Distances of transmission risk of Covid-19 inside dwellings and evaluation of the effectiveness of reciprocal proximity warning sounds

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ Esta es la versión aceptada del artículo publicado en: This is a accepted manuscript of a paper published in: Indoor air (2021): 14/10/2024 DOI: https://doi.org/10.1111/ina.12738 Copyright: cop. Wiley El acceso a la versión publicada del artículo puede requerir la suscripción de la revista. Access to the published version may require subscription. This is the peer reviewed version of the following article: [MarínGarcía D, Moyano-Campos JJ, Bienvenido-Huertas JD. Distances of transmission risk of COVID-19 inside dwellings and evaluation of the effectiveness of reciprocal proximity warning sounds. Indoor Air. 2021; 31: 335 347. https://doi.org/10.1111/ina.12738], which has been published in final form at [https://doi.org/10.1111/ina.12738]. This article may be used for noncommercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. 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Distances of transmission risk of Covid-19 inside dwellings and evaluation of the effectiveness of reciprocal proximity warning sounds David Marín-García1*, Juan Moyano-Campos 1, David Bienvenido-Huertas2 1Department of Graphical Expression and Building Engineering, Higher Technical School of Building Engineering, University de Seville. 4A Reina Mercedes Avenue, Seville 41012, Spain 2Department of Building Construction II, Higher Technical School of Building Engineering, University de Seville. 4A Reina Mercedes Avenue, Seville 41012, Spain *Corresponding email: [email protected] ABSTRACT One of the main modes of transmission and propagation of Covid-19 (SARS-CoV-2) is the direct contact with respiratory droplets transmitted among individuals at a certain distance. There are indoor spaces, such as dwellings, in which the transmission risk is high. This research aims to record and analyse risk close contacts in this scope, experimentally assessing the effectiveness of using electronic proximity warning sound devices or systems. For this purpose, the methodology is based on monitoring the location of the occupants of a dwelling. Then, the days in which a proximity warning sound system is installed and activated are compared to the days in which the system is not activated. The results stressed the significant reduction of time and number of close contacts among individuals when the warning was activated. Regarding the relation between the number and the duration of close contacts, together with the reductions mentioned, the possibility of making certain predictions based on the distributions obtained is proved. All this contributes to the progress in the prevention of Covid19 transmission because of close contacts in dwellings. KEYWORDS: Covid-19; dwellings; close contacts; proximity warning. PRACTICAL IMPLICATIONS -Currently, Covid-19 is a global epidemic that causes hundreds of thousands of deaths and high economic repercussions around the world. Despite advances in diagnosis and treatment, the global incidence remains high and additional strategies of prevention are urgently required. -This study provides the first empirical data showing that the application of an electronic proximity warning sound system is effective in improving the control of a Covid-19 outbreak in indoor environment of a dwelling, also being a breakthrough in recording systems and analysing the risk of close contacts in dwellings and in the possibility of predict of the number of such contacts. 1. INTRODUCTION Certain behaviours and characteristics of individuals are directly related to the level of infection risk 1 3, and close contacts are the main causes of the expansion of many viral respiratory diseases 4 9. These close contacts usually entails a certain interpersonal distance which facilitates the inhalation of viral droplets 10,11. Breathing, talking, coughing, and sneezing are activities that produce such droplets in various amount and size 12 16. The distance that these droplets can reach, and therefore the security distance among individuals, has been repeatedly studied for several types of situations 17 19. Since 31 December 2019, when the Wuhan Municipal Health Commission (Wuhan is in the province of Hubei, China) communicated to the World Health Organisation (WHO) the existence of patients affected by Covid-19, the world scientific community knows much better this virus, particularly its infectious behaviour. Similarly to other coronavirus, the main mode of transmission, although not the only one 20 22, is the contact with respiratory droplets, that is, the mucus of the mouth, nose or eyes 23, as the virus is found in nasopharyngeal secretions and saliva 24. Thus, the modes of transmission and propagation among individuals, if individuals do not use other means of protections, are directly related to the distance among them and the exposure time. A closer interpersonal distance implies a greater inhalation of the virus, thus increasing the risk of infection through the close contact route 25. After analysing the existing research studies trying to establish the most adequate security distance, it is clearly deduced that there is no unanimity when establishing an accurate unique distance 26 because many factors could vary the distance from a metre to eight or nine metres for specific situations, such as sneezes 27,28. However, various bodies and countries have recommended to keep minimum distances among individuals between one 29 