Note: This is part 4 of a planned 5 part series on sauna. Knowing my interest in indoor air quality an increasing number of people have been querying me about sauna ventilation so this a very quick draft of what I’ve learned so far. This as a living document and I will update it as I learn more.
Note: While much of this is applicable to all saunas regardless of heat source, some of the following is focused on electric heated saunas. Wood fire saunas function a bit differently and need somewhat different and potentially greater ventilation. Designing ventilation to attain a healthy CO2 level for bathers is applicable to all saunas.
IMPORTANT: I am not a doctor nor do I have any specialist medical or scientific training. I am a journalist. My job is to try to make complicated things more quickly and easily digestible – distill dozens or hundreds of hours of reading, research and interviews in to a 5-minute read.
If you put a plastic bag on your head and pull it tight around your neck, what will happen? After a few seconds you’ll start to feel a bit of confusion, then you’ll find it difficult to concentrate, you may feel dizzy or lightheaded, you’ll begin feeling like you’re suffocating (you are), you’ll pass out and eventually die.
What about something a bit larger? Perhaps being in a sealed up 6’x4’x3’ clear plastic box with a good book. Same as above except it will take a bit longer for our trip to heaven.
And a poorly ventilated sauna?
Those things we experienced above; the loss of our ability to think clearly, the feeling of suffocation, dizziness – are not due to lack of oxygen but too much CO2. It is too much CO2 in our blood that signals our autonomic nerve system to cause us to take a breath. We do need oxygen as well, but there is a huge amount of oxygen in the air so we nearly always get an abundance of oxygen with every breath. It’s getting rid of excess CO2 that’s the driver. (People in a sealed room will die from too high of CO2 levels before low O2 levels will begin to cause any negative effect.)
When we breathe in the partial pressure of O2 (pO2) in the air is greater than the pO2 in our blood and so O2 is transferred from the air to our blood. Similarly, going the other way, the partial pressure of CO2 (pCO2) in our blood is greater than that of the air so a bunch of CO2 passes from our blood to the air and is exhaled. The greater the pressure differential, the greater the amount of CO2 transferred with each breath and the lower the pressure differential, the less CO2 transferred per breath. Similarly, at high altitudes the percentage of oxygen is actually the same as at lower altitudes, it’s the lower partial pressure of it that causes us to get less per breath.
Our body likes to maintain a blood CO2 level of about 23 to 29 mEq/L (the actual amount isn’t important here but I wanted to provide some context). As CO2 exceeds that range we begin to experience physical and mental problems such as loss of our ability to concentrate or control our muscles and eventually we’ll begin to feel light-headed or nauseous. So, we want it to stay in that range.
Fresh outside air is comprised of 20.96% O2 and just 0.038% CO2 which is often expressed as 380 parts per million or ppm. (CO2 was once below 300 ppm based on analysis of glacial air pockets.)
We breathe in, a bunch of CO2 gets transferred from our blood to the air in our lungs and exhaled (and dispersed, taken up by plants, converted to O2, lather rinse repeat – a very very cool system). If all is going well the CO2 in our exhaled breath is 40,000-60,000 ppm – about 100x that fresh air we sucked in. The air we exhale then is now about 15.4% oxygen, but 5.6% CO2 (my actual). We’re getting rid of a lot of CO2 with each breath. And that CO2 is going in to the sauna… And then where?
If the air we breathe in has more than 380 ppm of CO2 (higher partial pressure) then there is less of a pressure differential to our blood so less CO2 from our blood gets transferred with each breath and our blood CO2 level rises. If the CO2 levels in a sauna (or gym, classroom, bedroom or wherever) are too high then we have more difficulty eliminating excess CO2 from our blood resulting in our brain and body not functioning so well. The following chart provides some idea of what we generally experience with what levels of CO2 in the air for lower levels of CO2.
Our physical and mental abilities likely begin to decline somewhat with any ambient level over 400 ppm but we don’t really see noticeable or measurable effects until higher levels. We begin to notice cognitive impairment at about 550 ppm, decreased athletic performance at about 600 and declining motor skills at 800. Sleep quality may begin to measurably decline at 500. If you have a typical poorly ventilated U.S. sauna then at the end of your last round try threading a needle as soon as you get out (this usually becomes more difficult at about 2400 ppm).
