ASHRAE Journal:
ASHRAE Journal presents.
Allison Hambrick:
Every year, millions of families take summer vacation in their family automobile to explore new destinations, relax in natural beauty and forge stronger bonds with one another. Typically, during the summer, vehicles are driven by setting the air conditioning system in recirculation mode to maximize cooling. Carbon dioxide exhaled by occupants remains in the cabin during this mode. The cabin carbon dioxide concentration starts increasing while oxygen concentration starts to decrease. The carbon dioxide inhaled by occupants goes into the bloodstream and changes blood chemistry and negatively affects health. Factors including the number of occupants and the amount of time the vehicle is driven in recirculation mode can affect the driver's alertness and comfort. This topic is covered by Gursaran Mathur in his January 2025 ASHRAE Journal technical feature, Control Strategy to Limit Cabin Carbon Dioxide Concentrations in Cars.
Hello and welcome to this episode of the ASHRAE Journal podcast. My name is Allison Hambrick and I'm an assistant editor at the ASHRAE Journal. Today I'm joined by Gursaran Mathur to discuss the content of his January 2025 ASHRAE Journal article Control Strategy to Limit Cabin Carbon Dioxide Concentrations in Cars.
Gursaran, how are you today?
Gursaran Mathur:
I'm doing good, Allison, thanks a lot, and hope you are doing good too.
Allison Hambrick:
I am, thank you. Now can you tell listeners a little bit about yourself?
Gursaran Mathur:
Yeah. As you stated, my name is Gursaran Mathur and I've been working in the automotive industry for many years, 25 years to be exact. I'm currently working for an American OEM and before that I worked for a Japanese OEM and I've been working in the development for heat pump system, air conditioning system, advanced engineering, refrigerants, and also indoor air quality in this area for both ICE vehicles as well as EV vehicles. And before the automotive industry, I worked in the residential commercial air conditioning industry.
Allison Hambrick:
Great, thank you. What inspired you to write this article?
Gursaran Mathur:
Yeah, this is a long time back, actually 25 years back when I moved to automotive industry, I found out that there is no real standard for the auto industry for IAQ and thermal comfort. As we know that, IAQ we have a ASHRAE Standard 62 and for thermal comfort we have ASHRAE Standard 52, but there was no real standard for cars, automobiles, and we are spending more and more time in our cars because of our daily drives. So I realized that there's a need to study in this area, and this is the reason why I actually started working in this area. It's been about 20 years I've been working on the indoor air quality for vehicles.
Allison Hambrick:
Interesting. You did specify summer road trips as an example in the article. So how should the AC system inside a vehicle be operated for, for example, a summer road trip?
Gursaran Mathur:
Yeah, I think typically for summertime, we operate the AC system in recirculation mode because when you are running in recirculation mode, you are maximizing the cooling because you are using the same air which is coming out from the cabin, which is good for energy efficiency but at the same time, if you have four or five people sitting inside the cabin, the people are breathing oxygen and they are exhaling carbon dioxide. So the carbon dioxide level keeps on building up within the vehicle and people start inhaling that. So I think that is not good for the indoor air quality and also for the occupants' health. And this is the reason why it's kind of important that we need to limit the buildup of carbon dioxide within the cabin. And this could be done by adding a carbon dioxide sensor within the cabin. And if the limit goes beyond a certain value, we can actually start the AC system in outside air mode to bring down the cabin CO₂ levels.
Allison Hambrick:
Okay. Is there anything that should be different for electric vehicles?
Gursaran Mathur:
Electric vehicles, the biggest issue is during the heating time. During heating season, we need electric heating. So electric heating takes a lot of energy and when it's taking lot of energy for heating, it's a electric heater, it's a PTC heater, so it takes lot of energy to actually heat and actually it reduces the total driving distance. So I think it's critical not only actually when you're running the AC system in recirculation mode, it actually expedites heating during initial startup, but also it starts to build up moisture within the cabin, especially when you have two or three people sitting inside. So moisture level builds up and CO₂ level builds up. So you have window fogging and not only window fogging, but also the CO₂ level builds up, which is not good for the occupants' health.