and two metres 30 because this is generally a reasonable measure to reduce the risk of infection as long as there are not other factors contributing to propagation, such as strong sneezes, air currents, infectious aerosols in closed spaces 31 or contacts with contaminated surfaces 32. Recent research studies 33 and experts like Bromage 34 indicated that most transmissions are the result of the spread of the infection at home. If a family member is infected outside the dwelling, then the other members usually become infected when the infected individual returns. This fact is confirmed by Qian et al. 33, who identified 318 outbreaks with 3 or more cases, involving 1,245 cases confirmed in 120 cities. The scientific literature related to transmissions in buildings 35, the spread of infection in family environments and households 36 and the behaviour of close contacts including the body positioning in detail 37 is almost unanimous in considering that in a closed architectural environment, such as dwellings and similar spaces, the risk of transmission due to proximity is high, particularly if an occupant is infected outside and lives with the other occupants of the dwelling. The main reasons are the characteristics of these closed spaces and the behaviour because there is usually a greater interaction among them 38, particularly in modest dwellings and in situations in which the cohabitation is intensified 36. In these cases, the transmission risk is closely related to the distance and position among individuals, the numbers of individuals, the actions related to the expulsion of contaminated droplets (sneezing, coughing, speaking or just breathing), and the exposure time, among other specific characteristics of these spaces. Nevertheless, the level of presence of the virus due to the exposure time is the tracking basis of the risk close contact. Although sneezing or coughing implies sufficient viral droplets to directly infect, some minutes talking face-to-face to an infected people 10,39 and even 20 minutes exposed to their breathing are necessary to be infected, all this based on the number of droplets expelled and received per minute 34. Various companies and entrepreneurs from the technological sector are today offering warning sound devices and systems which, with greater or lower success, intend to detect the reciprocal proximity among individuals. These devices or systems are applying several technologies and procedures and are presented in various formats, such as independent units, applications for smartphones, etc. This research is therefore focused on analysing the effectiveness of these devices by applying them in a dwelling whose dimensions, characteristics, and family unit are relatively common. For this purpose, this research is based on the studies on the human behaviour when there are non-verbal warning sounds 40,41. In addition, an experimental campaign is conducted in a real environment by using spatial position locators, and a system that generates a warning sound if reciprocal proximity is detected among individuals is established. It should be emphasized that if there is no infected individual in the dwelling, then family members can be close. For this reason, this research focuses both on the hypothesis that the risk exists when a family member meets other individuals, becomes infected and returns home and on the analysis of to what extent these devices or systems are effective to control and prevent infections related to social distance, such as the case of Covid-19. 2. SIMILAR WORKS Several research studies have recorded the real-time location of individuals and have analysed the duration and distance of contacts in a certain period and in various indoor spaces 2,37,42 46; however, no research analyses and compares the results from activating or not a warning sound device or system to avoid close contact and therefore transmission in a dwelling like that chosen. 3. MATERIALS AND METHODS As a test bed, a dwelling located in the south of Spain was used. Its characteristics and the family unit living in it are usual in this country 47. 3.1. Characteristics of the dwelling and family unit The dwelling has a hall, a living-dining room, a kitchen, three bedrooms, two bathrooms, a balcony, and a corridor, all made independent by walls and doors. The occupants of the family unit are four individuals (a man, a woman, a boy, and a girl). The adults are between 40 and 50 years old, and the subjects under 18 are 10 and 12 years old. All occupants have been living in the dwelling for more than ten years, so they are familiarized with the experimental environment, thus compensating any bias originated by that environment. The actual spatial characteristics, dimensions, uses, surfaces, and furniture of the dwelling are reliably represented with the lowest possible error by using a three-dimensional modelling Autodesk Revit 2019® 48. The model and representation of the floors corresponding to the dwelling (Fig. 1) are used in the graph of the location data obtained. 