Have you ever woken up in a stuffy bedroom or left a meeting with a headache, even though the room temperature felt comfortable? Often, the cause isn’t heat—it’s a buildup of carbon dioxide (CO₂) from people breathing in a space with limited fresh air. While outdoor CO₂ levels are typically around 400 ppm, indoor levels can rise much higher in tightly sealed homes, schools, and offices. Although these levels are not usually dangerous, CO₂ is an important indicator of ventilation and indoor air quality. Monitoring CO₂ can help you know when it’s time to bring in more fresh air.