Allison Hambrick:
So you mentioned earlier that there's not a specific standard that has to do with indoor air quality as it pertains to cars. How does ASHRAE Standard 62 come into play here?
Gursaran Mathur:
ASHRAE Standard 62 specifies the acceptable level of CO₂ concentration in the conditioned space and per ASHRAE Standard, if the CO₂ level is 700 ppm over the ambient conditions, it means when it's approximately 1100 ppm within the conditioned space, we need to bring in outside air to bring it down. So this is kind of important. The current ppm level for CO₂ on a global basis is about 422 ppm, which is about 0.04%. And when we are speaking here at the mouth, the CO₂ which I'm exhaling, the percentage varies from about 4 to 6%, which is about hundred times larger. And this is the reason why the CO₂ level builds up within the conditioned space. And so for ASHRAE Standard, if the ppm level goes above 1100, we need to introduce outside air. And this could be true for classroom, homes and automobiles.
And again, I still also have another requirement, which is a total flow rate requirement per person. It's 15 CFM per person, which is 0.43 meters cubed per minute, and assuming four people sitting inside the cabin in a vehicle, we have 60 CFM of airflow which needs to be brought from outside to the cabin. And in a typical midsize vehicle, the total airflow rate from the blower varies from eight to nine meters cubed per minute, which is about 280 to 320 CFM, and with an internal vehicle of about three meters cubed, which is about 97 feet cubed. So I think this is the critical reason why we need to use ASHRAE Standard 62 for vehicles.
Allison Hambrick:
Interesting. Okay. Next, would you care to discuss the physiological and bioeffluent effects of carbon dioxide on humans?
Gursaran Mathur:
Yeah, I think there've been a lot of studies done on this area. CARB, which is the California Air Resources Board, they have done a lot of studies on the physical activities for humans. And for a typical adult male, the lung capacity is about six liters per minute. That's the capacity on a given minute. And it depends on the activity. If the person is actually actively walking or doing exercise, the total volumetric flow rate which is coming out from the lungs, it increases and hence the CO₂ also increases. So essentially what we are doing is we are inhaling oxygen. Oxygen goes to the bloodstream through the lung, and then we exhale CO₂. In a typical environment right now in a vehicle, the oxygen level is about 21%, and when four people are sitting, and as a function of time, when we are breathing out CO₂, slowly we can see the CO₂ level starts to build up and the oxygen starts to go down.
And I think that's kind of very important to recognize that this is the reason why people starts to get dizziness, they starts to get dizziness. They also, again, as the ppm level goes up, they might experience headaches, they might experience symptom of dizziness, confusion at high level, and actually prolonged exposure to elevated CO₂ levels negatively affect comfort, alertness and overall health. And this is kind of very important, alertness, because the driver can make mistakes during driving and they can run into accidents, and there have been many accidents attributed to high levels of CO₂ in their bloodstream.
The other aspect of the IAQ within the cabin is the buildup of volatile organic compounds, which are the VOCs within the cabin. And this comes from the material within the cabin, for example, from carpet, from plastic parts and paints and adhesives. And majority of the people actually, they think this is a smell of a new car, but really it's really detrimental to the human health because we are inhaling the VOCs, which is affecting our health.
And the off-gassing of these VOCs actually becomes larger during summertime. It's a function of temperature. And we really need to avoid actually sitting in a hot car, in a brand new car. What we really need to do is if a car is new, and especially in a hot summertime, when we start the engine, we should roll down the windows so that in a few minutes all the VOCs are gone, then the family and the driver should sit inside, to make sure that these VOCs are not affecting the humans. And typically what happens in two to three years, these VOCs are gone. They almost become zero. They are never gone, but they're almost becoming zero or to levels of zero ppm or zero microgram levels.
