Fresh Air and Health: Why Indoor Air Can Make You Tired
What happens to the air in a closed room hour after hour, what CO2 might have to do with your thinking, and why airing out your room and taking a walk are more than everyday wisdom.
"Open a window" sounds like something your grandmother would say. I see something else in it: Air is a natural force you breathe in around 20,000 times a day. Its quality might help determine how clearly you think.
It's 3 p.m. You've been sitting at your desk since noon. The door is closed, and so is the window. You're reading the same paragraph for the third time. Your head feels like it's wrapped in cotton wool.
You reach for a coffee. Maybe something sweet. And you think: I'm just tired. That's normal in the afternoon.
Maybe that's true. But maybe it's also the air around you. In this article, we'll look together at what research shows about fresh air and health. We'll talk about CO2, about ventilation, about studies with surprising and contradictory results. And about why a walk may change your afternoon slump.
I know this pattern very well. People working from home, often in a small study, describe a slump that arrives at almost the same time every day.
Before we talk about air, we check the obvious medical causes, for example iron levels and thyroid, and ask about sleep and snoring. If this baseline is unremarkable, we look at everyday life. A simple CO2 meter on the desk then sometimes shows that the level in the afternoon is well above 1000 ppm.
The turning point is often unspectacular: regular burst ventilation and a short round outside after lunch. Some report after a few weeks that the slump comes less often and feels weaker. I can't claim causality, but I document the temporal connection. After all, sleep, stress and workload also change during this time.
The lesson: Sometimes it's worth looking not only inside yourself, but also around you.
Are you really short of oxygen in stuffy rooms?
Do you know the feeling of walking into a room where people have been sitting for a long time? The air feels "used up". Most people think of missing oxygen.
It's worth taking a closer look here. In a normal home or office, breathing lowers the oxygen content only minimally. What rises noticeably, on the other hand, is carbon dioxide, or CO2 for short. Outdoors it is around 400 ppm. That means 400 parts per million parts of air.
Imagine the air in a room as an aquarium. Every person in it releases something into the water with every breath: CO2, moisture, odours, tiny molecules from skin and clothing. Add to that emissions from furniture, paints and cleaning products. Without a water change, the aquarium gets cloudier hour by hour.
The ad hoc working group on indoor air guide values at the German Environment Agency (Umweltbundesamt) describes CO2 in exactly this way: as a lead parameter for the total amount of substances people release into a room. When CO2 rises, everything else usually rises with it.
Fresh air does not primarily mean "more oxygen". It means swapping used air for new air. CO2 is more like the gauge on the dashboard than the only engine of the problem.
And now you know why the word "oxygen deficiency" usually falls short when it comes to stuffy rooms.
What do studies show about CO2 and concentration?
May I ask you an uncomfortable question? How often do you judge your afternoon performance without taking your surroundings into account? Scientifically, this question is surprisingly young.
Usha Satish and a team led by William Fisk at Lawrence Berkeley National Laboratory exposed 22 adults in an office-like room to CO2 levels of 600, 1000 and 2500 ppm. Ventilation and temperature stayed the same, the additional CO2 was introduced in pure form, and neither participants nor test staff knew the condition. At just 1000 ppm, six of nine areas of decision-making performance declined moderately, and at 2500 ppm, seven areas declined markedly.
What this means for you: CO2 might influence complex decisions even at levels that are common indoors. The authors emphasise that these findings need to be confirmed.
Satish U et al. Environ Health Perspect. 2012;120(12):1671-7. DOI: 10.1289/ehp.1104789A few years later, a group at the Harvard T.H. Chan School of Public Health went even further. They wanted to know what a whole working day in different air changes.
Joseph Allen and colleagues had 24 people work for six full working days in a controlled office. They simulated a conventional building with many volatile organic compounds, a "green" building with few of them, and a green building with additional high fresh air supply. On the green air days, cognitive test scores were on average 61 percent higher, and with additional fresh air 101 percent higher, compared with the conventional day. CO2 and volatile organic compounds were each independently associated with the test scores.
What this means for you: It's not just one substance that counts. The mix of ventilation, CO2 and emissions might together influence how clearly you think.
