Myth: CO2 affects the perceived air quality
FALSE.
CO2 is a colorless and odorless gas
300,000 ppm (>30%) of CO2 can activate trigeminal nociceptors to produce a burning sensation in our mucous membranes (sharp, acidic odor)
CO2 levels, a quick recap:
Outdoors: ca. 420 ppm (0.04%)
Indoors: up to 5,000 ppm (0.5%), usually less than 2,000 ppm (0.2%)
Exhaled: 40,000 ppm (4%) to 56,000 ppm (5.6%)
Myth: CO2 at levels typical in non-industrial buildings affects health
FALSE
The threshold limit value (occupational) is 5,000 ppm (0.5%) averaged over the 8-hour work shift. The short-term exposure limit is 30,000 ppm (3%), not to be exceeded during any 15-minute work period.
Myth: CO2 then accumulates in the body which cause negative effects
FALSE
A body maintains acid-base homeostasis (pH 7.36-7.43). Enhanced respiration, and accumulation in bone marrow and extraction through kidneys remove CO2 from the blood; the body can accumulate up to 120 L of CO2.
Myth: CO2 must be removed from the air to improve perceived IAQ
FALSE
A new study used activated carbon to remove human bioeffluents other than CO2 and showed improved air quality with air cleaner even though CO2 remained at 3,000 ppm.
Source: Nami Akamatsu, Soma Sugano, Kanta Amada, Naho Tomita, Hidetaka Iwaizumi, Yuki Takeda, Pawel Wargocki, Bjarne W. Olesen, Shin-ichi Tanabe, Effects of a gas-phase air cleaner in removing human bioeffluents and improving perceived air quality, Building and Environment, Volume 257, 2024, 111540, ISSN 0360-1323, https://doi.org/10.1016/j.buildenv.2024.111540. (https://www.sciencedirect.com/science/article/pii/S0360132324003822)
Myth: CO2 is a marker of ventilation
TRUE
Once we know all variables influencing ventilation estimation using CO2 measurements, including emission rates of CO2, whether the steady state level was obtained, mixing, etc. It is a crude estimate.
Myth: CO2 is a marker of IAQ
FALSE
CO2 is, under specific conditions, a marker of ventilation, and is subject to the same restrictions as ventilation
Myth: CO2 is a marker of infection risk
FALSE
Emission rates of CO2 does not correlate with vocal activities (breathing, talking, sneezing, coughing) and neither with the emission of infectious aerosols.
Myth: CO2 should be kept at 1,000 ppm in buildings, also recommended by ASHRAE
FALSE
ASHRAE Standards do not recommend levels of CO2 for achieving good indoor air quality. Worldwide recommendations vary a lot with regard to CO2
Source: Mendell et al (2024)
Myth: Ventilation with outdoor air should keep CO2 in occupied spaces below 1,000 ppm
TRUE
Studies on learning in schools, sleep, and infection risk suggest that to avoid negative effects and risks the ventilation with outdoor air should be around 10 L/s per person to keep CO2 levels (a marker of ventilation) around 800 ppm (0.08 ppm) or lower.
Source: Wargocki et al (2020)
Source: Wargocki at al (2023)
Myth: CO2 is easy to measure
TRUE
With low-cost sensors that task is seemingly easy but the measurement is crude and interpretation depends on many factors such as the location of the sensor (usually on a wall so far away from a person), calibration (performance) of the sensor, mixing, and alike.
Myth: CO2 sensors are the solution to IAQ problems in buildings
FALSE
They will not indicate pollution penetrating from outdoors (PM2.5, NO2) and neither correlate with the product of chemical reactions (with ozone and other reactants) as well as other pollutants of which emission rates do not correlate with CO2.