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Key Summary

Indoor CO2 levels rise whenever people breathe in spaces that don’t get enough fresh air. Moderately elevated CO2 can make you feel drowsy, foggy, and restless at night, while very high levels can trigger headaches and more serious health concerns. You can track CO2 with affordable air quality monitors and bring levels back into a healthy range by increasing fresh air exchange with strategies like opening windows, improving exhaust, and installing balanced mechanical ventilation such as ductless ERVs and HRVs.

TL;DR

• Outdoor CO2 is typically around 420 ppm; aim to keep most indoor spaces under about 1,000 ppm, and ideally in the 600–900 ppm range when rooms are occupied.
• Symptoms of high indoor CO2 include sleep disruption, morning grogginess, headaches, trouble focusing, and a sense of stuffiness even when temperature and humidity seem fine.
• Airtight homes and bedrooms trap exhaled air, so CO2 can rise rapidly overnight—often exceeding 1,500–2,000 ppm in closed rooms with the door and windows shut.
• Use a CO2-capable air quality monitor to see how levels change throughout the day, especially in bedrooms, home offices, and basements.
• Simple fixes include cracking windows, undercutting interior doors, and running existing exhaust fans, but long-term healthy homes rely on mechanical ventilation.
• Ductless ERVs and HRVs provide continuous fresh air, transfer heat and (in ERVs) some moisture, and can dramatically improve indoor CO2, sleep quality, and comfort in tight homes.
• Treat ventilation as critical infrastructure—on par with heating and cooling—if you want a home that consistently feels clear, comfortable, and healthy.

Product Introduction

If your CO2 monitor regularly reads above 1,000–1,500 ppm, it’s a strong sign you need more than just open windows. Modern ductless ERVs and HRVs are designed specifically for today’s airtight homes, quietly exchanging stale, CO2-laden indoor air with fresh outdoor air while recovering heat (and, with ERVs, some moisture) so you stay comfortable and efficient year-round. Below, we’ll walk through how these systems work, where they fit best, and how to choose the right solution for your home or light-commercial space.

What Is CO2 and Why Does It Matter Indoors?

Carbon dioxide (CO2) is a colorless, odorless gas that we exhale with every breath. Outdoors, CO2 is relatively diluted in the atmosphere. Indoors, it can quickly accumulate—especially in well-sealed homes or crowded spaces. While CO2 itself is not toxic at the concentrations normally found in homes, elevated levels are strongly associated with how fresh, alert, and comfortable a room feels.

• We exhale CO2 constantly, so every occupied room is a CO2 source.
• Without enough fresh air exchange, CO2 builds up quickly in smaller or tighter spaces.
• CO2 is a useful proxy for ventilation effectiveness—higher levels usually mean the room is under-ventilated for the number of people inside.

Instead of thinking of CO2 as a mysterious pollutant, treat it as a simple, measurable signal: if CO2 is high, the air you’re breathing has been in other people’s lungs many times over and hasn’t been refreshed enough with outdoor air.

Safe Indoor CO2 Ranges: What’s Normal and What’s High?

Outdoor air today typically contains around 420 ppm (parts per million) of CO2. Indoors, levels almost always rise above that baseline because we’re breathing and many homes are designed to be airtight for efficiency. The question is: how high is too high for comfort, performance, and health?

Different organizations publish different guidelines, and there isn’t a single universal cutoff. But for most homes and light-commercial spaces, the ranges below are a practical way to think about indoor CO2:

• 400–700 ppm: Excellent / near-outdoor air. Often found in homes with windows open or in lightly occupied, well-ventilated spaces.
• 700–1,000 ppm: Good, typical for many occupied rooms with decent ventilation. Most people feel comfortable and alert here.
• 1,000–1,500 ppm: Borderline. You may start noticing stuffiness, mild drowsiness, or trouble concentrating, especially during long tasks.
• 1,500–2,000+ ppm: Poor ventilation. More people experience headaches, sleep disruption, and a sense that the air feels stale or heavy.
• 3,000+ ppm: Very poorly ventilated space. At this point, symptoms like strong fatigue, dizziness, or nausea are more likely, and urgent ventilation fixes are recommended.

For most residential and light-commercial settings, keeping occupied rooms under 1,000 ppm—and ideally in the 600–900 ppm range—strikes a good balance between comfort, practicality, and energy use. Bedrooms and home offices deserve special attention because the quality of air in those spaces has an outsized impact on how you feel and function every day.