We are still learning about CO2. For a long time it was not considered worthy of study. Twenty years ago it was of no concern for levels as high as 10,000 ppm. Six years ago we began getting some of the first research data on cognitive impairment. Today we know that an increase to just 550 ppm has a negative affect on us. And knowing how CO2 affects cognitive ability, companies with knowledge workers are investing considerable amounts to provide fresher air.
Except for most cases of death, these effects appear temporary and so perhaps of little concern other than discomfort during our sauna. However, we do not know what the long term affects of higher CO2 exposure may really be. (We do however know that people who sauna often in well ventilated saunas are healthier, less likely to suffer cardio problems or dementia and live longer.)
A poorly ventilated sauna is in this respect no different than a bag tied around our neck. If stale exhaled CO2 laden air is not removed and replenished by fresh outside air then CO2 levels (and potentially the levels of a variety of VOC’s) will rise and our blood CO2 will rise along with it. Our sauna needs to breath just as we do. It is in effect an extension of our own respiratory system.
That 6’x4’x3’ clear plastic box we’re sealed up in above? That’s how much space each person has in an average sauna.
Our first and primary ventilation goal then is removal of excess CO2.
Löyly (pronounced low-lou) is not just the steam rising from the rocks as many in North America believe but is a combination of a number of elements.
“There is no shortcut to perfect löyly, it is always about stones and proper ventilation.”
– Jesse Hämäläinen, Narvi Sauna Heaters, Finland, 2012
“Löyly is the Purity, Temperature and Moisture Content of the air contained inside the sauna as well as its thermal radiation.”
– 1988 paper on sauna health benefits
For Finn’s, and anyone in Scandinavia, Löyly is sauna. And proper ventilation is a core element of Löyly. No proper ventilation, no Löyly, not sauna. And as we saw above, for good reason.
That 1988 paper went on to say “The purity of the sauna air is above all a factor contributing to the enjoyment of the bathing experience. The sauna air must not contain any obnoxious extent gaseous impurities, particles, or micro-organisms. The purity of the sauna air is ensured primarily by effective ventilation.”
Today we know a lot more about CO2 and why proper ventilation is so critical to the enjoyment of sauna. We also know that proper ventilation may be much more important than just the enjoyment part of sauna. And this not just for CO2 but for other impurities such as mold or bacteria that need to be dealt with. Proper ventilation is key to good sauna and this is exceedingly obvious if you read much about sauna in Finland or talk to anyone in Finland about sauna – they will always mention proper ventilation.
U.S. Saunas Are Not Sauna.
What most people in North America experience is not sauna but a warm room with bad air.
“90% of saunas in North America are bad. The other 10% are worse.”
– Board Members, Finnish Sauna Society
– Mikkel Aaland
There are three areas where U.S. sauna’s seem to consistently fall short:
Ventilation: My guess is that most people in the U.S. leave a sauna not because they’ve had enough heat and löyly nor benefited from heat and löyly, but because they’re beginning to feel like they’re suffocating from high CO2 levels – and mistaking that for heat exhaustion. Little or none of the enjoyable or health benefits, no löyly, and all of the bad of being closed up in a tiny room of stale air. That is not a very enjoyable experience.
Overall Temperature: The Finnish Sauna Society recommends temps of 85-100°c at the bathers heads and at least 65°c at their feet – for health, enjoyment and hygiene. To insure this they often place thermostats for electric sauna heaters where they will best approximate the temp at users heads – typically on a wall 12” below the ceiling and 12-20” to the right or left of the heater.
Someone at Underwriter’s Laboratories in the U.S. appears to have decided that the highest temp for U.S. saunas is to be 90°c – and not at bathers heads but at the hottest point (3” below the ceiling directly over the heater). The result is maximums of 70-80°c at bather’s heads, 50-75°c at the sitting bench and 35-70°c at the foot (lower) bench. These temps are much lower than recommended for health and enjoyment. Most concerning though is that these temps are too low to provide for very critical hygiene. Saunas are breading grounds for mold and bacteria, the high heat and resulting low humidity is needed to kill them. The bad air we experience in the U.S. is not just suffocatingly high CO2 but mold and other stuff allowed to grow because of limitations on proper temperatures. Lower temps may also cause some people to stay in longer and so exacerbate our CO2 problems. In their ignorance of sauna, this person at UL is likely making our saunas actually less safe and less healthy.