Allison Hambrick:
We've talked a little bit about both what inspired you to write the article and the threat that carbon dioxide exposure poses to humans. Can you tell me a little bit more specifically about what kind of study you did for this project?
Gursaran Mathur:
Yeah. As I told you that I've been working in this area for a long time. So over the past 20 years, I've done actually testing on small cars, medium cars and SUVs, basically to measure the buildup of carbon dioxide within the cabin as a function of time. So I have done testing from 30 minutes to five to six hours of drive time. I've driven from here to Chicago to Toronto to measure these ppm levels during these drives. And locally, what I have done is actually I have driven on the same route many, many times. It's about 30 minutes drive. And the first drive I drove alone, then the second drive repeated the same drive, I took one more occupant, and in the third drive I actually added one more occupant, and so forth. So you can see the impact of adding more people during the same drive as a function of time.
So this was very important to understand. And 30 minutes is a typical drive time for an average Americans going to grocery store, going to post office, and so forth. And I also conducted wind tunnel test by injecting a known amount of CO₂ within the cabin with an average lung size of an adult. And not only on that in the wind tunnel, I did also control testing by adding physical people actually inside the cabin, adding one by one at the same location and same occupants to see the impact. And I wanted to make sure that sometimes you can see the impact of smaller, younger people versus larger people because of their lung capacities and I have noticed that. But it's very difficult to notice that. But I think if you do that test over and over, you can clearly see that difference.
Allison Hambrick:
So for this particular study, what kind of vehicle did you use and what were the testing parameters?
Gursaran Mathur:
I had used model year ‘20 production sedan, and the parameters which I used were the number of occupants, as I just discussed, vehicle speed. Vehicle speed has a big influence. So what happens is when I'm driving the vehicle at highway speed or at low speed, there's infiltration and exfiltration, or the body leakage from the vehicle. The body leakage comes from the doors, it comes from the door handles, it comes from the trunk.
So essentially what happens is the buildup of CO₂ happens within the cabin, but all these infiltration, exfiltration, it brings down the CO₂ level. It brings down to the level which is balanced by the infiltration, exfiltration and the amount of CO₂ buildup. So there is a balance. Then I also used blower speed by adjusting the blower speed, low blower speed to medium to high blower speed, to also to make sure that I have good mixing within the cabin so I can actually have a good record, good ppm measurement of the carbon dioxide. And also I added the recirculation mode as well. So I used partial recirculation to make sure that what is the minimum partial recirculation that I can still use for energy efficiency at the same time by maintaining ASHRAE Standard 62 limits for the CO₂.
Allison Hambrick:
Next question I had for you actually pertains to that. What is the minimum allowable partial recirculation factor to meet carbon dioxide criteria in cabins?
Gursaran Mathur:
So actually I did lots and lots of testing on different vehicles. Smaller, larger, medium vehicles, small SUVs, medium-sized SUVs. And I found out approximately 40%, it's about maybe 35% to 45% depending upon the vehicle, but if I take an average of 40% recirculation factor, that sufficed to meet ASHRAE Standard 62 requirements. So not only I did that, I also actually did simulation by actually develop a computer simulation program by adding the physics of this whole CO₂ being injected into the cabin and also infiltration, exfiltration, vehicle speed, number of occupants. And I was able to actually correlate that data very well. And also one other thing which I really should also mention is the age of the vehicle or the body seals also have a big influence on the buildup of CO₂. If you have a brand new vehicles, the body seals are very good. So the buildup of CO₂ is much more comparison to the same vehicle which is five year older. It means in five year older vehicle the body seals have deteriorated, so you have more infiltration, exfiltration, so your CO₂ level goes down at the same operating conditions.
Allison Hambrick:
What are some conclusions from the study?
Gursaran Mathur:
The most important thing which I really wanted to tell to the readers and also to understand myself, what is the minimum partial recirculation factor which I really need to actually determine that I'm meeting the ASHRAE Standard 62 to meet the CO₂ buildup. Actually, that also is extremely important for the drivers for maintaining their driver's alertness. The alertness is the most important thing because otherwise the people can actually get into accidents. So by using simulation data and also using experimental data, I systematically evaluated the impact of all the buildup in all the variables which I explained to you. And there was a very good agreement between the simulated and the experimental data. And by bringing in more than about 40% of the air from outside air, we are able to protect the driver, occupants from getting fatigued, resulting in an improved safety and driver's alertness.
