Ventilation Strategy for Proper IAQ in Existing Nurseries Buildings -Lesson learned from the research during COVID-19 pandemic

During the COVID-19 pandemic, many recommendations were made in the field of limiting the transmission of the SARS-CoV-2 virus, from which we can learn a lesson for determining ventilation strategies in strategic types of buildings (those whose closure during a pandemic is harmful to the economy, e.g., nurseries). The research was aimed at identifying recommendations in the field of ventilation and proposing a solution that would be applicable in existing buildings intended for the care of small children, and which would ensure the proper quality of the building environment, at the same time with low costs incurred by the owners. The outside air pollution (PM10) and the climate in winter (low air temperature) were also taken into account. A strategy was proposed based on the use of decentralized units, dedicated to single rooms, thanks to which the appropriate amount of air is supplied (per person), the air is cleaned and heated in the heat recovery exchanger. It has been shown that the use of heat recovery ensures that the costs of air heating will be significantly lower than during airing. The proposed solutions require two holes in the external wall with a diameter of 160-200 mm (depending on the number of people), which guarantees the technically possible application in existing buildings. The strategy provides suitable conditions for the functioning of nurseries, but can be used in many types of buildings, in cold and temperate climates, where airing of the rooms during winter is not possible, especially in the case of locations where the quality of outdoor air is very poor. The proposed strategy may be applied during a pandemic, but also on a daily basis, because by ensuring the proper quality of indoor air, young children will have healthy and hygienic conditions for development when they are not at home.

The location of the buildings and its climate are important for indoor air quality. This is especially important when the outside air is used to directly ventilate the rooms. The monthly average outdoor air temperatures in 2018 are presented in Figure S1. From January to April and from October to December, the outside air temperature is lower than 16 o C. Assuming that the temperature of the indoor air in the nursery should be around 21 o C, the temperature of the outside air is within this limit for June, July, and August. It can be assumed that in May and September, during the daytime, the temperature of the outside air is similar to the temperature of the air indoors. Due to the measured monthly concentration of PM and its direct seasonal relationship, it was assumed that in the heating season, the quality of the outside air is poor i.e., the concentrations of PM are above the recommended values (Environmental Protection Inspection., 2010). As such, in place of airing, alternative ventilation strategies should be used.

S2. Simulation of CO2 concentration in nursery rooms.
Step 1: The appropriate ventilation airstream should be designed in such a way as to maintain the specified air quality in a room. For the calculation, the concentration of outside air will be adopted as 420 ppm.
The simulations will show how CO2 concentration in the room changes during the use of the room and how CO2 emission is related to the breathing of children and sitters. Based on previously published work, it was assumed that CO2 emission for a child under 3 years old and a babysitter is 7.56 L/h and 18 L/h, respectively (Persily and de Jonge, 2017).
It was decided to analyze a wide range of airflows to select the smallest possible airflow that would facilitate appropriate average CO 2 concentration. From the recommendations indicated in various European countries, the following airflows were adopted: 30 m 3 /h/person, 15 m 3 /h/person, 10 m 3 /h/person (Basińska et al., 2019;Ratajczak and Basińska, 2021).
Step 2: To calculate the variability of CO2 concentration, equations (1) and (2) will be used. Equation (1) models CO 2 emission in response to the presence of children. Equation (2) models the situation where there are no emissions i.e., no children present.
Regarding the playroom, i.e., the room in which the children stay in the morning, vacate it for nap time, and then return to it later in the day, two equations will be used. Equation (1) will be used for calculations where the children are present in the room (morning and afternoon), and equation (2) will be used when the children are in the bedroom (nap time) and CO2 emission does not occur. For the sleeping room, only equation (1) will be used because the bedroom is used during the sleeping time only. Further, as the children only use the bedroom once per day and do not return to it, it is sufficient to ventilate this room only during nap time. This strategy will allow for the conservation of energy relating to fan operation and a reduction in costs.
Step 3: Average CO2 concentration seems to be the good indicator because it shows the values over the entire period of children's stay in the room, i.e., on average, it reflects what concentration of CO2 the children are exposed to throughout the day. For the playroom, the average CO2 concentration will be calculated over two periods of time. Additionally, the average CO2 concentration during nap time will be determined for the bedroom.
This approach is justified by the need to find a small enough airstream to keep the average CO2 concentration in an appropriate range. This should be done in new buildings. In existing buildings, where the CO2 concentration limit is exceeded by up to 4 times (Basińska et al., 2019), each improvement will make it easier to achieve better parameters. Therefore, it is important that the mean concentration be acceptably low, and that the maximum concentration not be higher than 10% of this value.
Step 4: Based on the average CO2 concentration values obtained, the design (VSU,ij), will be selected. The following possible single devices and their parameters are presented in Table S1. The above efficiencies were determined based on an overview of commercially available equipment. Specifying devices is not preferred, and the data has been prepared based on catalog data from various manufacturers. The proposed devices have a supply and exhaust system located on the same external wall resulting in U-shaped airflow which has been highlighted as a good set-up, even in hospital rooms (Ren et al., 2021).
Step 5: Steps 1-4 takes into account the real attendance of children during the heating season. In the design process, human presence coefficients are used to determine the ventilation airflow. Additionally, these coefficients take into account that the maximum load rarely occurs in most rooms. Further, this design aims to prevent oversizing the installation by using smaller crosssections of ducts or smaller devices.
A situation in which the selected airflow in Step 4 will work with a predetermined efficiency, and all children will be present was simulated. The rate of air quality deterioration and whether the average CO2 concentration over a given period (morning, nap, and afternoon).
Step 6: For the selected ventilation airflow (VSU,ij), the air quantity indicator per m 2 and m 3 of the room will be calculated. This will make it possible to compare rooms of different architecture and to select an optimal target airflow rate.
Step7: Based on the results of the modelled simulations, an appropriate strategy for ventilating nursery rooms will be determined. The final, proposed strategy will respond to literature recommendations and fill several gaps in the knowledge. The strategy will include: a) Determining the appropriate size of the ventilation airflow. b) Determining the maximum concentration of CO2 that may occur in the room. c) Field recommendations for mechanical ventilation efficiency and technical limitations of its installation. d) Defining actions in a critical situation i.e., exceeding the maximum CO2 concentration, or exceeding the density of children.

S3. Assessment of mechanical ventilation and airing -how much heat is necessary to warm ventilated air?
Formula (S1) models the amount of heat needed to warm up the air (QAIRING) from the average outdoor temperature for each month (Fig.S1) to room temperature. The air is supplied to the room by airing (without pre-heating) will be calculated by the symbol M (month):