Allen JG et al. Environ Health Perspect. 2016;124(6):805-12. DOI: 10.1289/ehp.1510037And now you know why researchers now seriously study indoor air as a performance factor.
Is CO2 itself the problem? The evidence is mixed
This is where it gets interesting, and honestly a little uncomfortable. Because not every study confirms the Berkeley results.
Xiaojing Zhang, Pawel Wargocki and colleagues at the Technical University of Denmark cleanly separated two things. In some conditions, they added pure CO2 up to 1000 or 3000 ppm, with otherwise very good ventilation. In others, they reduced the fresh air supply so that CO2 and all other human bioeffluents rose together. Pure CO2 showed no measurable effects. The bioeffluents at 3000 ppm, by contrast, worsened perceived air quality, increased headache, fatigue and sleepiness, and slowed some thinking tasks.
What this means for you: Possibly it's less the CO2 itself and more what accompanies it in the room. For your everyday life, this changes little: in both cases, airing out the room is the lever.
Zhang X et al. Indoor Air. 2017;27(1):47-64. DOI: 10.1111/ina.12284How does this fit together? A review by Bowen Du and colleagues from Toronto analysed 37 experiments. In the methodologically strongest studies, pure CO2 at everyday concentrations mainly affected complex decisions, measured with one specific test. Poorer ventilation overall, on the other hand, could reduce the speed of various thinking functions without worsening accuracy.
Contradictory studies don't mean the topic is unimportant. They show that we are still looking for the exact cause. The practical consequence stays the same: regularly exchange used air.
And now you know why a CO2 reading is a useful indicator, even if CO2 may not be the only reason for your slump.
Ventilation, wellbeing and learning: What large analyses show
Do you remember classrooms at the end of a long school day? Warm, cramped, windows closed. That's exactly where a lot of research has been done.
As early as 1999, Olli Seppänen, William Fisk and Mark Mendell summarised the literature on office and institutional buildings. More than 40 studies, each with around 30,000 people, were included. Almost all studies found that ventilation rates below 10 litres per second per person were associated with more complaints or air perceived as poorer. For symptoms of so-called sick building syndrome, such as headache, fatigue or irritated mucous membranes, the relative risks with low ventilation ranged from 1.1 to 6.
Ulla Haverinen-Shaughnessy and colleagues at the University of Tulsa estimated ventilation via CO2 measurements in 100 fifth-grade classrooms in the southwestern United States. 87 of the 100 classrooms were below recommended ventilation rates. For each additional litre of fresh air per second per person, the proportion of children passing standardised tests rose by about 2.9 percent in maths and 2.7 percent in reading.
What this means for you: Even small differences in ventilation might become measurable over weeks and months. A cross-sectional study, however, shows associations, not causes.
Haverinen-Shaughnessy U et al. Indoor Air. 2011;21(2):121-31. DOI: 10.1111/j.1600-0668.2010.00686.xIn a follow-up study with 140 classrooms and 3,109 children, the same group showed a similar association for maths. Interestingly, temperature also played a role. Each degree lower in the range between 20 and 25 °C was associated with slightly better results.
And now you know why airing out and a room temperature that isn't too warm should be considered together.
Bedroom air: The underestimated part of your day
You spend a third of your life in a room with the door closed. How fresh is the air there at four in the morning?
Peter Strøm-Tejsen, Pawel Wargocki and colleagues changed the night-time ventilation in student rooms for one week, either via an open window or a silent fan. With poor ventilation, CO2 averaged around 2400 to 2600 ppm, and with good ventilation about 660 to 835 ppm. With better ventilation, sleep quality measured by wristband, the perceived freshness of the air, next-day sleepiness and concentration, and a test of logical thinking improved.
What this means for you: Your afternoon slump might also begin the night before. The studies were small, but the approach is easy to test in your own daily life.
Strøm-Tejsen P et al. Indoor Air. 2016;26(5):679-86. DOI: 10.1111/ina.12254And now you know why a window opened a crack or an open door at night may be worth a try, if noise and cold allow it.
Fine particles, cooking and dry heating air
Indoor air is more than CO2. Two everyday sources are often overlooked: the kitchen and the heating season.
