HVAC & Control Engineering

VENTILATION AS ONE OF THE METHODS OF COUNTERING WITH THE CORONAVIRUS PANDEMIC

The risk of coronavirus infection can be predicted by the level of carbon dioxide CO2

The media gives a lot of advice on countering the spread of coronavirus, these are the rules of personal hygiene, maintaining distance, face masks, restricting communication, etc.

Unfortunately, inviting people to stay at home and not go out on the street, the question of how to reduce the risk of contracting a virus in a room is poorly explained. Let's try to fill this gap with the example of ventilation.

Theory.

It has been scientifically proven that the quality of the air we breathe in a room directly affects the likelihood of contracting airborne infections, such as tuberculosis, measles, influenza and rhinovirus infections. Among the latter, is the coronavirus (COVID-19), a pandemic which began in China in December 2019 and quickly spread to the whole world.

To predict the risk of transmission of infectious diseases transmitted by airborne droplets indoors, the Wells – Riley equation (1978) is used in modern science.

Where:

P - probability of infection;
D is the number of cases of the disease;
S is the number of people in contact in the room;
I - the number of sick people in the room;
p - human respiratory rate (m³ / s);
q is the rate of quantum generation of infection by a sick person (quantum / s);
t is the total time spent in the room (s);
Q is the amount of fresh air entering the room per unit of time (m³ / s).

However, in practice, the use of this equation is difficult, since it assumes the existence of established conditions and requires accurate measurement of the amount of outdoor air supply. Unfortunately, now in most apartments, shops, schools, offices and hospitals or there is no organized flow of fresh air through the supply ventilation or the rooms have ineffective ventilation, which cannot provide its required amount. In most cases, fresh air enters the premises through natural infiltration, which is very difficult to control.

In a scientific study by SN Rudnic (Department of Environmental Health, Harvard School of Public Health, Boston, MA, USA) and DK Milton (Department of Medicine, The Channing Laboratory, Brigham and Women's Hospital and Harvard Medical School, Boston, MA, USA) , the results of which are presented in the article “Risk of indoor airborne infection transmission estimated from carbon dioxide concentration”, the authors propose an alternative approach to predicting the risk of infection by airborne diseases. In the basic equation of the new theory for estimating the amount of fresh air in a room, the authors propose using the level of CO2 as an indicator of air quality. The equation obtained in the study is a non-stationary version of the Wells – Riley equation, which is now applicable for calculations in poorly ventilated rooms. The relationship between the average number of infections and air quality is also determined, which demonstrates the likelihood of an achievable critical proportion of inhaled air in the room, below which the airborne spread of respiratory infections and flu will not occur.

To assess the likelihood of infection spreading in people, S. N. Rudnic and D. K. Milton use the R0 indicator - the main reproductive number. This is the number of secondary infections that occur when one infected person is in the community of other people susceptible to this infection. If R0> 1, the infection will spread. The higher the R0 value, the greater the likelihood of an epidemic.

The reproductive number for infectious diseases spreading indoors (RA0) can be expressed as follows:

Where:

RA0 - reproductive number for infectious diseases in the premises;
n is the number of people in the room;
f - the proportion of air in the room exhaled by people in it;
q is the rate of quantum generation of infection by a sick person (quantum / s);
t is the total time spent in the room (s).

To assess the proportion of air exhaled by people and the rate of quantum generation of infection by a sick person, scientists suggest using the value of the CO2 level as the most adequate indicator.

As a result, the authors of the study give plots of the probability of infection depending on the CO2 content in the room air for three different types of infections.

The family of curves shown in the graph above describes the possibility of infection during a hypothetical outbreak of measles at a rate of quantum generation of measles infection q = 570 (1 / h). In this case, the reproductive number (RA0) increases almost linearly with the increase in the number of people at high concentrations of CO2. However, the reproductive number (RA0) does not increase directly with the increase in the number of people at low concentrations of CO2. In the latter case, the increase in the likelihood of illness with an increase in the number of people is much slower.