3.2. Detection devices For the experimental phase, detection devices were used for the real-time location of each subject. Given that these devices should provide readings as accurate as possible regarding the recording of the location from which the interpersonal distance could be deduced, the technologies not meeting these requirements were not considered, following the specificities of the scientific literature 42,49,50. As a result, the real-time locating system (RTLS) based on ultrawide band (UWB) was the most adequate technology for the purpose of this research 49,51 55. Based on both the comparisons conducted by other authors 56 and our analysis and available resources, a Decawave® 57 equipment was chosen as there were experimental precedents in several spaces and situations 58 65. This equipment is an ultra-wideband network constituted by fixed nodes (anchors) and moving nodes (tags). Regarding the accuracy, although ±10 centimetres and second intervals, other indications were also considered 66,67. However, these technical characteristics were adapted to the basic requirements for this type of experimentation in relation to the real-time collection of mode which each experimental subject permanently had, and the remaining anchors were placed in the walls of the dwelling near to the ceiling. The system generated real-time data which were collected by a mobile device and a computer though the procedure, the software and the APIs indicated by the manufacturer. On the other hand, an Excel® spreadsheet was used for the data compilation and processing, and Microsoft Visual Studio®68 for the programming of some algorithms, as well as Html, Php, and Mysql, among others. When an individual was getting closer to another or others from a certain distance, a not very high sound signal, but audible, was generated, whose tone and intensity were previously chosen by the experimental subjects to be as most pleasant as possible. To sum up, the system was designed by placing anchors in several points of the dwelling and tags of the experimental subjects (Fig. 2), so the location data of these subjects were generated. After data processing, the distances among subjects were determined, and all was coordinated by portable Bluetooth Speakers for the proximity warning sound system among individuals. The placing of the anchors therefore constituted a coordinate system to an origin corresponding to one of these anchors, so the distance from each tag to each anchor was calculated, thus allowing the tags to be always placed as the initial coordinates of the anchors were known because they were previously determined. Lastly, before starting the experimentation, the correct calibration of devices was checked by verifying if the offered data corresponded to actual data. 3.3. Distance and risk position considered This study only considered the distances among experimental subjects and not their corporal position due to the reasons mentioned below. To make the goal of the research feasible regarding the comparative study, the concept of risk contact or was initially established in a unique, clear and accurate way. Considering the authors aforementioned and the indications given by the most known international health organisations, such as WHO 29 or the European Centre for Disease Prevention and Control (ECDC) 30, the approaches at a distance less than 2 metres or f a household as long as the contact took place without protection (face mask, eye protection, etc.). Regarding time, although the duration of 15 minutes is used in certain scopes 30, the security margin was increased by using 10 minutes. The same occurred when 2 metres were chosen, instead of 1 metre, as recommended in some cases. As for the exposure time, there is no absolute certainty of how many particles should be inhaled and therefore how much exposure time is necessary in each circumstance to produce a transmission. For this reason, this study uses a time mark that may seem high but assures that the results reflect high-risk contacts. After counting close contacts, although they were first defined as episodes of 10 continuous minutes at less than 2 metres in some moments, some cases were not strictly suitable to these conditions. However, these cases were also considered because one of these conditions was fulfilled and the other was almost fulfilled. The goal of the research was not to establish whether transmissions would take place at other distances or exposure times different to those previously chosen. The main goal was to assess the effectiveness of proximity warning sound devices or systems with respect to behaviour when avoiding distances which could imply this transmission, regardless of whether the transmission was produced, so the reasonable and founded references chosen were enough. 3.4. Data collection and processing Regarding the data collection, all occupants were monitored from the hour in which the first occupant got up in the morning to the hour in which the last occupant went to bed at night, so the programmed time was from 8:00 a.m. to 24:00 p.m. (16 hours). When a subject got up, the location device was immediately placed and activated. However, the connection to the warning sound device was not activated until at least 2 subjects got up, around 9:00 a.m. On the other hand, the subjects on bed after 8:00 a.m. were assigned a fixed coordinate of their locations which coincided with the lying position in their respective bed until they also got up and the location devise was placed and activated. The experimental days coincided with the confinement days decreed by the Spanish Government in March 2020, so all family members stayed in the dwelling all the experimental hours. The internal temperature was between 21 and 23 ºC. According to the Spanish Building Technical Code, the ventilation rate of dwellings was 0.94 air changes per hour. In these experimental days, the distances of the tags of each subject to the anchors were obtained at each moment. With these data and by triangulation, the coordinates (x, y) of each individual