint, similarly to what’s done with zone thermostats and temperature control for different temperatures in different parts of the building. If the humidity in the singers’ zone is below the preferred setpoint level, a control action is taken to add moisture in that zone. The humidity is sensed periodically or continuously. One or more hygrometer sensors in each zone and the hygrostat control circuitry enable precise and accurate control.

The hygrostat is typically a microprocessor-based device that implements a software algorithm-controlled feedback loop, and is capable of communications over a local area control network (LACN). The control sequences can expand on basic proportional humidity control and include ‘integral-derivative’ control. In this case, the integral (area under the measured humidity-time curve) is used to calculate the amount (kg) of moisture that the humidity has deviated from the setpoint. The control action is limited to avoid overshooting the setpoint and oscillations and delays in control response, as often occur with proportional control. ‘Derivative’ or ‘humidity rate-of-change’ control can be used for dynamic applications where the ambient conditions change relatively frequently and dramatically, due to local weather or other factors—very much in the same way as derivative temperature control is done. Derivative control measures the slope of the humidity trend, and it adjusts the parameters of the control algorithm to respond to the slope changes.

As one possible approach, architects could create forced-air active-convection ‘air curtains’ between zones—adding ducting to create thin, downward planar ‘jets’ of air traveling tens of centimeters per second, directed from the ceiling to the floor, to create and maintain separate zones or microenvironments—one for the stage and one for the audience? Too radical, you think? Well, air-curtaining is routinely done in industry—such active ‘air curtains’ are used in clean rooms for medical device and pharmaceuticals manufacture, for example. And they could be practical in studios, to create separate ‘zones’ (a high-humidity one for the area occupied by the singers, separated from other zones for audience or other performers). But such air curtains would not be very practical for performance hall stages with very high ceilings. And there would be tremendous problems with acoustic baffling and duct noise. The high air velocities required for effective ‘air curtaining’ are normally noisy—they are designed for industrial purposes where turbulent flow patterns and noise are not important design considerations. To design air curtains for large halls that would achieve quiet, near-laminar flow patterns and acceptable acoustics for chamber music would require huge effort and expense. It could be done—but it would be very difficult. I’m unaware of performance facilities where multi-zone humidity control has been done.

But passive-convective multi-zone humidity control is done all the time in zoos. The reptiles and other animals have the atomizers, nebulizers, ultrasonic foggers, etc., controlled by multiple hygrometer sensors to create suitable microenvironments for them. Do singers merit less attention from architects and HVAC engineers than snakes?
How high should the humidity target level be, to be of significant help to singers? Well, normal breathing through the nose adds about 75% of the total water content before the inspired gas reaches the larynx, whereas with mouth-breathing, inspired gas is only about 25% saturated above the pharynx. Returning to our ICU ventilator analogy for guidance in suggesting an architectural design value, we know that, for respiratory tract health, the inspired gases should be delivered to the endotracheal tube or tracheostomy close to saturation at body temperature (i.e., giving a minimum RH = 75% when warmed [by passage into the airways] to 37 °C, AH = 32 mg/L). The new-generation HMEs are capable of providing 30 to 35 mg/L AH at 27 to 34 °C.

The rate of moisture loss from respiratory tract mucosa is proportional to the difference (P_sat - P_a) in water vapor partial pressure at the respiratory tract mucosal surface temperature and the average ambient temperature in the airway (mixed bulk air, near the airway centerline), respectively. If the evaporation rate exceeds the moisture secretion rate by the glands lining the respiratory tract, then drying of the mucosal lining will occur. For example, in a cool room, P_a will be low enough that moisture can easily evaporate from the respiratory tract mucosa even if RH = 100%. In fact, you lose about 7 g of water per m² of body surface area each hour via normal breathing at normal body temperature. This evaporative loss of moisture from the respiratory tract is nearly independent of ambient temperature below 30 °C (because the temperature differential is large, P_a is small, and the (P_sat - P_a) differential is therefore large). There’s no way around it! Unless you’re singing in the shower or under a waterfall, you’re always losing moisture.

Even the most efficient HME also allows a net loss of heat and moisture from the respiratory tract. But an efficient HME reduces the rate of loss to a modest, sustainable figure that the body can “keep up” with, without the mucosal surfaces getting dry or injured. HMEs constructed out of hygroscopic materials outperform hydrophobic HMEs in terms of keeping the respiratory tract well-hydrated. A singer-friendly architect’s design and HVAC engineering design would be ones that deliver air to zones that singer’s perform in such that the singers’ respiratory tract mucosal glands can “keep up” with the evaporative losses without getting dry or injured.

Assessing the adequacy of humidification is difficult. Excessive humidification may cause an increase in respiratory secretions volume, and insufficient humidification may result in a decrease in secretion volume because mucus becomes encrusted in the airways and retards further secretion. In my experience, there’s never “too much” humidity when it comes to singers’ vocal comfort and health. The biggest impediments to optimizing room humidity for singers are that (a) the comfort of others and the environmental requirements for some musical instruments (e.g., strings) entail lower RH, in the 45% to 65% range, and (b) the existing HVAC systems don’t have the necessary humidifier capacity to go above this range. Ideally, though, I believe we singers would want RH in the 65% to 70% range, if one of the HVAC approaches mentioned above could fit within building budgets. Secondly, we’d prefer that the performance hall temperature be set higher than normal, maybe 23 °C or so. That way, the air at RH 65%-75% will have a higher AH, will carry more moisture, resulting in higher RH values in our vocal tracts. But first, you need to find architect/engineering firms that’re ready to think a bit “outside the box,” to begin to serve the needs of singers.

Thanks again to all of you who emailed me, for your comments and questions!

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