2020.12.22

Monitoring carbon dioxide concentration in a room as an indicator of ventilation

Research and Development by

Shiro HARA, AIST

Corresponding Research Area

Use of ICT and IoT for contact reduction


Non contact technology for prevention of infection: Sensing of carbon dioxide concentration

ISSUE:
There is no clear knowledge on how to ventilate air-borne particles to prevent from being infected with coronavirus although how to prevent from the virus for two transmission routes via close contact and surface fomite is relatively well-known.

SOLUTION:
Exhaled air includes carbon dioxide (CO2).  So, by measuring the concentration of CO2 in a room, the quality of air and ventilation situation are known. Thus, using a CO2 sensor, people can manage ventilation so that CO2 level remains at a desired low level.

→In this study, we investigate CO2 concentrations in various kinds of rooms and halls to find out the validity of CO2 measurement as an indicator of ventilation.

Measurement of CO2 in a meeting room with windows

  • The CO2 concentration rises when the number of people increases.
  • The CO2 concentration reduces by ventilation with windows opening for 5 minutes.
  • Up to 700 ppm of CO2 it is felt that air is good. Over 1,000 ppm people begin to feel that the air is a little bit bad.
    !  Note that the sensitivity for a bad air is considerably different by a person.  The experiment of 1,000 ppm is not shown here.
  • CO2 of outside air is ~ 415 ppm.
  • In a relatively dense situation with 16 people, a little opening of two windows and two corridor side doors ventilate the room stabilizing the CO2 concentration to ~ 500 ppm.

→In a meeting room with a sufficient machine ventilation, the CO2 concentration can be reduced to 1,000 ppm or less in the situation implementing social distancing measures in the room.   In a meeting room without the sufficient machine ventilation, opening windows and doors are very effective for a quick ventilation.

Measurement of CO2 in a meeting room without a window (Room area is 40 m2, assuming a meeting in 8 people)

  • In a holiday when air conditioning is turned off, the CO2 concentration rises more and more without an end.
  • In a business day when air conditioning is turned on, the CO2 concentration saturates at ~ 1,000 ppm or less.   To keep CO2 under 1,000 ppm, it is necessary to reduce the density of the person to half.

!  Note that an air conditioner only circulates the indoor air inside the room. A building general ventilation system is needed to take fresh air into the building and to vent air with CO2 out to atmosphere.

In this experiment to prevent experimenter from coronavirus infection, the amount of CO2 exhalation for 7 people is diffused using seven CO2 gas generators.
Room size: 8.7m x 4.6m x h2.7m

→At a meeting the density of the person is recommended to be half of seating capacity.  At a meeting held on a holiday, a building general ventilation should be turned on.   Otherwise, the meeting should be ended within half an hour.

Measurement of CO2 in a large office (214 seats in 3,000 m2, 110 people are working)

  • The CO2 concentration may exceed 1,000 ppm in a typical office with the person density of 3 m2 / person.   In this type of cramped office, people should attend every other day. Or the amount of fresh air taken from outside should be doubled or tripled.
  • In the office shown in the right figure, where the person density is 15 m2 and attendance rate is ~50 % by work from home, the resultant density becomes ~ 30 m2, thus CO2 becomes much lower than 1,000 ppm.
  • In a recent office style with the person density is 8 m2,  the CO2 level of 1,000 ppm will be realized.

→In a large office, the person density of > 8 m2 or work from home with around half attendance rate  is recommended.

Measurement of CO2 in an adult daycare center

  • A constant ventilation by opening windows reduces CO2 to ~ 700 ppm for 14 people.
  • An intermittent ventilation by opening windows for 5 min / hour suppresses CO2 to ~ 1,500 ppm
  • A more frequent ventilation of 5 min every 30 min is recommended.
  • Chattering during teatime rises CO2 drastically.
  • If the ventilation management is neglected, CO2 increases more and more as time goes by.

→Opening window is quite effective for ventilation.   In winter, combination of ventilation and reinforcement of heating is recommended. 

Measurement of CO2 in a Karaoke box (One person sings and two persons listen in a box of eight capacity)

  • Emotional songs sung in a loud voice double energy consumption (calories) and CO2 generation than calming songs.
  • The CO2 concentration saturates at ~ 1,000 ppm within 15 min in a small room of 3 m x 2.5 m x h2.1 m with eight capacity.  If eight people of the full capacity song at the same time the CO2 concentration will increase up to ~ 2,000 ppm.
  • Since a Karaoke box has no window to reduce   noise, a smaller number of persons than the capacity and singing with a small voice  in turn one by one are recommended.

→Emotional songs sung in a loud voice double calories and CO2 generation than calming songs.  A small number of people of ~ 40% of capacity and singing with a small voice  in turn one by one lower CO2 to ~ 1,000 ppm.

Measurement of CO2 in a large cafeteria

  • A large cafeteria of 450 m2 we used for CO2 measurement has smaller lunch-time users because of work from home.  The present number of lunch-time users becomes around 100 people about half than the past before corona.
  • The performance of ventilating is not same as there are various forms  in restaurant, dining hall, cafeteria, fast food, etc.  To know the performance a CO2 sensor should be used.
  • Cooking produces a lot of CO2 gas as shown in the right figure.