But even at the lowest CO2 concentrations, the reproductive number is much greater than 1. This suggests that measles will spread very quickly even in buildings with very good ventilation.

The figure above shows graphs of the change in the reproductive number (RA0) for a hypothetical outbreak of influenza, characterized by a quantum generation rate of 100 (1 / h), where it is assumed that the infected person will remain in the building for 4 hours. The alignment of the reproductive number (RA0) is again visible, although this time it occurs even at high CO2 concentrations. At low CO2 concentrations, the reproductive number (RA0) drops below 1. The critical respirable fraction is 0.25%, which is equivalent to a CO2 concentration of approximately 500 ppm. Thus, very high outdoor air flow rates while limiting the number of people in a room can be effective in limiting the spread of influenza.

The figure above shows the family of reproductive number curves (RA0) for rhinovirus infections, which include coronavirus (COVID-19). In this case, the rate of quantum generation of infection by a sick person was taken based on experimental data at q = 4 (1 / h). It is also assumed that during the entire time an infected person spends a total of 24 hours in the building. On all the curves there is a level stabilization for the number of people over 20. And if the supply ventilation ensures a CO2 level of no higher than 600-700 ppm, we can count on prevention spread of infection.

Conclusions

Based on the results of the study, it becomes obvious that one of the effective ways to reduce the risk of the spread of rhinovirus infection, which includes coronavirus, is the organization of high-quality supply and exhaust ventilation, which will provide a CO2 level of no higher than 600-700 ppm.

If the room does not have a supply and exhaust ventilation system, this effect can be achieved by regular ventilation with constant monitoring of the CO2 level.

Recommendations
1. How to properly ventilate the premises in the face of a threat of respiratory infections.

The most correct and effective methods of ventilation will be the methods used in medical institutions, etc. "Clean rooms." In such systems, the flow of fresh air is carried out by a laminar flow through special air distributors. For example, through ceiling laminators in one direction, for example, from top to bottom. Air recirculation is completely eliminated, and the supply air is carefully filtered in HEPA filters. In addition to the set temperature and humidity, ventilation equipment and automation constantly maintain overpressure in the room. This prevents the entry of external polluted air.

Unfortunately, this method is expensive and has high operating costs.

Less costly is the use of standard supply and exhaust ventilation. In this case, fresh air is supplied through the air handling unit to the room. An exhaust unit draws exhaust air out of the room. In the supply unit, fresh air is cleaned of dust, heated or cooled.

Existing building codes regulate the frequency of air exchanges, which is calculated depending on the purpose of the room, the number of people, their occupancy and the amount of pollution that are released inside. The CO2 level is currently not regulated by current building codes. Therefore, if you want to control the risk of the spread of respiratory diseases, you need to measure the level of CO2 and adjust the performance of the supply and exhaust systems to maintain CO2 no higher than 600-700 ppm.

To measure CO2, you can purchase an indoor air quality monitor that measures at least temperature, humidity, and CO2. For example, such, Walcom HT-501

Such monitors will only inform you about the current state of the air in the room but they will not be able to change the situation. If the air quality is not satisfactory, you must decide on your own whether to ventilate the room or turn on ventilation, if any. Or maybe the other way around is to close the window if microparticles of pollution fly to you from the street.

When the issue of air quality concerns large public spaces, such as offices, schools, hospitals, etc. this method of monitoring air quality is ineffective.

Here you need to install automated measuring stations or air quality monitors with their connection to ventilation and air conditioning systems.

The principle of action is as follows.

Based on the results of constant measurement of air parameters, monitors generate commands for controlling the ventilation, conditioning, heating and humidification systems.

Our company offers the installation of such an air quality monitoring system for apartments, cottages, offices, educational and medical institutions. This system can control ventilation, air conditioning, heating and humidification. Also, air condition data is available from smartphones or a computer in real time and in recording. Sensors for monitoring air quality in each individual room are mounted in standard wall sockets. The sensors can communicate with the WEB server via wires using the Modbus RTU protocol or wirelessly via WiFi. In the latter case, it is sufficient to supply only 220V power to the installation site, for example, by installing sensors near a power outlet or switch.

