The Unseen Danger of Carbon Dioxide

The Unseen Danger of Carbon Dioxide

In today's world, nearly all of us are aware of carbon dioxide and its impacts globally. Not only is it one of the most common gases in the world, but it's gained a lot of attention due to carbon dioxide emissions being one of the key causes of climate change.

Carbon dioxide is essential for life. Indeed, plants can't photosynthesise without it, and it's one of the building blocks for life on earth. Considering its prevalence and the fact that it's all around us, many people take this to mean that it's harmless to human health. We live in a world full of carbon dioxide, and we are always surrounded by it - how can it be harmful to us?

However, carbon dioxide isn't this simple. While it is an essential chemical compound that is needed for life on earth to exist as it does today, it also has a darker side. Not only does the gas lead to changing climates on our planet, but it also directly impacts our health. This is no more true than when we are in confined, indoor spaces.

Interestingly, where the most significant and most well-publicised impacts of Co2 are found outside in the atmosphere, exceedingly high levels of the gas are most commonly found indoors. In enclosed environments lacking ventilation, we feel the greatest impact of carbon dioxide.

Few people consider the impacts of carbon dioxide indoors, and this is due to a large lack of awareness surrounding the matter. Where we are constantly hearing about the impacts of carbon dioxide on a macro level, we almost never hear about the impacts of carbon dioxide on our body on the micro level.

In today's article, we aim to bring some awareness to the issue of carbon dioxide indoors. We will look at how this gas impacts our bodies both in the short and long term. Afterwards, we will focus on how to stay on top of our Co2 levels through monitoring and acting against excess buildup.

What Level of Carbon Dioxide Is 'Safe'?

Image from Imperial College London


To begin with, we need to discuss what levels of carbon dioxide are deemed 'safe'. I use quotation marks because, realistically, there is no ‘safe’ level of carbon dioxide. Simply, the lower the carbon dioxide, the better. It's important to note that symptoms of excess carbon dioxide levels don't suddenly appear at a given concentration. Instead, they exist at all concentrations and gradually worsen. 

So then, what is considered a 'normal' level of carbon dioxide? Well, the best point of comparison is the ambient level of carbon dioxide that we can find outside. Unfortunately, this level is increasing yearly, so the new 'normal' is constantly changing. 

As of the time of writing, the outdoor level of Co2 is around 414 parts per million (ppm). Ideally, this is the concentration that we want to try and maintain indoors. However, this isn't always possible - even with good ventilation. For this reason, a higher limit of 1000ppm is considered to be the level of CO2 that should not be exceeded.

It needs to be emphasised that this is the upper limit of carbon dioxide that is considered safe. If it is at all possible, it’s best to keep carbon dioxide levels below 1000ppm and instead try to lower them to as close to ambient levels as possible.

Once 1000ppm is exceeded, many individuals begin to notice the worsening air quality and feel drowsy. Above 2000ppm, headaches and sleepiness can be noticed. Finally, concentrations above 5000ppm can lead to oxygen deprivation.

While we will dive into deeper detail regarding the health impacts later in the article, it’s important to note that at any level above 1000ppm, our bodies can feel both short-term and long-term impacts. Some people will even begin to feel impacts at lower concentrations.

If possible, all actions should be taken to prevent carbon dioxide levels from exceeding 1000ppm. Not only can some longer-term health impacts be observed, but being exposed to levels of CO2 above 1000ppm can put us into a state of perpetual drowsiness and lower cognitive performance.

Finally, before moving on, it's important to note that excessive Co2 concentrations can impact anyone. Even in large houses, I've personally seen concentrations as high as 3500ppm. On public transport and flights, concentrations as high as 4500ppm have been found.

This is what makes Co2 so harmful to humans. It's something we rarely consider, yet it's a chemical that can greatly impact how we feel - even if we don’t realise it.

When Is Carbon Dioxide an Issue?