→In a restaurant with a kitchen, CO2 produced during cooking should be differentiated from CO2 produced from people.  

Relation between energy and CO2 in exercises

  • It is well-known that the exhaled CO2 amount is roughly proportional to the energy consumption (calorie) of the human. In this experiment, we confirm that the relation between the energy and the CO2 generation rate as shown in the right figure.
    Energy consumption (kcal)        = METs × weight(kg) × time(hour) × 1.05

The CO2 generation rate per energy consumption for a deskwork were measured as follows:

CO2 generation rate/person= 0.253 L/(min・person)
[a deskwork by an adult male]

∴ CO2 generation rate(L/min) ~ 0.0044 × METs × weight(kg)

→The generation rate of CO2 is proportional to exercise intensity.   Ventilation should be strengthened as intense exercise.

Measurement of CO2 in a house

  • In a house with 24 hours ventilation prescribed by a Japanese law, the CO2 concentration may not exceed 1,000 ppm.
  • In the right figure, the vent hole and a fan were cleaned again after the former cleaning 11 months ago. The performance of the vent rose ~ 10% after the last cleaning.
  • The CO2 concentration was quickly reduced to half in 5 min when a window was opened after getting up in the morning.
  • Wind directions in each private room basically direct outward. The air flows in each room are independent.  But, living room, dining room and corridors are located at upstream positions of the private rooms. When an infected person appears, attention is necessary so that the line of flow with the infected person does not intersect.

→In a house cleaning of vents and fans are effective for a better ventilation.  Also, the directions of the fan tell us how to prevent uninfected people from infection.

Experimental results

  • Exhalation of CO2
    • The amount of CO2 tends to be proportional to the amount of exercise (~ calorie) of a person.
    • Singing in a loud voice increases calories and CO2 generation than singing moderately.
  • Ventilation
    • Opening windows are quite effective for quick ventilation.  Even in a calm weather with little wind, opening windows have 2 to 3 times higher ventilation performances than a mechanical ventilation in an office, or 20 to 30 times than 24 hours ventilation in a house.
    • A well-managed intermittent ventilation prevents the room from CO2  increase.
    • In a building with closed windows, it is effective that taking fresh air into the building and venting air with CO2 out to atmosphere.  Just an air circulation inside a room by an air conditioner has no effect for ventilation.
    • In almost all places, there is no information of ventilation performance ( or ventilation speed ).  In this unknown situation, there is no criterion about to what degree we should ventilate.  To know the ventilation performance, measurement of the CO2 concentration by a CO2 sensor is quite useful.  Once we use the CO2 sensor, we do not have to know the value of ventilation performance itself and we can get a good air quality with CO2 lower than 1,000 ppm by control opening windows, building general air conditioner, and the person density.

Ventilation performances in various kinds of places

→Amid the coronavirus crisis in Japan, most places we investigated have good air qualities by controlling opening windows and the person density with helps of voluntary action restraints by users.   But, in general, ventilation performances are different by individual circumstances.  A cramped place, a situation many people shouting, or a place with bad ventilation or no window is easy to raise the CO2   concentration.  Thus, in these places, it is recommended to use a CO2 sensor in order to control the ventilation so that the concentration is lowered than 1,000 ppm.

Summary

  • By measuring the CO2 concentration, ventilating situation in a room is clearly known for any number of people.
  • By using the ways to manage ventilation using a CO2 sensor in the types of places investigated in this study, the CO2 concentration is suppressed lower than 1,000 ppm prescribed by the Building Standards Act of Japan.
  • But, in general, ventilation performances are different by individual circumstances.  For more effective ventilating, the continuous measurement of the CO2 concentration is useful and it can optimize the ventilation way.
  • Even if ventilation performance is unknown, the combination of opening widows and turning up room heating so that more fresh air is taken in can be optimized by the CO2 measurement.
  • The CO2  measurement only useful for optimizing ventilation.    At the same time, as solutions for close contact, face mask, partition, and social distancing should be used.   Also, as solutions for surface fomite, hand washing and alcohol disinfection should be used.

How to ventilate

  • The CO2 concentration in a room can be an indicator to know the degree of ventilation in the room.
  • A good ventilation can suppress the CO2 concentration lower than 1,000 ppm in most types of room.
  • The following actions are quite effective to suppress the CO2 concentration lower than 1,000 ppm.
    • [1] open windows and doors
    • [2] turn up the building general air conditioner to take in fresh air and bring out dirty air.
    • [3] reduce the number of persons

Appendix:  criterions of CO2 concentration

  • House:  Indoor air should be exchanged once in two hours. (Or nominal air exchange rate should be 0.5 times/h), according to the Building Standards Act of Japan.
  • Office:  The concentration of CO2 should be lower than 1,000 ppm, according to the Building Standards Act, the Building Sanitation Law, and the Ordinance on Health Standards in Office of the Industrial Safety and Health Act.
  • School:  The concentration of CO2 should be lower than 1,500 ppm, according to the Standard for School Environmental Sanitation.

→These standards are criterions in terms of a general aspect of health.  Considering that there are many coronavirus cases under these laws, we should manage ventilation not only by the CO2 concentration but by "CO2 x staying time".