And the vehicles operating at idle condition is the worst case for the buildup of carbon dioxide. The reason is because at zero vehicle velocity, there is no infiltration, exfiltration. And this is the reason there is a big buildup of carbon dioxide within the cabin. And the peak concentration is a function of operating parameters, number of occupants. It starts to go up initially and then it starts to level off when the inflation, exfiltration actually matches with the buildup, there is levels off.
The concentration levels are lowest at the highest vehicle speeds because of the infiltration, exfiltration. And what I also did was I imposed ASHRAE boundary condition and the door movement for the wear. Essentially what I'm saying is the door moves if the CO₂ level goes beyond 1100, door goes to recirculation mode or outside air mode, and if the ppm level goes below 400, it goes to recirculation mode.
So basically, essentially I'm trying to also maximize efficiency, but at the same time, I'm also trying to maintain 1100 ppm. So what I found out is with blowers intake, when it operates between three to six and a half minutes from four to one occupant, so with four occupants, it takes about three minutes to go from outside air mode to recirculation mode. And with one occupant, it takes about six minutes, which makes sense intuitively because one person actually takes long time for the buildup of CO₂. Based on all the information I think the simulation, 40% is really the optimum partial recirculation factor for all these studies which I have done.
Allison Hambrick:
Okay. And finally, what are some of your recommendations from the study?
Gursaran Mathur:
Yeah, I think this is very important. I think the ASHRAE Journal article came at the right time in January, and this was perfect time because people would review this article and when people start making their plans for their summer vacation trip with their families, for example, going on vacation with two adults and two children with manual AC system. If they have manual AC system, it means older cars. They really need to change the modes from recirculation to outside air mode and vice versa every 10 to 15 minutes to maintain that we are maintaining 1100 ppm. And actually I have done testing myself, so this is what I'm saying, about 10 to 15 minutes. Again, it could vary, seven to 10 minutes, again, depending upon the vehicle age and so forth and vehicle volume. But with the auto system, it automatically goes in outside air mode once we have raised comfort condition. So maybe it's not an issue.
But if we see in auto condition if it actually gets stuck in recirculation mode, we should manually override and we should go in outside air mode and we should do that switching. And this is kind of very important to ensure that we are maintaining the health for the occupants and also driver's alertness. And also my recommendation to the OEMs especially for EVs, that occupant safety is priority than energy efficiency. Hence, HVAC system should not be operated in recirculation mode for extended periods of time for gaining energy because not only it affects the human health, but sometimes it also builds up moisture within the cabin and it could affect window fogging.
Allison Hambrick:
Well, that brings us to the end of our time today, but did you have any final thoughts?
Gursaran Mathur:
Yeah, I think this is an extremely important area, especially in the automotive sector. I know we have standards for the homes. I'm sitting here and you are sitting in the offices, but we do not have standards. So I think all the work which I have been doing is being investigated by many, many suppliers, many, many OEMs, other OEMs than OEMs I've been working for. So I think it's getting more and more attention, and I hope we are able to develop a standard so that one day we can actually develop one thing for automobiles and which will be helpful to the automotive industry.
Allison Hambrick:
Absolutely. Well, thank you, Gursaran, so much for joining us today.
Gursaran Mathur:
Oh, thank you very much.
Allison Hambrick:
Of course.
And to our listeners, thank you for tuning into this episode of the ASHRAE Journal Podcast.
The ASHRAE Journal Podcast team is editor, Drew Champlin; managing editor, Linda Rathke; producer and assistant editor, Allison Hambrick; assistant editor, Mary Sims; associate editor, Tani Palefski; and technical editor, Rebecca Matyasovski.
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