Carbon dioxide in itself is not harmful. It exists everywhere, and we breathe carbon dioxide in every breath we take - there's simply no way to avoid it. Luckily, the chemical in low concentrations is not harmful and hasn't been found to impact human health.

The danger of carbon dioxide is found when concentrations of the gas begin to climb. This is rarely ever an issue in an outdoor environment as natural circulation is constantly occurring. However, indoors is another matter.

As humans, we are constantly inhaling oxygen and exhaling carbon dioxide. In fact, each adult human exhales around 2.3 pounds of CO2 per day. This might not seem like much - 2.3 pounds is relatively little, or at least I thought so.

However, we must remember that carbon dioxide is exceptionally light. So light that a single pound of carbon dioxide translates to 8.7 cubic feet of the gas. Now, considering that we are constantly exhaling more than twice this amount daily, the issue becomes apparent.

This issue is no more obvious than in situations where many people share the same space. Areas such as office buildings, schools, public transport, and more are exceptionally prone to housing high concentrations of carbon dioxide.

It is in these situations that carbon dioxide begins to become a health concern. When there is an excess of people or a lack of ventilation, buildup quickly occurs, and it doesn't take long before carbon dioxide reaches detrimental levels.

In terms of when carbon dioxide is at its worst indoors, it's usually during winter. In summer, it's usual for windows to be open and to have some form of air circulation such as a fan or AC running. On the other hand, during winter, it's the opposite. In an effort to conserve heat, we often close windows and decrease ventilation.

Carbon dioxide is also a big issue in the morning. Many of us sleep with our windows and doors closed, and overnight we continue to exhale the gas. This means that in the morning when we wake up, there is a large concentration of CO2 in the room. 

This, in particular, is an issue because we all want to wake up fresh and rejuvenated after a good sleep. However, in a bedroom lacking ventilation, the CO2 buildup can mean that we wake up feeling drowsy - a feeling that can continue all morning.

Whether you live in a small studio apartment or a large house, CO2 can become an issue. For shared spaces with low ventilation, it can become an even more serious danger - consider public transport and classrooms as an example.

At the end of the day, two key factors cause high concentrations of CO2 to accumulate. One is overcrowding, as too many people exhaling carbon dioxide in a room will lead to high concentrations of the chemical compound, and the other is poor ventilation. When both of these are combined, we quickly see harmful carbon dioxide levels accumulate.

A recent news article in New Zealand highlighted the issue of carbon dioxide in public spaces. While the author approached the article from the perspective that carbon dioxide is representative of expelled respiratory particles and, therefore, COVID-19 in the air, the readings the author found with their portable monitor are very telling.

In two areas where people often spend a lot of time, classrooms and offices, CO2 levels were found to be 1373ppm and 1167ppm, respectively. While these readings aren't too far over safe levels, they are both areas where we spend a lot of time. Therefore, even with these concentrations only slightly in excess, they can still greatly impact us.

However, the article becomes much more interesting when the author carries their CO2 monitor into other commonly frequented areas. Both shops and cafes generally had acceptable levels of carbon dioxide, which didn't pose an issue. However, bars, gyms and taxis all had readings above 2000ppm. In the case of gyms, readings as high as 4228ppm were found!

The most impactful measurement that was taken, however, was in public transport. After being on a bus for 20 minutes, the author saw CO2 levels skyrocketing to over 5000ppm. This is a very unhealthy level of carbon dioxide and a level that greatly impacts us.

Excess carbon dioxide concentrations can be an issue in any indoor environment. If you feel any of the symptoms listed below, there is a chance that carbon dioxide is impacting you. If you want to know if the invisible gas impacts you, it might be worth investing in a CO2 monitor. However, more on that soon.

Health Impacts of Co2

Image from AirThings.


By this point, you're probably wondering about the health impacts of inhaling excess carbon dioxide. Until recently, it was widely accepted that long-term health problems only began to occur when concentrations reached higher than 5000ppm. However, more recent studies have shown that carbon dioxide can impact our health negatively at concentrations as low as 1000ppm.

At low concentrations, carbon dioxide can cause headaches, dyspnea upon mild exertion, and decreased cognitive performance over a period of a few hours. As concentrations increase to around 3000ppm, individuals can start to feel headaches, dizziness, sweating and increased blood pressure in a matter of minutes. 

At higher concentrations such as 6000ppm, you may start to see visual disturbances and begin to see tremors after a few hours (source). Although concentrations should never exceed 6000ppm in areas we frequent regularly, the fact that CO2 reached over 5000ppm on public transport is concerning, to say the least.

Perhaps the biggest symptom of carbon dioxide that most people feel, however, is drowsiness. There is no set concentration at which drowsiness begins to become an issue, but rather the symptom is gradually worsened as concentrations increase. If you find yourself regularly feeling drowsy, try to add more ventilation and see if perhaps CO2 was the issue.

There is a lot of debate over the longer-term health impacts of CO2. Many studies believed that there were few long-term impacts until CO2 concentrations reached exceptionally high amounts over 10,000ppm. However, more recent studies have pointed to CO2 having long-term health impacts at much lower concentrations.

Individuals exposed to carbon dioxide above 1000ppm for even a few hours can experience inflammation and reduced cognitive performance. Individuals with chronic exposure between 2000 and 3000ppm start to see far more worrying health impacts such as kidney calcification and bone demineralisation.

While these are the health impacts we are currently aware of, there are likely to be more coming to light in the future. As research into the health impacts of carbon dioxide is only recently becoming a more popular topic, we will likely begin to see more research covering CO2 shortly. 

Currently, we know that CO2 in concentrations above 1000ppm and likely even above 800 are detrimental to short-term human health. Not only does CO2 cause us to feel drowsy, seemingly with no fix, but it can slow down our brain and impact how we think.

How badly does it impact our cognitive performance? Well, a Harvard study completed in 2015 is very telling. The team found that on days of moderate CO2 levels (945ppm), cognitive performance decreased by around 15%. Considering that this is a level often seen in classrooms and office buildings, a big problem is highlighted. We are simply not performing at our best in these situations.

That’s not where the story ends, though. The same study found that at higher concentrations of CO2, such as 1400ppm, cognitive performance decreases by a whopping 50%. While these impacts of carbon dioxide are only short-term, we simply aren’t our usual selves when there are high carbon dioxide concentrations. 

This is perhaps the most significant danger of CO2 in our daily life. Although we can’t see it and rarely consider its impacts, carbon dioxide can greatly impact how we feel and think. For that reason, it’s a gas that we need to be conscious of the health effects of. 

Carbon Dioxide as a Proxy for COVID-19

Image by Zero COVID Thailand.

Before moving on, it's important to discuss another aspect of carbon dioxide's health impacts. This is that carbon dioxide alone isn't the whole story, as excess carbon dioxide concentrations can also indicate other issues.

Since humans exhale CO2, CO2 levels are one of our best ways to monitor the level of recently exhaled or 'recycled' air in an environment. We currently have no way to determine how many COVID-19 or other viral particles are airborne, so using CO2 concentrations is our best bet. 

Studies have shown how measuring CO2 concentrations in a given indoor environment can act as a proxy for the chances of catching COVID-19 (or other viruses that spread through expelled respiratory particles).

A study researching the risk of catching COVID-19 relative to carbon dioxide concentrations found that as carbon dioxide concentrations double, so too does the chance of catching COVID-19. While the conclusion that CO2 concentrations can indicate the risk of catching COVID-19 aren’t surprising, the exact relationship between the two is.

Of course, one key caveat is that you must be in a shared environment. If you are at home and monitor CO2 levels of 1500ppm in your bedroom, your chances of catching COVID-19 are very low or non-existent (depending on if you share the room or not). The relation between CO2 concentrations and COVID-19 risk is irrelevant in these cases. 

