key: cord-293097-poh1y6o7
authors: V, Antony Aroul Raj; R, Velraj; Haghighat, Fariborz
title: The contribution of dry indoor built environment on the spread of Coronavirus: Data from various Indian states
date: 2020-07-02
journal: Sustain Cities Soc
DOI: 10.1016/j.scs.2020.102371
sha: 
doc_id: 293097
cord_uid: poh1y6o7

Coronavirus spread is more serious in urban metropolitan cities compared to rural areas. It is observed from the data on the infection rate available in the various sources that the cold and dry conditions accelerate the spread of coronavirus. In the present work, the existing theory of respiratory droplet drying is used to propose the mechanism of virus spread under various climates and the indoor environment conditions which plays a greater role in the virus spread. This concept is assessed using four major parameters such as population density, climate severity, the volume of indoor spaces, and air-conditioning usage which affect the infection spread and mortality using the data available for various states of India. Further, it is analysed using the data from various states in India along with the respective climatic conditions. It is found that under some indoor scenarios, the coronaviruses present in the respiratory droplets become active due to size reduction that occurs both in sessile and airborne droplet nuclei causing an increase in the spread. Understanding this mechanism will be very useful to take the necessary steps to reduce the rate of transmission by initiating corrective measures and maintaining the required conditions in the indoor built environment.

Immediately after the 100th anniversary of Spanish flu [1] , the deadliest outbreak in the recorded history, wherein approximately 50 million people were killed which is more than the death toll from the First World War, now the new corona pandemic is threatening the entire world. The other major pandemics during the 20th century were Asian flu , and Hong Kong flu which killed 1 to 4 million people during each era. In the last two decades, the epidemics of infectious diseases are occurring more often and spreading faster and further than ever. The reason is due to change in the lifestyle that leads to climate change leading to a biological imbalance in the system and thus we continuously learn about the unpredictable power of nature. There has been some suggestion by the authorities that the most effective approach to prevent the transmission of COVID 19 among the population are: keeping social distance and washing hands. The rationale for such a policy is that virus transmission occurs through direct contact and droplet transmission, and public health professionals believe that using facial masks and gloves is an efficient way in preventing virus spread. It is a general belief that population density is an important parameter in deciding the spread of the COVID-19 virus; however, it is observed that any single case [3] in India. Jonathan Kay [4] has prepared a comprehensive international database of large COVID-19 infection clusters or super spreading events in the spread of coronavirus for February and March 2020. The commonality of the outbreak observed by him is that all the infections were indoor with people closely packed. The main sources of infections are home, workplace, public transport, social gathering, and restaurants. Top 50 outbreaks have happened in the prison, religious houses, meat packing facilities, weddings, funerals, business networking events, call centers, and choir. Outbreaks happened outdoor is a small percentage of around 10%.

The author has also mentioned that studies have to be done on the effect of indoor conditions such as temperature and RH in the spread of the virus. The various other factors are classified as:

 Minimum Infectious dose of virus: A minimum dose of about 1000 SARS COV2 infectious virus particles is required to start a pathogenesis cascade that can cause new infection [4] . The successful infection is a product of the dosage of virus and time for exposure. A person can get infected by a strong dose of a virus for a shorter period or a weak dose of the virus for a longer period. For example if 1000 virus particle is required for infection it can happen by 10 particles in 100 minutes or by 100 particles in 10 minutes.

 Number of droplets released and speed of release: A cough by a person releases about 3000 droplets at 50 mph, a sneeze releases 30000 droplets at 200 mph and during the breath, about 5000 droplets are released at very low velocity [5] . The droplets released by sneeze and cough can travel a longer distance and stay in the air for a longer period of time [6] .

 Age of the infected person: The amount of the virus released depends on the age of the infected person. It is lesser if the age is less than 20. It increases up to 40 and then decreases with age. When the viral load builds up the person becomes symptomatic [5] .

 Enclosed environments: Any crowded indoor environment with poor air circulation and high density of people with high breathing and panting due to physical activity of the infected person such as loud talking, laughing, singing, dancing and playing has resulted in an increase in breathing rate and the number of droplets released causing high spread rate [5] and [6] .

 Social and large public gathering: Physical contact between the individuals as part of social and public gathering procedures such as hugging, handshaking, kissing also has resulted in fomite transmission.

