Twenty years ago a colleague gave me a portable carbon dioxide (CO2) monitor to use during my training sessions. These sessions were often in small, inadequately ventilated rooms loaded with too many breathing trainees. Tony suggested that I try to determine at what CO2 level I noticed my trainees’ attention started drifting away. After following Tony’s suggestion for a few months, I determined that my trainees were “gone” when the CO2 in the room reached 1000 to 1200 parts per million (ppm). Although I used the CO2 monitor with at least a dozen different groups, the results were always the same. When this level was reached they would become glassy-eyed, stop paying attention, and their response rates would go down.

After lugging around this CO2 monitor for too long, I gave it back to Tony. I had used it to determine the CO2 threshold that affected the apparent attention of my trainees. After I returned the device, I found I was using my trainee’s level of attention to determine the approximate CO2 level in the room; the reverse. This was an interesting anecdotal survey. I forgot about it until late 2015 when some significant research brought back the memory.

What is Carbon Dioxide

CO2 is a natural component of our atmosphere, usually expressed in parts per million (ppm). Plants utilize CO2 and we exhale it. A typical adult’s breath contains about 35,000 to 50,000 ppm (3.5 to 5 percent). At low levels, carbon dioxide is harmless, but at high levels – 100,000 ppm or more – it can cause unconsciousness or death (refer to Table 1).


This article focuses on this rather unique contaminant that cannot be eliminated from our interior environments because humans generate it. The only option is to control its levels to within a healthy range. CO2 is similar to moisture in our interior air; we exhale it, so it is present in our dwellings. But too much of it can be harmful.

Carbon Dioxide in the Atmosphere

As a result of news and research about climate change, we all know that the concentration of CO2 in the outdoor air is slowly increasing. Historically the levels were in a narrow band from about 200 to 300 ppm. Since 1988, global levels of CO2 have never gone below 350 ppm and are currently just above 400 ppm [NOAA] and rising.

In some locations, outdoor levels of CO2 can be significantly higher than global averages. CO2 “domes” over many large cities can be 100 to 200 ppm above the current global average of 400. This results from auto emissions, the use of heating fuels, electricity generation, and weather conditions.

Most of us are aware of the climatic impacts of these increased CO2 levels and many of us are working to reduce our “carbon footprint” by making our homes energy efficient, driving efficient cars, and avoiding air travel. Stabilizing the CO2 level in the atmosphere is significant to our future on this planet. Almost 200 countries recently agreed at the 2015 United Nations Climate Change Conference in Paris to limit CO2 emissions. This significant global agreement recognized the hazards of CO2 and other greenhouse gases, and the negative impacts they are bringing to life on earth. As it turns out, there is even more to know about the hazardous impacts of CO2!


Figure 1. CO2 levels at Mauna Loa, Hawaii. Source NOAA.

Carbon Dioxide in Our Homes

Recent research has shown that CO2 levels in our homes and workplaces affect us at lower levels than previously thought. Years before I had found that increasing levels of CO2 affected the attention of my trainees. What is more striking is that we are now discovering that fairly low levels of CO2 in our interior environments can adversely affect human cognition, or the way we think.

For many decades, we used CO2 concentrations in interior environments as an indicator of ventilation needs. For example, a CO2 monitor/control in a office building might turn on the ventilation system if the CO2 concentration reached 1000 ppm, a common threshold value for commercial buildings. Conversely, if CO2 levels reach a low level, the ventilation system is shut down.

As a result of studies completed late last year, researchers found that once-acceptable levels of CO2 in offices, schools, cars, and homes can adversely affect human cognition and decision-making. The authors of a significant Harvard School of Public Health study[1] found “statistically significant and meaningful reductions in decision-making performance” in tested subjects as CO2 levels increased from a baseline to 600 ppm to higher levels. Of the seven cognitive domains that were studied, the researchers found “the largest effects were seen for Crisis Response, Information Usage, and Strategy, all of which are indicators of higher level cognitive function and decision-making.” Researchers are now suggesting interior CO2 levels from 800 to as low as 600 ppm for good cognitive health.

This research not only has significant implications for offices and other interior environments where people work while expelling CO2, but also has implications for the air quality in our dwellings. CO2 levels of 1000 had not previously been shown to negatively impact cognitive abilities. While additional field research is needed to confirm these findings, this Harvard study is cause for concern. 