Symptoms of High Indoor CO2: How It Affects Sleep, Focus, and Comfort

Many people don’t realize that recurring issues like groggy mornings, afternoon brain fog, or a chronically stuffy-feeling home can be tied to elevated CO2. Because CO2 is invisible and odorless, the first clues usually come from how you feel rather than what you see or smell.

Impact of high CO2 on sleep quality

Bedrooms are often the most CO2-challenged rooms in a home. A closed bedroom with one or two people sleeping for 7–8 hours is a perfect recipe for CO2 buildup, especially in newer, airtight houses. Common signs that your bedroom is under-ventilated include:

• Waking up feeling unrefreshed despite a full night’s sleep.
• Morning headaches or a heavy-headed feeling that clears after you leave the room.
• Night sweats or feeling “suffocated” even when the thermostat is set reasonably.
• Restlessness, tossing and turning, or frequent awakenings without clear cause.

Research has linked elevated bedroom CO2 with poorer sleep efficiency, more awakenings, and reduced next-day performance on concentration and decision-making tasks. When CO2 climbs above roughly 1,500–2,000 ppm overnight, most people notice they simply don’t sleep as deeply or feel as clear in the morning.

Impact of high CO2 on focus and productivity

High CO2 doesn’t just affect how you sleep; it can also make your waking hours less productive. Home offices, classrooms, and conference rooms are notorious for CO2 spikes because they pack people into relatively small spaces for long periods.

• At moderately elevated CO2 (around 1,000–1,500 ppm), many people experience slower reaction times and mild brain fog.
• At higher levels (above 1,500–2,000 ppm), complex decision-making, problem-solving, and concentration can noticeably decline.
• Students and remote workers often report feeling tired, unmotivated, or “sluggish” in poorly ventilated rooms even when they slept enough and had caffeine.

If your home office feels fine in the morning but starts to feel heavy, stale, or tiring by mid-afternoon, a CO2 monitor may reveal that rising CO2, not just screen time, is dragging you down.

General comfort symptoms of high indoor CO2

Even when you’re not working or sleeping, elevated CO2 can subtly damage how pleasant your home feels. Common comfort-related complaints in high-CO2 spaces include:

• Persistent stuffiness or “stale air” even when temperature and humidity seem fine.
• More frequent tension headaches or mild dizziness in certain rooms.
• A sense that the air feels heavy, thick, or harder to breathe during gatherings or in small rooms.
• Discomfort that improves quickly when you step outside or open a window.

These symptoms overlap with other indoor air quality issues like volatile organic compounds (VOCs), high humidity, or allergens. That’s why measuring CO2 is so valuable: it helps separate ventilation problems from other air quality challenges and guides you toward the right fix.

Why Airtight Homes and Bedrooms Often Have High CO2

Modern building codes and energy standards have pushed homes to be more airtight than ever before. This is great for cutting drafts and saving on heating and cooling costs—but it also means that indoor air, including the CO2 we exhale, has a harder time escaping.

From leaky houses to tight envelopes

Older homes often had so much natural infiltration—gaps around windows, doors, and framing—that they “breathed” constantly, even if no one intended them to. That uncontrolled airflow wasted energy but also diluted indoor CO2 and other pollutants without any equipment. In contrast, today’s high-performance homes are wrapped with continuous air barriers, taped sheathing, and advanced windows and doors. The result:

• Far less uncontrolled air leakage, which is excellent for comfort and energy bills.
• A much greater dependency on deliberate ventilation strategies to keep indoor air fresh.
• Higher risk of CO2 buildup in bedrooms, basements, and interior rooms if mechanical ventilation is undersized or missing.

If you’ve upgraded windows, added insulation, and air-sealed your home but never updated ventilation, you’ve likely improved energy efficiency at the expense of indoor air freshness. A CO2 monitor is often the first tool that reveals this trade-off.

Why bedrooms are CO2 hotspots

Bedrooms are usually smaller rooms with closed doors at night. One or two people can easily push CO2 levels above 1,500 ppm within a few hours in a well-sealed bedroom if there is no active fresh air supply. CO2 build-up is even more dramatic in:

• Basement bedrooms or bonus rooms with fewer windows.
• Rooms with tightly weather-stripped doors and no dedicated supply or return vent.
• Bedrooms with closed HVAC vents in attempts to “save energy,” which can reduce airflow even further.

A common CO2 pattern in tight homes is a smooth climb overnight while everyone sleeps, peaking just before waking, then dropping quickly when the bedroom door opens or windows are cracked. If your mornings start with grogginess that lifts as you move around the house or step outside, this pattern may be playing out in your home.

Crowding, gatherings, and light-commercial spaces

Any space with more people than it was designed to ventilate will see CO2 spike—even if it’s not a bedroom. For example:

• Home gyms with closed doors and multiple people working out.
• Conference rooms or small offices with several occupants and minimal ventilation.
• Retail or light-commercial showrooms that weren’t designed with continuous fresh air in mind.

In these settings, CO2 monitors often reveal rapid spikes during busy periods and slow declines once people leave, underscoring the need for properly sized and controlled ventilation rather than relying on chance air leakage.

How to Measure Indoor CO2: Monitors, Placement, and Interpretation

The good news is that understanding your indoor CO2 profile is straightforward. Affordable sensors make it easy to log CO2 levels throughout the day and see exactly where and when your air gets stale. Once you see the data, it’s much easier to justify investing in better ventilation—and to verify that your fixes are actually working.

Choosing a CO2-capable air quality monitor

Look for consumer-grade air quality monitors that specifically list CO2 (not just “air quality index” or VOCs). Key features to prioritize include:

• True CO2 sensing, often based on NDIR (non-dispersive infrared) technology, which is more accurate than CO2 “equivalent” estimates from VOC sensors.
• A clear ppm (parts per million) display so you can see actual CO2 levels at a glance.
• Data logging or app connectivity, which lets you view CO2 trends overnight or over weeks and see how changes in behavior or equipment affect your air.
• Additional sensors for temperature, humidity, and sometimes PM2.5 or VOCs, giving you a fuller picture of air quality.

If you’re serious about creating a healthy, high-performance home, consider at least one high-quality CO2 monitor that you can move between key rooms, or multiple monitors to permanently track bedrooms, main living areas, and a home office.

Where to place CO2 monitors in your home

Placement matters, because CO2 levels can vary within a room and throughout the house. To get meaningful readings:

• Mount or place the monitor at breathing height—typically 3–6 feet above the floor, similar to where someone’s head would be when seated or lying in bed.
• Avoid placing the sensor right next to windows, doors, supply vents, or exhaust fans, which can skew readings with localized air currents.
• Keep it away from direct sunlight, heat sources, or enclosed corners that might trap air and misrepresent the room’s average CO2.
• For bedrooms, place the monitor near the head of the bed or on a nearby nightstand to capture the air you actually breathe overnight.

You can also temporarily move the monitor to different rooms to map out CO2 “hot spots” across your home—bedrooms, basements, offices, and rooms that consistently feel stuffy are great starting points.

How to interpret CO2 readings and patterns

The absolute CO2 level at a single moment is helpful, but trends tell the real story. Pay attention to how CO2 changes with occupancy and ventilation:

• Overnight bedroom patterns: Does CO2 start near outdoor levels, then slowly climb above 1,500 ppm by morning? That’s a clear sign you need more nighttime ventilation.
• Workday office patterns: Does your home office hover around 600–900 ppm when the door is open but spike to 1,500+ ppm when closed during calls? That suggests you need better airflow while you work.
• Event patterns: Do CO2 levels soar when you host gatherings and take hours to drop afterward? If so, your home’s effective ventilation rate is too low for the number of people during those times.

Use these patterns to prioritize fixes. For example, if only bedrooms show high overnight CO2, a ductless ERV serving the bedroom wing may solve most of your problem. If CO2 spikes throughout the house whenever people are home, you may need a whole-home ventilation strategy integrated with your existing HVAC.

Practical Ways to Lower Indoor CO2: From Quick Wins to Long-Term Solutions

Once you’ve confirmed that CO2 is elevated, the next step is to increase fresh air exchange. Some strategies are quick and free, while others involve upgrading your home’s ventilation infrastructure. The right mix for you depends on your climate, budget, and how consistently you want to maintain low CO2 without constant manual effort.