Frying produces many fine and ultrafine particles. In a Korean measurement study in 13 homes, researchers fried 100 grams of pork for just nine minutes. Without ventilation, the median fine particle concentration over two hours was 4.5 mg/m³, and with the range hood running 0.5 mg/m³. A measurement study by the US National Institute of Standards and Technology also showed that a range hood can reduce ultrafine particles from a gas stove and oven by about 31 to 98 percent, depending on its power. The rear burners benefited more.
In winter, dry heating air is added to the mix. Peder Wolkoff of the Danish National Research Centre for the Working Environment describes in a review that very low humidity may be associated with irritated eyes and a feeling of "dry air". Slightly higher humidity might have a favourable effect on perceived air quality, eye complaints and possibly also work performance. Rooms that are too humid, on the other hand, encourage mould. Burst ventilation exchanges both: used air and excess moisture.
Your home is not a neutral place. It's a small ecosystem that you help shape with every breath, every meal and every heating season. That's not a burden, but room to act.
And now you know why cooking with a range hood and airing out after cooking may contribute to a good indoor climate.
The four KPNI lenses: How air touches your whole system
In Clinical Psychoneuroimmunology, or KPNI for short, we don't just ask whether your head is tired. We ask which systems are interacting. Air touches all four.
Nervous system
The brain reacts very sensitively to the chemical balance of the blood. Even small shifts in CO2 can influence blood flow to the brain. Whether this plays a role at everyday indoor levels is still open. What is clear: in studies, thinking speed and concentration respond to ventilation.
Immune system
Fine particles and irritated mucous membranes can trigger local defence reactions in the eyes and airways. In Seppänen's review, poor ventilation was also associated with more respiratory illness, with relative risks of 1.5 to 2.
Metabolism
Concentration is energy work. When the brain works more slowly under unfavourable conditions, many people reach for coffee and sugar. Fresh air and movement might be part of the answer before the next snack comes.
Hormonal system
Daylight outdoors is the strongest timekeeper for your internal clock, which regulates cortisol in the morning and melatonin in the evening. A walk gives you both: air and light. In a pilot study, office workers without windows reported less vitality and poorer sleep.
This is exactly what makes this topic so valuable to me. It's not about a device or a rule. It's about clarity of mind, about energy for the people and things that matter to you. Energy is not a luxury. It's a piece of freedom.
And now you know why fresh air is considered a resource in KPNI, not a side issue.
Why a walk may change your afternoon slump
Imagine two scenarios. In the first, you stay seated, open the window and carry on. In the second, you go outside for 20 minutes. Both bring fresh air. But only the second brings more with it.
Gregory Bratman and colleagues from Stanford sent healthy adults on a 90-minute walk, either through a natural environment or along a busy road. After the nature walk, participants reported less rumination. At the same time, activity was lower in the subgenual prefrontal cortex, a brain region associated with rumination. This was not seen after the urban walk.
What this means for you: It's not only the air but also the place that counts. A path through greenery might offer your mind a different kind of break than a trip to the supermarket.
Bratman GN et al. Proc Natl Acad Sci U S A. 2015;112(28):8567-72. DOI: 10.1073/pnas.1510459112Marily Oppezzo and Daniel Schwartz, also from Stanford, examined in four experiments what walking does to creativity. While walking, 81 percent of participants scored better on a test of creative idea generation than while sitting, and this boost persisted for a short time afterwards. Walking outdoors produced the most original results.
A systematic review by Jo Thompson Coon and colleagues from Exeter compared exercise outdoors in nature with the same exercise indoors. In eleven studies with 833 adults, outdoor exercise was associated with more energy, a greater sense of revitalisation and less tension. The authors emphasise, however, that the quality of the studies is low and larger investigations are lacking.
A break outside is not lost working time. It might be the time in which your mind gathers new clarity. Many good ideas don't arise at the desk, but on the way there.
And now you know why air, light and movement together may be a stronger lever than any one of them alone.
When should you air out the room? CO2 monitor and burst ventilation
The tricky thing about used air: if you're sitting in it, you hardly notice it. Your nose gets used to it. Only someone coming in from outside smells the difference.
That's why a measuring device can be useful. In 2008, the ad hoc working group on indoor air guide values of the Indoor Air Hygiene Commission of the German Environment Agency and the Supreme State Health Authorities published health and hygiene based guide values for CO2. It explicitly suggests using them as a kind of ventilation traffic light.