Further, it is understood from various data and literature that cold and dry conditions accelerate the rate of spread of coronavirus [7] - [11] . This infers that climate severity is an important parameter in the spread of the virus. Hence the major objective of the present work is to propose the mechanism of virus spread under various climates and the indoor environment conditions maintained through the existing theory of respiratory droplet drying. Further, it is aimed to perform a statistical study on the dependence of mortality and infection in the Indian States with respect to four major parameters such as population density, climate severity, volume of indoor spaces, and air-conditioning usage based on monthly data for March and April. This study is done to confirm whether the trend and correlation from the analysis support for the drying of respiratory droplets under dry conditions proposed in the present work. Understanding the mechanism of production of dry pockets in various indoor air environments by the heating, ventilating and air conditioning (HVAC) systems is very essential to take appropriate measures and to reduce the rate of transmission.

The mechanism of drying of respiratory droplets and transmission potential is explained initially using the various sources available in the literature. The present work shows how the conventional cooling and heating mechanism in the indoor environment creates the possible drying potential for the respiratory droplet using the psychrometric chart for different climatic zones such as (i) hot and humid zone, (ii) cold and humid zone (iii) cold and dry zone (iv) hot and dry zone. The effect of this drying potential on the size reduction of the droplets and its effect on the spreading of viruses is also analysed.

In order to assess the concept proposed, the number of infected persons and the mortality from several states of India where different climatic conditions prevail during March and April, 2020, are compared using the selected parameters. The population density is one of the parameter chosen as there is a general agreement that the population density is the reason for its spread.

The effect of climate severity in a particular state is brought through the parameters such as monthly average relative humidity, monthly average peak temperature, and monthly average minimum temperature. In any country, 40 % of the total power generated is utilized in the building sector in which 50 % of the energy is utilized for building air-conditioning. Hence 20 percent of the total energy generated in the country is used by the HVAC system. Hence the parameter energy J o u r n a l P r e -p r o o f utilisation is considered as another parameter which is proportional to the dry pocket creation during the period of analysis through the HVAC system. Power infrastructure [12] , is also used as a parameter to indicate the volume of the contained indoor built environment in the state.

Population density, power generation capacity [13] , and CORONA infection and mortality data [3] were taken from Wikipedia and the weather details taken from the ISHRAE weather data [14] . The The size of the droplet will not undergo size reduction if the surrounding is in high temperature and high humidity due to less potential difference for heat and mass transfer. The virus in the droplet nucleus is less active if the size remains larger with more water content. The droplet coming from the infected person will be initially at the body temperature of 37 o C. When it tries to attain equilibrium with the surrounding lower temperature the heat available from the droplet itself will be utilized for evaporation. When the surrounding air is dry (low Relative humidity) and

cold, the size of the particle is reduced by combined heat and mass transfer [16] - [18] . In hot and dry conditions the heat transfer potential is less, however, the mass transfer potential is high. The mass transfer effect will be more pronounced when the relative humidity is very low. If the J o u r n a l P r e -p r o o f surrounding air is humid, water cannot evaporate and the droplet size is not reduced. Water content in the particle plays an important role in diluting the virus so that it is less active and less harmful. Also, the salt (sodium chloride and potassium chloride) in the respiratory droplet is hygroscopic and acts as a desiccant to absorb the moisture from the humid air to increase the size of the droplets. The virus gets diluted with water content and become less active. Thus, a humid environment naturally makes the virus less active [15] . Further, in hot and humid conditions, the mucus membrane in the respiratory tract will be wet which will humidify and dilute the droplet nuclei. However in dry locations, the mucus membrane becomes dry and the fluid over lining the cells becomes more viscous and the little hair CILIA which normally protects our lungs from deep settling of particle, cannot filter out the virus. After entering the lungs, the virus can cross the one cell layer separating the air path in the lungs and the blood path, and it can infect the blood very easily.

The 

In the hot and humid as well as warm and humid zone regions, the surrounding air has high temperature and humidity, and hence the temperature and humidity potential available for reduction in the size of the droplet is lesser. However, when the air for the indoor environment is conditioned, the air is cooled and dehumidified in the air handling unit and so the potential 

In cold and humid zones (a condition referred as CH0 with 5 o C, 90% RH), although there is no potential for drying, a small amount of sensible heating from CH0 to CH1 (15 o C, 50 % RH)

represented with a drying potential (S1-CH1) and on further heating to comfort temperature CH2,

(25 o C, 25% RH) the droplet drying potential (S2-CH2) increases greatly due to relative humidity of 25%. Under such circumstances, the sensible heaters used for the heating system of the room will reduce the relative humidity of the room and will lead to the size and weight reduction of the droplet nuclei and increases the virus activity. The droplet nuclei containing the virus may become airborne( Figure 2 ). Thus in cold and humid places, the indoor environment with low relative humidity will serve as pockets for virus survival. Thus, the virus can be active in cold and humid regions in these moderately dry pockets mentioned above.