Quick fixes: opening windows and improving airflow

The simplest way to reduce CO2 is to bring in outdoor air. Even short bursts of window opening can drop CO2 significantly, especially in mild weather. Consider these low-cost strategies:

• Nighttime window venting: Crack a bedroom window by an inch or two at night (where safe and comfortable) to allow fresh air in and CO2 out.
• Cross-ventilation: Open windows on opposite sides of a room or home for 10–15 minutes to flush out stale air quickly.
• Door undercuts or transfer grilles: Ensure bedroom doors have enough gap at the bottom or install passive transfer grilles so that air can flow to and from hallways and common areas.
• Use of ceiling and portable fans: Fans don’t add fresh air, but they can help mix indoor air, allowing areas with fresher air to dilute CO2 more evenly while windows or vents are open.

These approaches are most practical in shoulder seasons or mild climates. In very cold or hot/humid climates, leaving windows open long-term can be uncomfortable and inefficient, which is where mechanical ventilation shines.

Using existing exhaust fans and HVAC to help

Even if you don’t yet have a dedicated fresh air system, most homes have some mechanical tools you can leverage:

• Bathroom exhaust fans: Running these fans more often (or installing quieter, continuous-rated models) can pull stale air out, encouraging fresh air to seep in through the building envelope and open windows.
• Range hoods: Properly ducted range hoods that exhaust outdoors can provide periodic high-volume ventilation, especially useful during cooking and gatherings.
• HVAC fan circulation: Setting your furnace or heat pump fan to “on” or using a scheduled circulation mode can help distribute fresh air from rooms with open windows to the rest of the house, reducing CO2 hotspots.

These measures can reduce CO2 indirectly by increasing overall air movement and encouraging infiltration, but they usually don’t guarantee consistently low levels in tight homes. For predictable results, especially in bedrooms and home offices, dedicated fresh air systems like ERVs and HRVs are far more effective.

Why mechanical ventilation is the long-term answer

If you want a home that is comfortable, quiet, and efficient in all seasons, relying solely on windows and random leakage is not enough. Mechanical ventilation gives you control over how much outdoor air you bring in, where it goes, and how it mixes with indoor air. The two main categories for homes and light-commercial spaces are:

• Exhaust-only systems: Use fans to pull air out, relying on building leakage for make-up air. Simple and low-cost, but can lead to pressure imbalances and may not evenly distribute fresh air.
• Balanced systems (ERVs and HRVs): Bring in fresh air and exhaust stale air at roughly equal rates, often recovering heat (and with ERVs, some moisture) between the two air streams for comfort and efficiency.

For most airtight homes and high-performance renovations, balanced mechanical ventilation with energy or heat recovery is considered the gold standard. It offers predictable, year-round control over indoor CO2 without sacrificing comfort or driving up energy bills.

How ERVs and HRVs Help Fix Indoor CO2 Problems

Energy recovery ventilators (ERVs) and heat recovery ventilators (HRVs) are specialized systems that continuously exchange stale indoor air with fresh outdoor air, while transferring heat—and in the case of ERVs, some moisture—between the two streams. This lets you keep your home tightly sealed for efficiency while still enjoying low CO2 and consistently fresh air.

HRV vs. ERV: What’s the difference for CO2?

From a CO2 perspective, both HRVs and ERVs do the same fundamental job: they lower CO2 by bringing in outdoor air and exhausting indoor air. The key differences between them relate more to moisture control and climate:

• HRV (Heat Recovery Ventilator): Transfers heat but not significant moisture between outgoing and incoming air. Often favored in colder, drier climates where moisture control is easier or where dehumidification is a priority.
• ERV (Energy Recovery Ventilator): Transfers heat and a portion of moisture, helping stabilize indoor humidity. Often recommended in climates with humid summers or for homes where humidity swings are a concern.

In either case, the process of exchanging indoor and outdoor air steadily dilutes CO2. Instead of CO2 climbing steadily overnight, you’ll see a much flatter curve on your monitor, staying closer to your target range even with doors closed and windows shut.

Ductless ERVs and HRVs: A practical option for existing homes

Traditional whole-house ERVs and HRVs rely on ductwork, which can be challenging to retrofit into existing homes or small commercial spaces. Ductless ERV and HRV units solve this by packaging the fresh air and exhaust pathways into a compact wall-mounted unit that typically requires just a couple of penetrations through an exterior wall.