What Does Ventilation Have to Do with It?

In older, leaky dwellings, outdoor air naturally diluted the indoor air, especially during cold (increased stack effect) or windy weather. This outdoor dilution air is often referred to as infiltration. As a result of our efforts to make our dwellings more energy efficient, infiltration rates have been reduced over the last three decades. In new or weatherized dwellings, mechanical ventilation is now accepted as the best method of introducing a healthy amount of outdoor dilution air. It does cost a bit to operate, but it is controllable and much more dependable in all weather conditions than haphazard infiltration. It is always healthier to build a house tight and ventilate it properly than to intentionally build a leaky house.

When a home is occupied, indoor CO2 levels will always be higher than outdoor levels. As occupants exhale CO2, indoor levels continue to climb unless the indoor levels are diluted by outdoor air. The difference between indoor and outdoor CO2 levels is commonly referred to as the “differential”. In order to reduce this differential, the ventilation rate can be increased for a given number of occupants (refer to Table 2).


Outdoor Levels are Climbing, Healthy Indoor Levels are Falling: What is a Homeowner to Do?In commercial buildings, such as offices, the ventilation rate is often regulated with CO2-activated controllers, turning on or increasing ventilation when CO2 levels increase. In these buildings, CO2 is used as an indicator of the need for ventilation; the greater the number of occupants, the greater the ventilation rate required to keep CO2 levels in check. For other occupant-based indoor contaminants, such as body odor, CO2 levels are an effective surrogate also. On the other hand, CO2 is not an effective surrogate for contaminates that originate from non-occupant sources, such as building materials and furnishings.

Installing and using a mechanical ventilation (fresh-air) system in a new or weatherized existing home is now common. The Department of Energy requires ventilation for homes weatherized by their low-income program and states are adopting ventilation requirements. Mechanical ventilation is good for the health of the occupants and costs very little to operate. The primary ventilation standard used in the U.S.A. is Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings, ASHRAE 62.2-2013. If a ventilation system is installed according to this minimum standard, it is likely that CO2 and other contaminants will remain at healthy levels in the indoor air.

An important fact to remember is that we cannot experience CO2 levels inside our dwellings that are lower than outdoor levels, there will always be a differential. This differential depends on a number of things, including the mechanical ventilation and infiltration rates, the number of occupants, and the occupant activity (metabolism) level.

There are two issues making CO2 more of a concern. First, outdoor levels are climbing (refer to Figure 1). We have now passed a global average of 400 ppm and some large population areas experience levels that are significantly higher. Remember, our MINIMUM indoor level is dependent on the outdoor level.

Second, research is indicating that we might be adversely affected by CO2 levels that were once thought to be benign. If it is substantiated that CO2 levels as low as 600 ppm can adversely impact human cognition, what will this mean for people occupying homes, school, and offices? If the outdoor CO2 level is 600 ppm in a large city and the differential between outdoors and indoors is 500 ppm with a 20 CFM ventilation rate (rather high), the resulting indoor CO2 level is 1100 ppm. Is the collision course clear between increasing outdoor levels and this recent study demonstrating the need for lower indoor levels?

In addition to using a mechanical ventilation system, it is recommended by some specialists that homeowners monitor CO2 levels in dwellings. A number of table-top monitors are now available for just over $100. Many of these inexpensive devices record high and low values. For $20 to $30 more, data-logging features are available, so that a homeowner can view time-stamped historic CO2 values. These devices are usually accurate enough for home use and often simultaneously monitor temperature, relative humidity, and other gases. The use of one of these monitors makes it clear to a homeowner that CO2 levels increase directly with the number of occupants and inversely with the ventilation rate.

With outdoor CO2 levels climbing and research indicating that benign levels are lower than previously thought, the healthy CO2 range for folks on earth is narrowing. At the least, we must study the impact of CO2 on human cognition and determine if ventilation rates should be increased to lower indoor CO2 levels.

[1] “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” by Joseph G. Allen, Piers MacNaughton, Usha Satish, Suresh Santanam, Jose Vallarino, and John D. Spengler, October 2015.

Additional Resources: