What You Need to Know About ASHRAE’s 2025 Position on Indoor Carbon Dioxide

Indoor carbon dioxide (CO₂) levels have long been used as a shorthand for indoor air quality (IAQ) and ventilation performance—but new guidance from ASHRAE (approved February 12, 2025) offers a more nuanced take. If you’re an HVAC professional, building designer, or facilities manager, this update could reshape how you monitor and manage indoor environments.

In this post, we’ll unpack what ASHRAE’s updated position means, why it matters, and how you can align with best practices going forward.
​

Why This Update Matters

ASHRAE has officially clarified that CO₂ is not a reliable, all-purpose indicator of indoor air quality. That’s a major shift in perception for many professionals and organizations that have relied on CO₂ levels as a go-to metric.
Here’s what’s changing:

• CO₂ can help evaluate ventilation but shouldn’t be treated as a standalone measure of IAQ.
• Health effects linked to typical indoor CO₂ levels are inconsistent and don’t justify changes to ventilation standards—yet.
• Consumer-grade CO₂ sensors can mislead if not properly calibrated or positioned.
  ​​
  

The Misconception: CO₂ = IAQ?

Historically, CO₂ levels have been used to infer whether a room is "stuffy" or under-ventilated. But ASHRAE’s 2025 position document emphasizes this can be dangerously simplistic.
While rising CO₂ concentrations may correlate with increasing levels of other indoor pollutants (because they all rise in poorly ventilated spaces), CO₂ is not the pollutant to worry about. In fact:

• Many indoor pollutants, like volatile organic compounds (VOCs) and particulates, aren't emitted in correlation with CO₂.
• Removing CO₂ alone does not improve overall air quality.

​

What ASHRAE Now Recommends

ASHRAE's revised position includes several critical recommendations for research and practice:
1. Use CO₂ Only as a Supplemental Tool - CO₂ can still help assess outdoor air ventilation rates—but only if applied correctly, with validated assumptions and proper sensor placement.
2. Avoid Over-Reliance on CO₂ for Infection Control - CO₂ is often promoted as a proxy for airborne virus risk, but its relationship to disease transmission is complex and indirect. Infection risk also depends on occupancy patterns, space types, filtration systems, and more.
3. Do Not Use CO₂ to Justify Air Cleaning Alone - Technologies that remove only CO₂ may interfere with ventilation controls and give a false sense of safety about indoor air quality.​

Best Practices for Building Professionals

If you work in HVAC, construction, education, healthcare, or real estate, here’s how to adapt:

• Calibrate CO₂ sensors regularly and ensure proper placement.
• Use CO₂ alongside other IAQ indicators—not in isolation.
• Educate clients and stakeholders about the limitations of CO₂ as a ventilation metric.
• Stay current with ASHRAE Standard 62.1, which now includes updated CO₂ setpoints for DCV—not as health limits, but for operational guidance.

​

A Future with Rising Outdoor CO₂

​As outdoor CO₂ levels continue to rise due to global climate change, indoor readings will shift as well. This reality makes it more important than ever to distinguish between indoor-generated CO₂ and ambient levels, especially when fine-tuning ventilation systems.
​

Final Thoughts

ASHRAE’s new position on indoor CO₂ is a call for smarter, more holistic air quality management. By understanding the limitations of CO₂ as an IAQ metric—and by investing in comprehensive, evidence-based monitoring—you can make better decisions for occupant health, safety, and performance.

See How Your Building Measures Up