It’s also important that we remember to fully warm up our sauna’s because not doing this means that we miss out on important heat radiation.
Temperature Stratification: Sauna should ideally be about the same temp from head to toe. Traditionally this has been partially accomplished by having the foot bench at or slightly above the top of the rocks. The temperature differentials are much less above the top of the rocks (12°c in mine) than below (39°c in mine). New ventilation strategies such as mechanized exhaust below the benches are also improving on this.
A major sauna manufacturer wanted me to build just such a warm room with bad air. I and my dealer (thanks Devin and Jey) had to fight them every step of the way as they wanted the benches too low, overall temps too low and a ventilation system that doesn’t ventilate well resulting in high levels of CO2.
But it’s not just this one manufacturer. These problems are pervasive throughout North America. Many people in North America build saunas with zero ventilation and most of the rest with poor ventilation.
Do we want a Finnish Sauna or American warm room with bad air? I think that for most people in the US experiencing real lõyly, good pure sauna air, for the first time will be a revelation.
A Reference Point
One problem we face in North America is that good information on sauna is difficult to come by. Not just how to build a sauna or how to take a sauna but what sauna is and what sauna should be or feel like.
Much of the knowledge of how to build a good sauna, how it should feel and how to ventilate it is contained in the heads and experience of people who build saunas – in Europe. For many it’s knowledge passed down to them from those who came before and it’s a lifetime of experiencing proper sauna embedded deep within them. They know bad sauna air when they breath it. There is little written about how to do it properly and much of that is in Finnish, Swedish and German (I’ve become intimately familiar w/ translate apps). Unless you talk to these knowledgeable people directly and really, unless they build your sauna for you (because there are a lot of details to get it right), it is extremely difficult to do it right here.
This is compounded by changes to how saunas are built. Historically sauna’s were quite leaky and breathed well on their own. Just like with our houses though, sauna’s today are much more tightly sealed up than ever before and with that we’ve sealed out a lot of natural ventilation and löyly. Planned ventilation is more important than ever before. And this is perhaps compounded further by the U.S. being behind on building/space ventilation in general.
And made worse by diagrams on the internet that show hoped for airflows that are simply impossible with actual physics.
And worse yet – our being unfamiliar with what true sauna should feel like. Our lack of a reference point embedded deep in us from a lifetime of real sauna. We think we’re experiencing sauna… Until we do.
“I took 50 saunas in 12 days in Finland, and I was never dizzy.”
Glenn Auerbach, saunatimes.com, 2019.
Those who’ve only experienced sauna in North America think that’s it. That’s their reference point. They think that the feeling of suffocation they experienced is the heat. They think it’s normal. But it’s not. It’s not normal and it’s not the heat. And they don’t know that until they experience proper sauna with proper ventilation.
Even for those of us who travel to and from Europe frequently (Ex-Covid) I think it is impossible to have a real reference point, a gauge of a sauna here vs one in Finland. After saunas there we spend hours on a plane with not so great air, we live in houses and work in offices that are poorly ventilated and have too high levels of CO2 and then we get in a sauna and the feeling of suffocation seems normal. It’s not. Glenn’s comment is about as good and accurate as we can get and he’s been experiencing and building saunas for 30 years.
Sauna in a properly ventilated sauna gets better and more refreshing with each round. That’s different than what many people in America have told me that they experience.
In considering ventilation it occurred to me that the problem is not how to ventilate, but what we’re aiming for. What are we trying to accomplish? What’s our goal?
Without objective measures I don’t believe it is possible for us to build saunas in North America properly.
What follows is not a proscription for how to ventilate a sauna. Because right now I don’t know what that is. I’ve searched and, at least in English, there is a lot more inaccurate and bad information than good. I also don’t believe there is necessarily a single best way to accomplish it. It can successfully vary from one sauna to the next and this is part of why people say that each sauna has its own soul.
Rather, this is focused on better understanding what we are trying to accomplish – what is good air? What is the purity element of löyly? And how can we know. How can we have a reference point so that we get it right? So that we don’t build a sauna and use it for five years and then experience a proper sauna and realize how bad ours had been. How can we do it correctly from the start?