• They’re ideal for bedrooms, home offices, and main living areas where CO2 and stuffiness are most noticeable.
• Installation is usually simpler and less disruptive than adding new duct runs, making them an excellent choice for retrofits and renovations.
• Many models offer multiple speed settings, timers, and sometimes even built-in humidity or CO2 controls.

By placing ductless ERVs or HRVs in your home’s key problem zones, you can dramatically reduce CO2 and improve overall air freshness without a full HVAC overhaul. In multi-room layouts, a combination of several ductless units can often achieve whole-home coverage with good design and placement.

How much can ERVs/HRVs lower CO2?

The impact of an ERV or HRV on CO2 depends on its airflow rate (usually measured in cubic feet per minute, or CFM), your home’s size, and how many people occupy the space. In general:

• Properly sized systems can keep typical occupied spaces under 1,000 ppm for most of the day, even in tight homes with doors and windows closed.
• Bedrooms served by dedicated ductless ERVs often see overnight CO2 levels drop from 2,000+ ppm down to the 700–1,000 ppm range depending on settings and occupancy.
• When combined with strategic door management and passive airflow paths, a central ERV or a network of ductless units can maintain consistently low CO2 throughout the home.

The best part: once set up, ERVs and HRVs work in the background. Instead of constantly opening windows or remembering to run bathroom fans, you can rely on a quiet, efficient system to keep CO2 in check every day and night.

Designing a Ventilation Strategy Around CO2 and Comfort

Lowering CO2 is important, but it’s only one piece of a truly healthy home. Good ventilation design also considers temperature, humidity, pollutants, and noise. The goal is a ventilation strategy that quietly supports your comfort rather than demanding constant adjustment.

Step 1: Map your home’s CO2 profile

Start with your CO2 monitor and gather data for at least several days in different rooms:

• Track bedroom CO2 overnight with doors and windows in their typical positions.
• Measure your home office or main living area during peak occupancy.
• Test CO2 during gatherings, workouts, or busy weekend times when the house is fuller.

Make note of which rooms regularly exceed your target range. These are your priority zones for ventilation improvements, product upgrades, or both.

Step 2: Set target CO2 levels for key spaces

Different rooms can have different CO2 goals based on how you use them. As a starting point:

• Bedrooms: Aim for overnight CO2 below about 1,000 ppm, and ideally around 600–900 ppm if practical with your climate and equipment.
• Home offices: Target 600–1,000 ppm during working hours to support focus, clear thinking, and consistent energy.
• Living areas: Because occupancy varies, staying under 1,000–1,200 ppm during typical family use is a reasonable goal.

Use these targets to size and select ventilation solutions. Remember that you can always start by improving the highest-priority room (often a bedroom) and expand later as budget allows.

Step 3: Choose the right mix of ventilation tools

With your CO2 goals and trouble spots identified, you can assemble a toolkit of solutions that might include:

• Ductless ERVs/HRVs in bedrooms and offices that regularly exceed target CO2 levels.
• A central ERV or HRV integrated with your existing HVAC for whole-home coverage in larger or newer homes.
• Upgraded, quieter bathroom fans rated for continuous operation, running on low most of the time and boosted when needed.
• Smart controls or timers that adjust ventilation when occupancy or CO2 levels rise.

Think of this as building a “ventilation stack” that can adapt to different situations: overnight sleep, workdays, parties, and vacations. CO2 monitors then serve as your real-time dashboard, confirming that the system is doing its job.

Ventilation as Critical Infrastructure for Healthy Homes

In the past, ventilation was often an afterthought, overshadowed by heating and cooling equipment. But as homes have become tighter and people spend more time indoors, it’s clear that controlled fresh air is just as essential to comfort and health as a good furnace or heat pump.

Indoor CO2 is a powerful, easy-to-measure reminder of this. High CO2 levels signal that your home is holding onto exhaled air and other pollutants instead of regularly refreshing its indoor environment. When you lower CO2 with better ventilation, you’re not just changing a number—you’re usually improving sleep, focus, mood, and how your home feels day to day.

By treating ventilation as critical infrastructure—planning for it in new builds, prioritizing it during renovations, and upgrading it in existing homes—you create a living environment that supports long-term health and comfort, not just short-term energy savings.