How to make a simple device for measuring the level of CO2 in the air yourself.

This information is for those who are not afraid to hold a soldering iron in their hands and even know a little about programming, for example, they are familiar with the ARDUINO controller family.

The following is a description of the manufacturing procedure of a portable meter (monitor) of the level of carbon dioxide CO2 in the atmosphere.

To do this, you will need:
• carbon dioxide sensor MH-Z19B;
• microcontroller Arduino PRO MINI, or similar, for example Arduino UNO;
• USB-UART adapter CP2102. If you will use an Arduino microcontroller with a USB connector, you do not need this adapter;
• 7-segment indicator on HT16K33;
• MT3608 boost converter or similar, to provide 5V power from two AA / AAA batteries. You can also use two 18650 Li-Ion batteries to power the meter. Then you need a DC-DC step-down voltage converter, for example, on an LM2596 chip. If you do not plan to make the meter portable, you can power it via a USB cable from a computer or power it from Power Bank.

Assemble the circuit as shown in the figure below.

Download the program listed below at the microcontroller. To work, you will need several libraries:

SoftwareSerial.h - library for working with the UART software port. This library is usually present by default. If not, download it from https://github.com/PaulStoffregen/SoftwareSerial

MHZ19.h - library for working with the CO2 sensor MH-Z19B. Download it from https://github.com/strange-v/MHZ19

Adafruit_LEDBackpack.h, Adafruit_GFX.h - libraries for working with the 7-segment indicator. Installed in the Arduino IDE through the menu Tools - Library Management.

#include "MHZ19.h" //https://github.com/strange-v/MHZ19
#include <SoftwareSerial.h>
#include <Wire.h>
#include "Adafruit_LEDBackpack.h"
#include "Adafruit_GFX.h"

#define ledPin1 13 //светодиод на плате контроллера
#define DEBUG
#define DISPLAY_ADDRESS 0x70

SoftwareSerial mySerial(10, 11); //Объявляем программный порт на выводах 10(RX), 11(TX)
MHZ19 mhz(&mySerial); //Объявляем датчик CO2 MHZ-19B
Adafruit_7segment clockDisplay = Adafruit_7segment(); //Объявляем 7-сегметный дисплей HT16K33

uint32_t timer_co2 = 0;
int MHZ_CO2 = 0;
int MHZ_T = 0;
boolean switchDisplay = false;

void setup(){
//Инициализируем 7-сегментный дисплей
clockDisplay.begin(DISPLAY_ADDRESS);
clockDisplay.clear();
clockDisplay.setBrightness(1);
//Инициализируем программный и аппаратный порты
mySerial.begin(9600);
Serial.begin(9600);
while (!Serial) { }
Serial.println("Start programm!");
//Устанавливаем диапазон <2000ppm для датчика СО2
mhz.setRange(MHZ19_RANGE_2000);
}

void loop() {
if ((millis()-timer_co2) > 5000) {
    MHZ19_RESULT response = mhz.retrieveData();
    if (response == MHZ19_RESULT_OK) {
      MHZ_CO2 = mhz.getCO2();       //Получаем уровень углекислого газа в воздухе, CO2
      MHZ_T = mhz.getTemperature(); //Получаем температуру воздуха, CO2
      if (switchDisplay) clockDisplay.print(MHZ_CO2, DEC);
      else clockDisplay.print(MHZ_T, DEC);
      clockDisplay.writeDisplay();
      Serial.println("CO2="+String(MHZ_CO2)+"ppm, T="+String(MHZ_T)+"°C");
      digitalWrite(ledPin1,(not digitalRead(ledPin1)));
      switchDisplay = not switchDisplay;
    } else {
      Serial.println("MHZ19 EROOR = "+String(response));
    }
    timer_co2 = millis();
}
}

The average current consumed by the circuit is 30 mA. Periodically, when measuring CO2 levels, the MH-Z19B uses a pulse current of up to 150 mA. For correct measurement, it is necessary to let the sensor warm up for 30-60s, after which it will show the current CO2 content in the surrounding atmosphere.