Some aspects of löyly cannot be measured. Some bits come from the soul of each individual sauna. One element, moisture content, is perhaps purely personal preference. But two elements, temperature and air purity, can and should be measured. They are definitive and air quality much more so than temperature. There is considerable personal preference in temperature but air quality is either good or bad – our body doesn’t care what our personal preference or opinion is. How we accomplish them is a separate discussion. Let’s first figure out what we are trying to accomplish.
Some CO2 Measurements
Over the past 6 months I’ve done informal testing in our sauna with my GasLab meter. This is a 6 person sauna of 12.6 m3 (444 cubic ft) or about 2 m3 (74 cubic ft) per person. It’s heated by a 9kw Tylö-Helo Himalaya. This image and the one at the beginning should give you a rough idea of the setup. My meter is at 1m above the sitting bench, approximately where a friend would be sitting and what is the generally accepted standard for temp measurements. (A friend would be much more enjoyable but with Covid we must make do and the meter is it.) Measurements from the floor (black tile) are 19” to the standing platform + 18” to the foot bench + 18” to the sitting bench + 43” to the ceiling. Due to height constraints we were unable to get the foot bench to above the rocks as we’d wanted. You can see the exhaust vent in the ceiling.
1) With my current venting (traditional Finnish/European venting of 4” gap below the door, convection vent to outside in the opposite corner ceiling) and just me in the sauna – the CO2 rises from an initial 397ppm to 651 ppm after 15 minutes. At 20 minutes it was 704 and 30 minutes 783.
That was just me alone and measured at the seating positions with the best airflow. With six people in there for 20 minutes the increase in CO2 would be about 1842 ppm resulting in an ambient of 2239 ppm (and higher for seating positions farther away from the exhaust vent).
2) When I repeated this with a venting solution recommended by Tylö-Helo U.S. in their manual the CO2 levels rose faster. From 400 at the start to 873 after 20 minutes and 1082 after 30 minutes. With six people then we’d expect the rise in CO2 after 20 minutes to be 2838 for an ambient CO2 level of 3235 ppm. This by the way is nearly the same result as when I closed off venting entirely, which isn’t surprising as the design of their ventilation solution would be expected to result in no effective ventilation. Again, that is with just me in a 6 person sauna.
These levels are all much too high and will have many bathers feeling suffocated and lightheaded.
It’s also important to note that the CO2 levels will often decline slowly. If you do rounds of 15 minutes in + 15 minutes out then the CO2 level will likely not return to 400 ppm before you start your next round. It will likely be higher when you start round 2 than when you started round 1. And a bit higher yet starting round 3. And so on.
With just me alone I saw cumulative increases of about 80 ppm per round. Four of us would be an increase of about 320 ppm per round or 960 after 3 rounds. Better ventilation will significantly reduce or hopefully eliminate this cumulative effect.
For comparison, a sauna of 2 cubic meters per person, 6 air changes per hour and good air distribution/mixing (E.G., a good Finnish sauna) with bathers spending 15 minutes in and 15 minutes out for 3 rounds calculates to CO2 levels of very roughly 582, 594 and 606 ppm. I say very roughly because I don’t know what the actual mix rate would be. I assume it is very high but we need a lot more tests to know for sure. Given my experience in Finnish saunas this seems about right. I felt better in Finnish saunas (≈600 ppm CO2) than in my current sauna alone (≈700-800 ppm CO2) and both of these better than our current sauna w/ multiple people.
Do we want a Finnish Sauna or American warm room with bad air?
Goals of Sauna Ventilation
1) First and foremost, removal of excess CO2 and replacement with fresh outside air.
2) Removal of all or as much of air impurities as possible. This includes a variety of VOCs, mold & bacteria and perspiration / body odor.
3) Removal of humidity and perspiration after the end of the days sauna.
4) Maintain fresh air during periods of non-use.
How Much Ventilation Is Needed?
Some? A lot? 15 CFM per person? 6 changes per hour?
If nobody ever told you what temp a sauna should be but only said ‘hot’, what is the likelihood you’d get it right? Or they said that you need 9kw of heat? Turn on that 9kw and leave it on? Air freshness is much more difficult to gauge than temp. I’m still surprised how often someone is told to keep windows open to reduce the CO2 levels in their bedroom at night and how surprised they are with how much better they sleep and how much better they feel the next day. Just as with our saunas, they’d no idea how bad their air was.
