What is carbon dioxide (CO2), and what does CO2 in my home mean for my health?

What is carbon dioxide?

Carbon dioxide (CO₂) is a natural, colorless, and odorless gas, also referred to as a “greenhouse gas”. Outdoors, CO₂ is a vital part of the earth’s ecosystem, necessary for plant photosynthesis and life itself. But indoors, it is a completely different story.

What should the CO₂ level be inside my home?

CO₂ is measured in parts per million (ppm). As of 2026, baseline outdoor air typically hovers around 420 ppm, so generally speaking, the lower the CO₂ indoors, the better.

To understand CO₂ levels in your home, first you need to measure them with a CO₂ monitor. Here’s an overview of what the different levels mean for your air quality:

• Under 800 ppm (excellent): This is the ideal zone. Your home is well-ventilated, and your air is fresh.
• 800 to 1,000 ppm (fair): The air is starting to get stale. You might begin to feel mild drowsiness. It’s time to consider opening a window or running a fan.
• 1,000 ppm and above (poor): At this level, cognitive function drops, and the air feels noticeably stuffy. Headaches and restlessness become common. Immediate ventilation is required.

How does CO₂ inside my home affect me?

CO₂ can make us feel drowsy, affect our sleep quality, and reduce cognitive function, often referred to as the “brain fog” feeling. Because we spend up to 90% of our time indoors, often in highly insulated, energy-efficient homes with poor ventilation, the CO₂ we exhale can build up quickly.

A study from Harvard University found that with every 500 ppm increase, response times to a simple cognitive test slowed by between 1.4–1.8%, and throughput dropped by between 2.1–2.4%.

Here is everything you need to know about indoor carbon dioxide, how it affects your body, and how to keep your home’s air fresh and healthy.

Where does CO₂ inside my home come from?

Indoor carbon dioxide concentrations come from people, bad ventilation, or combustion (burning things). Here’s a breakdown of each:

1. People (respiration): Every time someone breathes out, they release CO₂. In enclosed spaces like bedrooms, classrooms, or home offices, human breathing is the primary source of CO₂ buildup.
2. Poor ventilation: Modern homes are built to be airtight to conserve heating and cooling energy. While great for your utility bill, this traps exhaled air inside, leading to CO₂ build-up. Many HVAC systems simply recirculate the same stale air rather than pulling in fresh air from outside.
3. Combustion: Burning fossil fuels indoors, such as using gas stoves, space heaters, or even burning candles, also releases additional CO₂ into your living space. Learn how other sources of combustion impact air quality in our guide to fine particulate matter (PM2.5).

Rising co2 levels caption

Is CO₂ the same thing as CO?

No, they are completely different. CO₂ is carbon dioxide, which at worst will make you drowsy, less productive, and disrupt your sleep. CO is carbon monoxide. This is a highly dangerous, potentially fatal gas caused by faulty appliances. Always keep an active carbon monoxide alarm in your home.

How do high CO₂ levels affect your health?

CO₂ is not toxic in typical indoor amounts. However, high CO₂ levels do directly impact how you feel, work, and sleep:

1. Brain fog and poor focus and productivity

If you find yourself losing focus or feeling sluggish in your home office during the afternoon, CO₂ is likely the culprit. A landmark study by the Harvard T.H. Chan School of Public Health found that lowering indoor CO₂ levels can boost cognitive function test scores by 101%. In schools, well-ventilated classrooms with lower CO₂ levels are consistently linked to faster learning and higher test scores in math and reading.

2. Restless and poor quality sleep

We spend a third of our lives in our bedrooms. With the door closed and windows shut, CO₂ levels can easily reach way above 2,000 ppm. Research shows that high CO₂ concentrations in bedrooms lead to restlessness, reduced sleep efficiency, and a groggy feeling the next morning.

Learn more about how CO₂ and other indoor air quality issues can impact sleep in our article, How indoor air quality is ruining your sleep, and how to fix it.

3. Physical symptoms

When CO₂ levels go above 2,000 ppm, your body needs to work harder to get the oxygen it needs. Common physical symptoms of very high indoor CO₂ include:

• Drowsiness and fatigue (cue lots of yawning)
• Headaches
• Increased heart rate
• Mild shortness of breath or “stuffy” air sensations

Does lower CO₂ have other health benefits?

As the US CDC (Centers for Disease Control and Prevention) states, good ventilation in your home, specifically with low CO₂ levels, can also help to reduce the risk of virus transmission. While more significant in workplaces and schools, keeping your home well-ventilated in virus season, especially when people are gathering together, can have the added benefit of helping to stop the spread of respiratory viruses, such as colds, flu, and COVID-19.

