Carbon dioxide (\(\text{CO}_2\)) is an odorless, colorless gas that is a natural component of the Earth’s atmosphere. It is constantly produced by human respiration, making it a primary byproduct within any occupied, enclosed space. While \(\text{CO}_2\) itself is not typically a direct toxin at levels commonly found in buildings, its concentration serves as an accurate, real-time indicator of how much fresh outdoor air is entering a space. High indoor concentrations signal inadequate ventilation, which allows not only \(\text{CO}_2\) but also other pollutants and airborne pathogens to accumulate. Understanding what constitutes a safe level is done by measuring the volume of the gas in parts per million (PPM).
Understanding Ambient and Ideal Concentrations
Current typical concentrations in the ambient outdoor atmosphere are approximately 400 to 425 PPM. This outdoor level represents the lowest possible concentration that can be achieved indoors through maximum ventilation.
Maintaining indoor \(\text{CO}_2\) levels close to the outdoor concentration is the goal for achieving the highest quality indoor air. An ideal indoor concentration for prolonged occupancy is often considered to be below 600 PPM. Levels ranging from 400 to 1,000 PPM are typical in occupied spaces with effective air exchange systems. Keeping levels in this lower range indicates excellent ventilation that is consistently introducing sufficient fresh air.
Defining Standard Indoor Air Quality Thresholds
The most widely accepted safety guideline for indoor \(\text{CO}_2\) levels is set by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). ASHRAE standards suggest that indoor \(\text{CO}_2\) concentration should not exceed the outdoor level by more than about 700 PPM. Given the outdoor baseline of approximately 400 PPM, this translates to an upper acceptable limit for prolonged occupancy of about 1,100 PPM.
Levels between 800 PPM and 1,000 PPM are not immediately dangerous but are considered a clear sign of poor ventilation that requires attention. These levels should prompt immediate action to increase the introduction of outdoor air to prevent further concentration increases.
The U.S. Occupational Safety and Health Administration (OSHA) sets a Permissible Exposure Limit (PEL) of 5,000 PPM, averaged over an eight-hour workday for industrial or occupational settings. This high limit is designed to prevent acute health harm in a healthy working adult population and should not be confused with the air quality targets for residential or educational buildings.
Health Risks of Elevated \(\text{CO}_2\) Exposure
As indoor \(\text{CO}_2\) levels rise above the ideal range, people begin to experience noticeable physiological and cognitive effects. In the mild exposure range of 1,000 to 2,000 PPM, occupants frequently report feelings of drowsiness, fatigue, and a general perception of stale or stuffy air. Scientific studies have linked these concentrations to a measurable decline in cognitive function, including reduced concentration and impaired decision-making abilities.
Moving into the moderate exposure range of 2,000 to 5,000 PPM, the symptoms become more pronounced and uncomfortable. Individuals may experience headaches, increased heart rate, and slight nausea. Sustained exposure in this range forces the body to work harder to compensate for the change in blood acidity, which can contribute to long-term issues like increased oxidative stress and potential cardiovascular strain.
Exposure to extremely high concentrations, particularly those exceeding 5,000 PPM, carries severe and immediate risks. Concentrations above 40,000 PPM are considered immediately dangerous to life and health, as \(\text{CO}_2\) can displace oxygen, leading to asphyxiation, loss of consciousness, and even death.
Strategies for Monitoring and Improving Indoor Air
The only reliable way to know the concentration of \(\text{CO}_2\) in an indoor space is through monitoring with a dedicated device. The most accurate and reliable consumer devices use Non-Dispersive Infrared (NDIR) sensor technology. NDIR sensors work by measuring the specific wavelength of infrared light that \(\text{CO}_2\) molecules absorb, providing a highly precise reading of the PPM concentration.
When monitoring reveals elevated \(\text{CO}_2\) levels, the most effective solution is to increase the introduction of fresh outdoor air into the space. Simple actions like opening windows and doors provide immediate relief by allowing natural air exchange. In buildings with mechanical ventilation systems, the air handling unit’s fresh air intake rate must be increased to bring the \(\text{CO}_2\) levels back down below the 1,000 PPM threshold.