In addition to the amount of ventilation we also need to pay attention to the dispersion or mixing within the sauna to insure that all bathers get sufficient fresh air. We could have great air for people in a direct line between the supply and exhaust vents but poor air for those farther away. The ventilation plan recommended by my manufacturer would have resulted in much or all of the limited fresh air flowing across the floor to the exhaust vent without benefiting bathers. Even with powered exhaust – plenty of air but largely useless.
An Objective Standard: 500-700 ppm CO2 For All Bathers.
How can you know if you’re being affected by healthy heat or by bad air? If you’re leaving the sauna because you’ve received max benefit from heat and löyly or because your blood is suffocating from too much CO2?
Our body’s response to bad air is fairly objective. X level of ambient CO2 results in Y level of blood CO2 and produces Z effect on our cognitive and physical systems.
What we want is fresh air to breath – so let’s measure that.
Based on what we currently know, if CO2 at bathers faces is below 500 ppm then you are likely experiencing the full benefits of sauna heat and löyly. If CO2 is above 700 ppm then you are likely experiencing bad air, not heat and löyly. It’s as simple as that.
Good ventilation that maintains healthy low levels of CO2 applies to our homes (particularly bedrooms at night), offices, gyms, restaurants and other indoor spaces as well. It may be more important in a sauna because of the heat stress our bodies are going through but it’s still important elsewhere.
Why Measure CO2?
1) CO2 is itself undesirable at levels above 500 ppm. We want to know what it is so that we can keep it low.
2) CO2 is an indicator gas for ventilation in occupied spaces. If CO2 levels are kept low then we know that we are getting relatively good ventilation, that any VOCs or impurities present are likely being exhausted and kept at healthy levels.
3) It takes the guesswork out of ventilation. Good ventilation requires exhausting stale air, supplying fresh make up air and insuring that the air throughout the sauna is being mixed well so that all bathers have good pure air Löyly. It can be complicated and easy to miss judge. We can debate this or that ventilation scheme until the cows come home but until we measure it we really have no idea. Measuring CO2 makes this much simpler, it’s absolute, no guessing – If under 500 ppm then we’re good, if over 700 ppm then we’ve work to do.
4) It’s affordable and easy to measure. (Be careful to observe the maximum operational temperatures of various measuring devices).
How Much Ventilation Is Needed?
Enough to insure that we do not get excessive build up of CO2 in our blood and experience negative effects from it. Enough to insure that other air impurities are exhausted.
Ideally then a sauna should have ventilation that results in CO2 remaining below 500 ppm but up to perhaps 700 is likely acceptable. Above 700 means that bathers are being negatively affected by bad air and the higher the CO2 level the greater the negative affect on bathers so over 700 should be avoided if possible.
Focusing on CO2 gets us to where we want to be – fresh pure healthy air for bathers – Löyly. It eliminates problems of too little ventilation or poor dispersion. If we’re maintaining proper low levels of CO2 and proper heat then the rest should be taking care of itself. There’s no need to question if this ventilation scheme or that ventilation scheme does or does not work (except for those of us who enjoy exploring those things) – if it results in maintaining appropriate levels of CO2, which in turn should be exhausting other impurities, then it works.
Realistically, staying below 500 ppm may prove difficult. We may have to find a balance between air purity and temperature.
Aiming for 6 air changes per hour or 8 CFM’s per person can likely work in many cases so long as people use the proper vent configuration to insure good dispersion. However, a clear problem we have in North America is a lot of diagrams of vent configurations that even with proper airflow will not provide sufficient fresh air to bathers. A CO2 measurement eliminates this problem.
The Analyzed Sauna
I’ve studied the effects of indoor air quality and CO2 enough to know that getting this right in our sauna will make for a much more enjoyable and beneficial experience for me and guests and be well worth the investment of time and money. To better understand what’s happening I’m hoping to embark on a bit of further analysis. The goal is to better understand heat, fresh air ventilation and how they interact in a sauna. This will involve several elements.
Temp logger and probes. I have a two port logger and probes but I’d ideally want 6 or 8 probes so that I can consistently track temps across many tests and routine sauna sessions. Test points might include; 3” below ceiling directly over heater (UL thermostat location), 12” below ceiling & 12-20” to side of heater (EU thermostat location), 2 or more seating locations at bather’s head height (1m above seat), foot bench, some lower point. Air for ventilation has the potential to create temperature problems so it’s critical to monitor temps with various ventilation schemes.