However, in shared environments, the findings from this study hold true. If you are on public transport and observe CO2 concentrations around 2000, the chances of catching COVID-19 are approximately double what they would be if the carbon dioxide concentrations were around 1000ppm.

This is a significant finding because it shows that high concentrations of CO2 indicate more than just potential health impacts caused by the gas. Instead, they can also indicate various other issues that can have greater implications on human health.

Although COVID-19 is the current focus, these findings likely apply to other airborne viruses too. If you are sharing space with someone carrying an airborne illness, as carbon dioxide concentrations increase, so do the chances of catching the illness in question.

How Can We Monitor CO2?

Whether looking at the short-term or long-term impacts of carbon dioxide, it’s clear that we need to avoid excessive concentrations of this gas wherever possible. Unfortunately, knowing when carbon dioxide levels exceed safe levels requires specialised equipment. Luckily, this equipment is now more affordable than ever and is available for a modest sum. 

This newfound accessibility to carbon dioxide monitors has led to a recent trend where individuals carry CO2 monitors on them in a range of different situations. This allows individuals to assess not only the CO2 levels in an area but also the ventilation and COVID-19 risk.

It’s first worth discussing whether a CO2 monitor is worth it. The short answer is yes. If you’re worried about the levels of CO2 that you are inhaling and want an accurate summary of the current air quality, a carbon dioxide monitor is the way to go.

Carbon dioxide monitors are also the only way to stay on top of CO2 concentrations regardless of where we are. These little monitors easily fit into a pocket alongside a wallet or phone, and they constantly read the ambient air quality to let you know when you should be worried.

With that being said, if you are only concerned about the air quality in your home or another indoor environment that you have control over, a CO2 monitor may not be essential. While you will still have no accurate reading as to what the CO2 levels are, you can prevent them from rising by ensuring that there is always good ventilation. Of course, a carbon dioxide monitor is the way to go if you want further insight or the ability to monitor CO2 on the go.

When it comes to which monitor to pick, you’ll want to look for the most accurate device within your price range. Not all devices are accurate, and you will want to avoid no-name monitors from websites such as Aliexpress. All of this is to say, carbon dioxide monitors are one of the products where the brand name is important.

Currently, some of the most popular (and trustworthy) devices on the market are the Aranet 4 and the TheatreCaps Mini Carbon Dioxide Monitor. While the Aranet 4 is significantly more pricey, it’s considered the standard that CO2 monitors strive to reach.

Both of these devices rely on batteries, meaning they are portable and can be used to take readings anywhere. However, a docked monitor might be a better idea if you’re looking for a carbon dioxide monitor for an indoor environment such as an office space, classroom, or house. 

Many of these monitors feature the same internals, meaning they are equally accurate and will provide similar readings to portable monitors. However, they are better suited to static use as many require USB or wall socket power to operate.

uHoo is a great monitor for anyone wanting an all-around indoor air quality monitor. Not only does it monitor CO2 levels, but it also monitors other pollutants such as particulate matter, VOCs, carbon monoxide, and more. While it doesn’t have a screen, it’s a fantastic monitor for keeping an eye on your indoor air quality.

Whichever form of monitor you opt for, you will be able to get accurate readings on current CO2 levels. Unfortunately, outside of these niche devices, there is no effective way to monitor carbon dioxide levels. Luckily, these devices are becoming more accessible; some can be found for under $100.

How Can We Reduce CO2 Indoors?

Now that we know how to monitor carbon dioxide levels, it’s time to consider how to reduce and manage our CO2 concentrations. It’s important to note that there is, unfortunately, no easy way to manage CO2 when out and about. We have very little control over air quality when it comes to public transport, gyms, bars, and otherwise.

Luckily, in areas over which we do have some control, such as in our homes, there are a few steps we can take to minimise carbon dioxide levels. Let’s take a look at them:

Ensure good ventilation

The best way to decrease CO2 levels indoors is to ensure good ventilation is present. This means opening windows and doors or making sure that your ventilation system is on and working correctly. 

This step is more important for smaller rooms and houses but is always beneficial - even in vast office spaces or classrooms. There is no better way to decrease carbon dioxide levels than to open the windows!

If you have a fan, air conditioning, or some other device that can provide airflow, these might be worth using too. While opening the windows is the best bet, there are times when this isn’t possible. In these situations, try to maximise airflow as best you can.

Place plants around your home

Although plants are not as effective at reducing CO2 as good ventilation, they can still benefit indoor air quality. Not only are they a benefit to CO2 concentrations, but plants have been proven to have many other positive effects, such as decreasing stress.

While all plants will decrease carbon dioxide, the best plants are snake plants, areca palms, peace lilies and ferns such as the Boston fern. However, to make a noticeable difference, you will want to place a decent number of plants around your home. Even when many plants are present, ventilation will still prove the more effective way to decrease CO2 levels.

Plants are also effective at improving indoor air quality in other ways. They are good at decreasing VOCs, and they can significantly improve the quality of the air we breathe. If you don’t already have some plants around your home or office, consider adding some for their air-purifying benefits!

Reduce overcrowding

If you are in an office, classroom, or another area with many people, consider the impacts of overcrowding on carbon dioxide levels. Indoors, humans are the most significant contributors to carbon dioxide levels, and this is especially true in overcrowded environments. 

If you are having trouble controlling CO2 levels indoors, even with ventilation, the chances are that your workspace or classroom is overcrowded. While there may not be an easy way to solve this issue, it’s worth acting on if possible.

Prevent/Minimise the impact of CO2 sources

Although humans are the biggest contributors to carbon dioxide indoors, there are a couple of other sources that can cause a significant difference in indoor air quality levels. The two primary sources of CO2 indoors are open fires and smoking.

If you cook with a gas stove or otherwise have open flames in your house, it’s worth ventilating well during cooking or when the fire is active. If you share office space or home with someone who smokes, ask if they can do so outside. 

Both smoking and open fires are big contributors to indoor CO2 concentrations and will be detrimental to indoor air quality levels. Not only do they increase carbon dioxide concentrations, but they also fill the air with VOCs and other harmful chemicals.

A note about air purifiers: air purifiers will NOT decrease CO2 levels indoors. These devices cannot filter carbon dioxide and won’t make a difference to CO2 levels. While they are effective against VOCs and other indoor air pollution, ensuring adequate ventilation is the best way to reduce CO2.

Can Masks Protect Against CO2?

We get this question often, so it’s worth addressing in its own point. Can masks such as KN95s and N95s filter against carbon dioxide? Unfortunately, they cannot. Whether you are using a filtering face piece respirator or any other form of air-purifying respirator, carbon dioxide will not be filtered.

This means that the best way to protect yourself from the adverse impacts of carbon dioxide while out and about is to avoid areas of high concentrations as much as possible. While we appreciate that this isn’t always possible, it’s the only action we can take when we can’t control our local environment.

That said, masks are still worth donning whenever carbon dioxide concentrations are high. As we discussed earlier, carbon dioxide concentrations can often act as a proxy for the risk of catching COVID-19 and other viruses. For this reason, masks are still well worth wearing whenever concentrations exceed 1000ppm.


Carbon dioxide is a gas that surrounds us. It’s everywhere, and its impacts are rarely publicised on a micro-scale. While we often hear about the impacts of CO2 on global warming and climate change, we rarely hear about how this chemical impacts our bodies and minds.

However, as we hope this article has elaborated, carbon dioxide is something that we need to consider. It can impact both our health and feelings and all without us knowing. With something impacting our lives so much, it’s almost unbelievable that its dangers are so rarely discussed.

Luckily, there are ways that we can reduce the impacts of the chemical compound. When indoors, ensuring good ventilation is present is key; with a bit of management, it’s possible to significantly lower our carbon dioxide concentrations. 

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