were automatically obtained in real time, and the coordinate (z) was not required, as previously justified, considering the goal. Not all the coordinates were the result of the data record provided by the location devices because these were updated based on the activity of the movement sensor. In addition, certain disparities were turn entails the distributions and their respective graphs. This research showed that short close contacts were the most usual, thus decreasing the frequency as the duration increased by following a logarithmic distribution. The transmission risk increased by reducing the interpersonal distance, and vice versa 18,69 71. Although the location of individuals could not be accurately quantified at each moment, human behaviour usually varied according to certain factors already mentioned 2,3. Based on the results, all subjects responded to the warning in a similar way most of the time. Given the family unit and the dwelling analysed, it would be interesting and even in certain cases necessary to experiment with several dwelling typologies and family units to diversify the test bed and to obtain more data to verify the results, and even to add conclusions. Regarding the results of the means of the distances in the day when the warning sound was not activated, the mean personal distances in case of more affective subjects were the most frequent and sometimes even inevitable, although the warning sound was connected. These risk contacts were usual according to what some authors indicated 25,37. As for the activities and the way of circulation of subjects throughout the day, although habits strongly influenced the probability of close contacts, the results also showed that these habits could be modified by the warning sound, thus greatly reducing these contacts, at least in the short term. In this regard, the subjects, who at the same time were carrying out different activities, usually recorded a low probability of contact among them, thus indicating that the design of dwellings should influence this aspect contributing that each subject has their own space according to the activity. In this manner, subjects could carry out their activities in far zones, particularly if they were aware of a possible close contact, such as the case when the warning sound was activated. On the other hand, the zones with comfortable seats were usually hotspots to contribute to close contacts, and if these seats were big, such as the sofa, and playful activities could be carried out, they turned into a space where contacts were frequent even with the warning sound connected, as subjects usually sat together. Another aspect was a high probability of short close contacts when the subjects went from a point to another of the house as the subjects who coincided in the passing places of the dwelling did not generally have enough space as these zones were not wide enough to keep the distance required. Nevertheless, contacts were short and sporadic in these cases. The performance of the sensors was generally adequate; however, to rely on the motion sensors, these were checked at intervals similar to the times established to consider close contacts. Data were not generally corrected, but when the tags were away from the anchors or when there were more obstacles between them data were corrected. Finally, these results could be useful to analyse the risk of infection at a greater scale. For instance, if the number of dwellings, their typology, and family units in the households of a certain zone are known, the number of close contacts which could be reduced by applying warning sound devices or systems could be calculated. 6. CONCLUSIONS The probability of risky close contact at a distance less than 2 metres is very high in closed spaces, such as dwellings, and the human behaviour inside them is a determinant factor for the transmission risk. Close contacts could be decreased with these warning sound devices or systems, making the contacts almost null. Both when the warning device was disconnected and connected, the living-room of the dwelling, and particularly the sofa, the zones in which people usually eat, and the kitchen were the zones where close contacts were more frequent and therefore there was a greater probability of proximities at a distance less than 2 metres for more time. In this regard, the subjects carrying out different activities at the same time usually recorded a low probability of contact among them, thus indicating that the design of dwellings should facilitate that each subject has their own space for the respective activities. However, there was an important decrease of close contacts in these places when the warning sound was connected. On the other hand, if the warning was disconnected, the results indicated that the relation between duration data and the sum of contacts followed logarithmic distributions. If the warning was connected, the distribution was linear after around 5 minutes, or exponential including these 5 minutes, so predictions could be made with a determination coefficient greater than 80%. Based on the data of some of these two variables, the other variable could be deduced with a probability of success of 80%, although this limit should be increased by studying a more extensive casuistry. Regarding a more long-term feasibility of using warning sounds in a dwelling, and according to what experimental subjects manifested, it could be less feasible if it is intended to be used for a long time. 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