Revisiting the 1,000 ppm CO2 Limit

The 1989 revision of ANSI/ASHRAE Standard 62, Ventilation for Acceptable Indoor Air Quality, states that "comfort (odor) criteria are likely to be satisfied if the ventila-tion rate is set so that 1,000 ppm CO2 is not exceeded ";1 an accompanying, informa-tive Appendix D describes the mathematical relationship between ventilation rates and steady-state indoor carbon dioxide (CO2) levels. However, the recommendation regarding CO2 has changed over the years. Standard 62-1999 replaced the original limit with a guideline stating that indoor CO2 concentrations should be 700 ppm or less above the ambient outdoor concentration. Standard 62.1-2019 entirely removed both the guideline and Appendix D. Over the years, many practitioners concerned about air quality have mistakenly continued to refer to the 1,000 ppm CO2 absolute limit of Standard 62-1989 as an indicator of acceptable indoor air quality(2)-but then again, it may be not a mistake after all.