How do I know what my home’s CO₂ levels are?

Using a dedicated air quality monitor with a CO₂ sensor is the only way to know what your home’s actual CO₂ levels are. A monitor like the Airthings View Plus or Wave Enhance can continuously track CO₂, alongside a range of other air quality factors, and send notifications to you if it’s time to open a window.

Read our dedicated guide on how to test air quality in your home to get more tips on perfecting your indoor environment.

How do you reduce CO₂ levels in your home?

Ventilation is the best way to reduce CO₂. However, it’s impossible to manage carbon dioxide levels without measuring it. A first step is using a smart indoor air quality monitor, like the Airthings View Plus. It gives you real-time data and historical trends on CO₂, alongside other important indoor air quality factors, such as particulate matter (PM2.5), radon, and VOCs. It will send push notifications to your phone when it’s time to take action, such as opening a window or door.

Once you know your levels are rising, use these simple strategies to bring them down:

• Ventilate: The easiest fix is opening a window or leaving interior doors ajar. Even cracking a bedroom window a few inches at night can drastically lower CO₂ and improve your sleep. If you can’t leave a window open due to security concerns or poor air quality outside, leaving your bedroom door open can also help.
• Upgrade your HVAC filters: Ensure your home’s central air system is pulling in fresh air rather than just recirculating stale air. Run bathroom and kitchen exhaust fans, which pull indoor air outside.
• Limit open flames: Reduce the use of gas stoves without range hoods, and limit burning candles or using wood fireplaces, especially if you’re not able to ventilate with fresh air.
• The house plant myth: While plants do absorb CO₂ during photosynthesis, you would sadly need an indoor jungle of hundreds of plants to make a measurable dent in your home’s CO₂ levels. Rely on ventilation, not vegetation, for fresh air.

Your home should be a healthy, relaxing, and productive place to be. By monitoring your air quality to understand and control carbon dioxide, you can breathe easier, sleep better, and think clearer.

Sources

• Harvard T.H. Chan School of Public Health (COGfx study)
• US EPA (Indoor Air Quality)
• US CDC (Improving Ventilation in Your Home)

Frequently Asked Questions (FAQ)

Does CO₂ make you sleepy?

Yes. High levels of indoor carbon dioxide displace oxygen and cause a “stale” air environment. This leads to drowsiness, lethargy, and a lack of focus, particularly in enclosed spaces like offices and bedrooms.

Where does CO₂ in my home come from?

The main source of CO₂ in your home is from humans breathing out. However, using gas stoves and fireplaces can also contribute to CO₂. CO₂ will build up if your home is not well-ventilated.

What is a dangerous level of CO₂ indoors?

While rarely fatal in residential settings, CO₂ levels above 1,000 ppm can cause headaches, brain fog, and restlessness. Levels exceeding 2,000 ppm indicate severe ventilation issues and will cause significant discomfort and nausea.

How do I test for CO₂?

An indoor air quality monitor with a CO₂ sensor is the best way to find out your home’s CO₂ levels. Use it in the spaces you spend the most time, such as a bedroom or home office.

Does high CO₂ affect children more than adults?

Research suggests children are more vulnerable to the effects of high CO₂ than adults. A study found that attention levels of school-aged children decreased by approximately 5% when exposed to a CO₂ concentration of 2,909 ppm compared to that at 690 ppm.

What’s the best way to reduce CO₂ in my home?

The easiest and best way to reduce CO₂ is to make sure you have good ventilation, either from passive/natural sources, such as opening a window, keeping doors open, and ensuring your home’s ventilation system (if it has one) is drawing fresh air in from outside, rather than recirculating stale air.

Do air purifiers reduce CO₂?

No. Standard HEPA air purifiers are designed to capture solid particles (like dust, pollen, and PM2.5). They do not remove gases like carbon dioxide. To reduce CO₂, you must physically replace the indoor air with outdoor air through ventilation.

What is the difference between carbon dioxide (CO₂) and carbon monoxide (CO)?

Carbon dioxide (CO₂) is a naturally occurring gas we exhale, which causes stuffiness and fatigue in high concentrations. Carbon monoxide (CO) is a highly toxic, potentially deadly gas produced by the incomplete burning of fuels from faulty furnaces, water heaters, or cars.

How do I know if I have high CO₂?

While you can often tell if something is “off” with your air, such as a room feeling stuffy or stale, monitoring your indoor air quality with a device like Airthings View Plus can tell you what your CO₂ levels actually are.

Can plants reduce CO₂ in my home?

No, unfortunately you would need hundreds of plants in your home to reduce your CO₂ levels. Opening a window will have far more impact on your CO₂.