The cold and dry zone (a condition referred to as CD0 with 5 o C, 40% RH) is a highly unsafe condition since the drying potential already exists at low temperature. A small sensible heating without humidification, in this case, increases the drying potential CD1 (15 o C, 20 % RH) and further heating will lead to CD2 (25 o C, 10 % RH), which increases the drying potential to a higher level and there is an additional possibility of the virus becoming airborne. In an environment with low humidity and low temperature, due to combined high heat and mass transfer potential leads to fast drying and size reduction of the respiratory droplets and the virus is almost active in all the locations. 

In hot and dry climates, the drying potential for the droplet is higher at HD0 and HD1. If a cooling and dehumidification system is used in a hot and dry climate this will increase the drying potential and thus the virus activity to a dangerous level. Cooling with humidification like spray washer will improve the humidity level and will reduce the risk of viral contamination.

Drying of the respiratory droplet due to heat and mass transfer potential existing in various zones mentioned above moves the droplet nuclei to the region of low specific density as explained in the psychrometric chart so that they become lighter, buoyed up and become airborne.

It is understood from the theory of respiratory droplets drying under various climatic zones that the virus is very active in cold and dry environment irrespective of conditioned or non-conditioned space [11] . Hence this climatic zone has the most vulnerable transmission potential for the coronavirus even in the outdoor environment. In the other zones, based on the condition maintained by the HVAC systems the transmission potential will differ.

The various states and the union territories in India are shown in Figure 3 and the climatic zones prevailing in India in various regions are shown in Figure 4 . Table 1 shows the population density, - [21] . This confirms that the decrease in RH which increases the drying potential causes the increase in viral spread. Hence it is understood that humidity will play a major role than the temperature, as the mass transfer potential for the respiratory droplets will be high at low humid conditions. It is also a well-known fact than for any drying phenomenon, mass transfer is the major influencing factor than the heat transfer. The effect of population density is the least from the correlation due to lock down and social distancing procedure. This will play a major role if the lockdown is released in the state. Table 4 Correlation and regression for infection -April 2020 April, the sun position is straight to these states that leads to large usage of the HVAC system.

Hence the spread rate is very high. However, questions may arise that in the neighboring states of Kerala and Karnataka, the high spread rate is not reported which is due to the following reason:

In all these states the major spread is reported in the capital city where the air conditioning system usage is the highest. In Karnataka, Bangalore, the capital city is located at a higher altitude (partial hill station) and hence the air-conditioner usage is less and avoided the high spread. The state

Kerala normally referred to as "God's own place" because of its lush greenery (full of forests,  It is concluded from the present study that the region with low humidity has a higher spread potential even in the outdoor environment. Hence the high spread states like Maharashtra, Gujarat, Madhya Pradesh Rajasthan, and Delhi with hot and dry weather conditions, the spread may continue till south-west monsoon begins in the middle of June.

Hence the existing lockdown cannot be released till Mid-June in those states particularly in the cities like Bombay and Delhi.

 In the southern Indian states, the spread rate due to the outdoor environment is very low.

As the summer intensity increases, the effect due to the outdoor environment will further decrease. However, the increase in the use of air-conditioners in the built indoor environment may increase the spread rate. In these states, as summer continues till August, proper measures should be taken while using HVAC systems with the correct set conditions mentioned particularly in the cities like Chennai.

 It is also concluded from the present study that the spread rate may reduce in the cold countries in the northern part of the world as summer begins. However, before the next winter starts, it is advisable to modify the kind of heating system adopted to avoid dry conditions in the indoor environment. Presently, the humidity control is not available in most of the commercial window, and split type air-conditioners and experimental research outcomes are also not available to prescribe the safe demarcation of the relative humidity in the HVAC systems. Hence, it is suggested now to set higher temperatures and relative humidity without compromising the comfort. Also, it is advisable to avoid spending long hours in the closed dry environment until the corona spread is controlled.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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