Is this a story that you can relate to? 

A public health inspector flagged an issue at an older library facility. A worker complaint process resulted in an investigation into carbon dioxide (CO2) levels in various spaces. The readings were varied: 416 parts per million (ppm) in an unused office space; 1,222 ppm in the primary study area; and 1,563 ppm in a frequently utilized meeting room. Staff is now concerned. Does this range of CO2 concentration pose a human health hazard? 


CO2 concentrations and ventilation requirements

Carbon dioxide is a natural constituent of the air we breathe everyday. It is a colorless, odorless, non-flammable gas produced by the combustion of fossil fuels and through metabolic processes (such as the breathing you are doing right now). 

CO2 concentrations are frequently a point of concern in the context of indoor air quality. It is relatively easy to measure and is commonly included in indoor air quality monitoring procedures.


CO2 concentrations as a proxy for occupancy levels 

Carbon dioxide concentrations can serve as a proxy for occupant density in a space. The more people in a space, the higher the CO2 concentration. As such, CO2 monitors are frequently tied to demand-controlled ventilation (DCV) systems that adjust the amount of outdoor air brought into a space based on the occupancy. The concentration setpoint for DCV systems can vary based on specific applicable codes, standards, or rating systems being adhered to. However, the most common CO2 setpoint for DCV systems is 1,000 ppm.

Why 1,000 ppm?


Nonproblem buildings frequently exhibit CO2 concentrations over 1,000 ppm 

A number of studies have revealed that even in nonproblem buildings, CO2 concentrations can drift up around 1,500 ppm - and it would not be uncommon to achieve roughly double this concentration.

Carbon dioxide is usually not found at hazardous levels in indoor environments and is not considered a toxic gas at typical indoor concentrations. However, the U.S. Environmental Protection Agency (EPA) warns that prolonged elevated concentrations of CO2 in indoor air can cause discomfort, drowsiness, and decreased cognitive function.


A frame of reference for CO2 concentrations

How much is too much when it comes to CO2 levels in a building? Consider the following ranges.


300 - 500 ppm

According to ANSI/ASHRAE Standard 62.1-2022, Ventilation and Acceptable Indoor Air Quality, the CO2 concentrations in acceptable outdoor air typically range from 300 to 500 ppm. 


1,000 - 1,200 ppm

Indoor levels are usually higher than 500 ppm, due to the CO2 exhaled by building occupants. Thus, ASHRAE Standard 62.1-2022 recommends maintaining a steady-state CO2 concentration in an indoor space no greater than 700 ppm above the outdoor air levels - a recommendation long-maintained by Standard 62.1. This spots acceptable indoor CO2 limits around 1,000 to 1,200 ppm.


3,000 ppm

According to the National Collaboration Centre for Environmental Health, human metabolism alone can lead to CO2 levels in excess of 3,000 ppm, especially in poorly ventilated rooms. Indoor combustion appliances, in particular gas stoves, can also increase CO2 levels. 


5,000 ppm and beyond

The Occupational Safety and Health Administration (OSHA) has established a Permissible Exposure Limit (PEL) for CO2 of 5,000 ppm averaged over an 8-hour work day (time-weighted average or TWA). 

Similarly, the Centers for Disease Control and Prevention (CDC) cites an American Conference of Governmental Industrial Hygienists (ACGIH) recommendation of an 8-hour TWA Threshold Limit Value (TLV).  

Arguably, sustained and limited exposure to CO2 concentrations at or below 5,000 ppm should be considered the upper limit of regular "safe" exposure. The U.S. Department of Agriculture (USDA) acknowledges the increasingly severe physiological effects of prolonged exposure of CO2 concentrations beyond 5,000 ppm up to 80,000 ppm.


Setting a better CO2 concentration target

Data suggest many of us dwell in buildings that fail to meet acceptable ventilation rates by today's best practices. If your project team is meeting the minimum ASHRAE standard, you should expect CO2 levels under 1,000 ppm. However, the recent Harvard COGfx Study suggests that increasing ventilation in our buildings such that carbon dioxide levels are kept at / under 600 ppm may result in significantly improved cognitive function.

The emerging scientific evidence is compelling: lower CO2 concentrations support our general health and improve our cognitive function.


Figure: Continuous carbon dioxide (CO2) monitorization over a 21-day period in a newly-constructed high-performance home. The spike in CO2 concentration levels during the third week coincided with a series of open-house events. Figure by Daniel Overbey.