CO2 in indoor air: guide values according to the German Environment Agency (2008)
The value of about 1000 ppm is a guide value for ventilation, not a sharp health limit. Source: Ad hoc working group on indoor air guide values, Bundesgesundheitsblatt 2008.
How to air out your room sensibly
- Burst ventilation instead of tilting: Open the windows wide for a few minutes. Tilted windows exchange little air and in winter mainly cool down the wall.
- Cross ventilation, if possible: Two opposite windows, or a window plus an open door, create a draft and a faster air exchange.
- Rhythm instead of chance: How often you should air out depends on room size and the number of people. A CO2 monitor shows you how quickly the level rises in your room.
- Device with a real CO2 sensor: Look for a sensor that measures CO2 directly, often labelled NDIR. General "air quality sensors" often only estimate CO2.
- After cooking and showering: Use the range hood and then air out deliberately to get rid of particles and moisture.
And now you know that a small display on your desk can be a surprisingly honest mirror of your indoor climate.
When the tiredness stays: Please don't just blame the air
As valuable as fresh air is, it's one building block, not a diagnosis. I don't want you to overlook a serious cause because you open the window and wait for things to improve.
Persistent tiredness can have medical causes that can be investigated well. These include, among others:
- Iron deficiency: even without anaemia, especially in women with heavy periods or with a plant-focused diet.
- Thyroid disorders: an underactive thyroid can show up gradually with tiredness, feeling cold and concentration problems.
- Sleep apnea: loud snoring, pauses in breathing, a dry mouth in the morning and daytime sleepiness are warning signs.
If airing out, light and movement change little over several weeks, or if the tiredness clearly limits you, talk to your doctor.
Three levers you can use starting today
- Air out your room in the rhythm of your day. A few minutes of burst ventilation before starting work, before the afternoon and after cooking. If you like, a CO2 monitor can show you whether you're staying in the green zone.
- Go outside after lunch. 15 to 20 minutes of walking, through greenery if possible. That way you combine air, daylight and movement in one break.
- Take a look at your bedroom. Air it out thoroughly before going to bed and try whether a window opened a crack or an open door changes your morning.
And now you know why "open a window" is a small gesture with a surprisingly well researched foundation.
Frequently asked questions about fresh air and health
Why does stale indoor air make you tired?
In a closed room, exhaled CO2, human bioeffluents, volatile organic compounds and particles build up with every hour. In chamber studies, poorly ventilated conditions were associated with more headache, fatigue, sleepiness and slower thinking. CO2 mainly serves as an indicator of overall air exchange. Which component contributes how much has not yet been fully clarified.
Is there a lack of oxygen in closed rooms?
In normal homes and offices, breathing lowers the oxygen content only minimally. Tiredness in stuffy air therefore probably has little to do with a lack of oxygen. Research focuses on the rise in CO2 and the accumulation of other substances from exhaled air, skin, furniture and cooking. So fresh air mainly means exchanging used air.
At what CO2 level in ppm should I air out the room?
The ad hoc working group on indoor air guide values at the German Environment Agency (Umweltbundesamt) rates CO2 below 1000 ppm as hygienically harmless, 1000 to 2000 ppm as hygienically noticeable and above 2000 ppm as hygienically unacceptable. Above 1000 ppm, the room should be aired out; above 2000 ppm, it must be aired out. The working group explicitly suggests using these values as a kind of ventilation traffic light.
Does CO2 alone impair concentration?
The evidence here is mixed. In a study by Satish and colleagues, decision-making performance declined at just 1000 ppm of pure CO2. In a Danish study by Zhang and colleagues, by contrast, pure CO2 up to 3000 ppm had no measurable effect, while human bioeffluents at the same CO2 level worsened performance and wellbeing. A review of 37 experiments concludes that pure CO2 might mainly affect complex decisions, while poor ventilation overall tends to affect the speed of thinking.
How do I air out my home office properly?
Burst ventilation has proven useful: open the windows wide for a few minutes, ideally with a cross draft through opposite windows, instead of leaving a window permanently tilted. In winter a few minutes are often enough, in summer it takes longer. How often depends on room size and the number of people. A CO2 meter can show you how quickly the level rises again in your room.
Is a CO2 monitor worth it for your home?
A CO2 monitor can be a useful tool, because we are poor at noticing stale air ourselves, especially when we have been sitting in the room for a while. It shows you when the guide value of 1000 ppm is exceeded. What matters is a device with a real CO2 sensor (often labelled NDIR), not just a general air quality sensor.
Does bedroom air affect sleep?
In a Danish intervention study with students, the bedroom CO2 level with the window closed averaged around 2400 to 2600 ppm, and with better ventilation about 660 to 835 ppm. With better ventilation, objectively measured sleep quality, the perceived freshness of the air, next-day sleepiness and performance on a logical thinking test improved. The groups were small, but the finding is plausible.
Why might a walk help with the afternoon slump?
A walk combines several stimuli: fresh air, daylight, movement and a change of scenery. In experiments from Stanford, walking increased creative idea generation, and walking outdoors produced the most original results. In another study, a 90-minute nature walk reduced rumination and the activity of a brain region linked to rumination. This may explain why you can often feel clearer after a round outside.
Can tiredness have causes other than poor air?
Yes, and this is important. Persistent tiredness can have medical causes, for example iron deficiency, a thyroid disorder or sleep apnea. If airing out and movement change little over several weeks, or if the tiredness clearly limits you, it should be checked by a doctor. Fresh air is one building block, not a substitute for diagnostics.
Is dry heating air a problem?
A review by Peder Wolkoff describes that very low humidity may be associated with irritated eyes and a feeling of dry air. Slightly higher indoor humidity might have a favourable effect on perceived air quality and eye complaints. Air that is too humid, on the other hand, encourages mould. Regular burst ventilation therefore also makes sense in winter, because it exchanges moisture and used air at the same time.
How this topic connects with others
Air is closely linked with light, movement, sleep and the medical causes of tiredness. Here are the connections I consider most important.
More articles from the Natural Forces Guide
- Forest bathing (Shinrin-Yoku): What happens in the body when we go into the forest
- Forest bathing for stress and exhaustion
- Chronobiology: How morning light regulates the internal clock and sleep
- Breathing techniques for the nervous system
- The 4-7-8 breathing technique for falling asleep
- Grounding: Walking barefoot and earthing
- Sun, serotonin and mood
- Winter depression and lack of light
- How much sun does the body need?
- Vitamin D deficiency: why the sun isn't enough in winter
- Heat therapy and heat hormesis
- Sauna and the heart: How often is healthy?
- Does cold strengthen the immune system? What hormesis means
- Cold showers in the morning
- Ice bathing for beginners
- Contrast showers and Kneipp in everyday life
- Red light therapy and photobiomodulation
- See all articles in the Natural Forces Guide
Sources
- Satish U, Mendell MJ, Shekhar K et al. Is CO2 an indoor pollutant? Direct effects of low-to-moderate CO2 concentrations on human decision-making performance. Environ Health Perspect. 2012;120(12):1671-7. DOI: 10.1289/ehp.1104789 [RCT-like design: blinded chamber study with balanced order, n=22]
- Allen JG, MacNaughton P, Satish U et al. Associations of Cognitive Function Scores with Carbon Dioxide, Ventilation, and Volatile Organic Compound Exposures in Office Workers: A Controlled Exposure Study of Green and Conventional Office Environments. Environ Health Perspect. 2016;124(6):805-12. DOI: 10.1289/ehp.1510037 [RCT-like design: blinded controlled exposure study, n=24]
- Zhang X, Wargocki P, Lian Z, Thyregod C. Effects of exposure to carbon dioxide and bioeffluents on perceived air quality, self-assessed acute health symptoms, and cognitive performance. Indoor Air. 2017;27(1):47-64. DOI: 10.1111/ina.12284 [RCT, n=25]
- Du B, Tandoc MC, Mack ML, Siegel JA. Indoor CO2 concentrations and cognitive function: A critical review. Indoor Air. 2020;30(6):1067-1082. DOI: 10.1111/ina.12706 [Review, critical review of 37 experimental studies]
- Seppänen OA, Fisk WJ, Mendell MJ. Association of ventilation rates and CO2 concentrations with health and other responses in commercial and institutional buildings. Indoor Air. 1999;9(4):226-52. DOI: 10.1111/j.1600-0668.1999.00003.x [Review, 41 studies, each with around 30,000 people]
- Haverinen-Shaughnessy U, Moschandreas DJ, Shaughnessy RJ. Association between substandard classroom ventilation rates and students' academic achievement. Indoor Air. 2011;21(2):121-31. DOI: 10.1111/j.1600-0668.2010.00686.x [Real-world, cross-sectional, 100 classrooms]
- Haverinen-Shaughnessy U, Shaughnessy RJ. Effects of Classroom Ventilation Rate and Temperature on Students' Test Scores. PLoS One. 2015;10(8):e0136165. DOI: 10.1371/journal.pone.0136165 [Real-world, multilevel analysis, 140 classrooms, n=3,109]
- Strøm-Tejsen P, Zukowska D, Wargocki P, Wyon DP. The effects of bedroom air quality on sleep and next-day performance. Indoor Air. 2016;26(5):679-86. DOI: 10.1111/ina.12254 [RCT, field intervention study, n=14 and n=16]
- Wolkoff P. Indoor air humidity, air quality, and health: An overview. Int J Hyg Environ Health. 2018;221(3):376-390. DOI: 10.1016/j.ijheh.2018.01.015 [Review]
- Lee S, Yu S, Kim S. Evaluation of Potential Average Daily Doses (ADDs) of PM2.5 for Homemakers Conducting Pan-Frying Inside Ordinary Homes under Four Ventilation Conditions. Int J Environ Res Public Health. 2017;14(1):78. DOI: 10.3390/ijerph14010078 [Real-world, field measurement in 13 homes]
- Rim D, Wallace L, Nabinger S, Persily A. Reduction of exposure to ultrafine particles by kitchen exhaust hoods: the effects of exhaust flow rates, particle size, and burner position. Sci Total Environ. 2012;432:350-6. DOI: 10.1016/j.scitotenv.2012.06.015 [Real-world, measurement study in a test house]
- Bratman GN, Hamilton JP, Hahn KS, Daily GC, Gross JJ. Nature experience reduces rumination and subgenual prefrontal cortex activation. Proc Natl Acad Sci U S A. 2015;112(28):8567-72. DOI: 10.1073/pnas.1510459112 [RCT, randomised allocation to nature vs. urban walk, with imaging]
- Oppezzo M, Schwartz DL. Give your ideas some legs: the positive effect of walking on creative thinking. J Exp Psychol Learn Mem Cogn. 2014;40(4):1142-52. DOI: 10.1037/a0036577 [RCT-like design: four experiments with students]
- Thompson Coon J, Boddy K, Stein K et al. Does participating in physical activity in outdoor natural environments have a greater effect on physical and mental wellbeing than physical activity indoors? A systematic review. Environ Sci Technol. 2011;45(5):1761-72. DOI: 10.1021/es102947t [Systematic review, 11 studies, n=833]
- Boubekri M, Cheung IN, Reid KJ, Wang CH, Zee PC. Impact of windows and daylight exposure on overall health and sleep quality of office workers: a case-control pilot study. J Clin Sleep Med. 2014;10(6):603-11. DOI: 10.5664/jcsm.3780 [Real-world, case-control pilot study, n=49]
- Ad-hoc-Arbeitsgruppe Innenraumrichtwerte der Innenraumlufthygiene-Kommission des Umweltbundesamtes und der Obersten Landesgesundheitsbehörden. Gesundheitliche Bewertung von Kohlendioxid in der Innenraumluft. Bundesgesundheitsblatt Gesundheitsforschung Gesundheitsschutz. 2008;51(11):1358-69. DOI: 10.1007/s00103-008-0707-2 [Official document, announcement by the German Environment Agency]
The evidence on CO2 and concentration is mixed: some experiments found effects at just 1000 ppm of pure CO2, others only with poor ventilation involving many other substances. Many studies are small, observational or conducted under laboratory conditions. The German Environment Agency's guide values are hygienic reference values, not toxicological limits. The clinical observation described is a composite pattern and not evidence of cause and effect. This article does not replace individual medical advice.