Ideally I’ll have enough that they can be consistent through numerous tests and routine sauna sessions without moving any. Even so I’ll do some sessions with probes all along seating areas to better understand these differences and arrayed from ceiling to floor to better understand stratification.
These loggers are accurate to ±0.5°c which is much better than typical thermometers and thermostats that can be off by ±7°c or much more.
CO2 Logger and sensors. I’m currently using my handheld GasLab meter. It’s quite accurate but cannot log data when temps are over 60°c. And even though GasLab told me that it’s OK to use it in a sauna up to 99°c I’m a bit leary. And it’s only one measuring point.
I’m working with them on a setup of three sensors that will send data back to a logger located in a cooler environment such as the floor of the sauna or in the changing room. This will allow us to compare CO2 levels and thus fresh air ventilation at multiple locations. Mostly at head height along the seating areas but at some point also other parts of the room.
Smoke. There’s no better way to understand airflow (and lack of airflow) than smoke. I’m somewhat concerned about polluting my sauna though. That’s kind of funny when you think about the popularity of Smoke Sauna but airflow testing smoke is usually chemical based and I want to make sure I don’t create any long term odor/chemical problems so want to find a way to do this that does not leave any unpleasant or unhealthy after smell. Any ideas on this would be very welcomed. Dry Ice (CO2) sort of works but not very well.
Energy Use. This will come from IotaWatt and be correlated with various ventilation schemes. How much in energy use will better air quality cost?
CFM’s. I would like to know with some accuracy the actual CFM’s of exhaust. Measuring or logging this accurately on an ongoing basis may prove difficult given my duct layout but hopefully I can figure out something that is somewhat accurate.
Humidity. My industrial hygrometer can only work to 50°c. I may see if I can find budget to include humidity on one of the temp loggers. All of the inexpensive sauna hygrometers I’ve tried so far have been grossly inaccurate. This is, I think, very non-critical and more for curiosity. Two or three scoops of water on the rocks every so often produces a very comfortable level of humidity and I think that’s all we’re after.
Air Pressure. This is another fun to have for my own curiosity but not really necessary. Using a digital pressure differential manometer to measure pressure changes at various levels.
Maybe after Covid is over we can see about hauling some of this to other saunas to see how they perform.
Conclusion / Next Steps
Now that I’ve a good, measurable and attainable goal rather than something nebulous I can focus on doing it.
My goal is 500-600 ppm CO2 for all bathers which will make for a much more pleasant experience than the higher levels we currently have and be more similar to a properly ventilated sauna in Finland. I want to achieve this while maintaining sufficient and even temps throughout and with as little energy use as possible. So for me the next steps are some analysis to better understand sauna temps, air purity, and air movement with various ventilation schemes. Is keeping CO2 below 500 ppm a good and reasonable goal? Or is staying below 800 ppm difficult enough? What is a good goal for CO2 levels in a sauna?
I also want to better understand our physiology and how respiration works at 96°c vs 20°c and in a dry vs moist environment. Do our lungs transfer O2 and CO2 as effectively in a hot sauna? Dr. Jari Laukkanen is an expert on sauna health and I’ve a friend who’s an anesthesiologist at Mayo Clinic so hopefully more to come on this soon.
I would also like to better understand, besides bather comfort and enjoyment, what affects poor ventilation and high CO2 have specific to sauna. Does higher blood CO2 lower the affect of sauna on relaxing muscles after a workout? Or lower the affect on diminishing heart disease or dementia? Higher CO2 levels are likely to cause bathers to spend less time in the sauna due to a feeling of suffocation or dizziness, what affect does that have on health and other benefits? Or increased blood pressure, heart rate or heart rate variability?
In a sauna there is a bit greater air pressure near the ceiling than at the floor (stack effect, heat rises, etc.). Is it enough to make a difference in respiration? In theory we’d take in more O2 and eliminate less CO2, but how much? Does vent placement matter with this?
I want to better understand löyly. Some I can do here but mostly I require a trip to Finland this next winter! 🙂
Finally, thanks to numerous people whose knowledge, insight and good humor have improved my understanding and this article including Dr David Johnson, Risto Elomaa, Jarmo Lehtola, Kimmo Raitio, and several others.
Respiration (PDF): https://www.draeger.com/Products/Content/co2-measurement-bk-9097450-en.pdf