How Rise-Style Ventilation Solutions Support Healthy Indoor CO2 Levels

A modern, sustainable home doesn’t stop at efficient heating and cooling. It also includes thoughtfully designed ventilation solutions that keep CO2 and other indoor pollutants in check while respecting your energy and comfort goals. That’s where carefully selected ductless ERVs, HRVs, and smart air quality tools come in.

Ductless ERVs for bedrooms and home offices

For many homeowners, the most transformative upgrade is a compact, wall-mounted ductless ERV in the rooms where they spend the most time—bedrooms, nurseries, and home offices. These units:

• Continuously bring in filtered outdoor air and exhaust stale indoor air, reliably lowering CO2.
• Recover heat and some moisture between outgoing and incoming airstreams, so you gain fresh air without huge energy penalties or cold drafts.
• Operate quietly, often on low background speeds, so they don’t disrupt sleep or work.

If your CO2 monitor shows that one or two rooms are consistently problematic, this type of targeted, ductless ventilation can be a fast, effective way to reclaim comfort and clarity without reworking your whole HVAC system.

Whole-home ERVs and HRVs for total coverage

In larger or newer homes, a centralized ERV or HRV connected to ducts can provide even, whole-home ventilation. By distributing fresh air to main living spaces and bedrooms while drawing stale air from bathrooms and other exhaust points, these systems create a controlled airflow pattern that keeps CO2 and other pollutants diluted throughout the entire house.

• Integrating a central ERV/HRV with your existing HVAC can simplify controls and make it easier to manage comfort holistically.
• In high-performance homes built to advanced energy codes or certifications, whole-home balanced ventilation is often a requirement rather than an option.
• Combining central ventilation with room-level CO2 monitoring lets you fine-tune airflow and verify that all major spaces stay within target ranges.

Whether you choose ductless units, a central ERV/HRV, or a hybrid approach, the goal is the same: design your home’s fresh air system with the same care you give to heating, cooling, and insulation.

Pairing ventilation with smart CO2 monitoring and controls

Finally, remember that ventilation is most effective when it responds to actual conditions in your home. Pairing ERVs and HRVs with smart CO2-capable air quality monitors can unlock powerful control strategies:

• Boosting ventilation automatically when CO2 crosses a threshold during gatherings or busy evenings.
• Scheduling higher nighttime ventilation in bedrooms to protect sleep quality while running lower speeds during unoccupied hours.
• Adjusting ventilation seasonally to balance CO2 targets with comfort and energy use.

For homeowners and light-commercial operators, this combination of measurable data, responsive controls, and efficient ventilation hardware is what transforms indoor air quality from a guessing game into a predictable, manageable part of building performance.

Putting It All Together: A Practical Path to Healthy Indoor CO2

If you’re just beginning to think about indoor CO2 and ventilation, the process can seem technical. In practice, improving your home’s air is usually straightforward when you break it into clear steps:

• Measure: Get a reliable CO2-capable air quality monitor and spend a week learning your home’s patterns in bedrooms, offices, and main living areas.
• Experiment: Use windows, doors, and existing fans to see how simple changes affect CO2. This builds intuition about what your home needs.
• Plan: Decide on CO2 targets for sleep, work, and everyday living, then identify which rooms fall short most often.
• Upgrade: Add ductless ERVs/HRVs in priority rooms or consider a central system to bring the whole home into a comfortable CO2 range year-round.
• Verify: Keep an eye on CO2 trends after upgrades to confirm that your investment is delivering better sleep, sharper focus, and a consistently fresher-feeling home.

By following this path, you’re not just checking a box for ventilation—you’re turning your home or light-commercial space into a healthier environment that supports how you live, work, and rest every day.

Sources

• ASHRAE — Ventilation for Acceptable Indoor Air Quality (Standard 62.1/62.2) https://www.ashrae.org
• U.S. EPA — Indoor Air Quality: Carbon Dioxide, Health, and Ventilation Guidance https://www.epa.gov
• NIOSH — Carbon Dioxide Health Hazard Evaluations and Workplace Exposure Guidelines https://www.cdc.gov/niosh
• Building Science Corporation — Ventilation Strategies for High-Performance Homes https://buildingscience.com
• Lawrence Berkeley National Laboratory — Studies on CO2 Concentrations, Cognitive Function, and Ventilation Rates https://eta.lbl.gov