**1. Introduction**

Today, in developed countries, 80–90% of people's time is spent indoors [1], especially in their homes. The duration spent in the home varies between 60% and 90% of the day, and 30% of the time is spent sleeping [2,3]. Since homes contain air that is inhaled, the greatest exposure to potential air pollutants is in these interior spaces. The indoor environment of the home should facilitate rest and recovery [4,5]; therefore, as poor indoor air quality (IAQ) has harmful effects on health, it prevents these beneficial effects from being realised.

Since the energy crisis of the 1970s, buildings have become increasingly airtight, leading to the appearance of IAQ-related diseases, such as sick building syndrome (SBS) [6]. In addition, a relationship between air movement in buildings due to ventilation and the spread of infectious diseases has been demonstrated [7,8]. In this context, the benefits of indoor air exchange have been confirmed, although the influence of ventilation on the spread of infectious diseases is not clear [6]. Consequently, since the influence of airflow rates on health has not been quantified, the ventilation rates specified in different regulations are usually set according to comfort criteria (perceived conditions) [9–12].

However, a healthy indoor environment can be achieved by applying strategies necessary to improve the COVID-19 pollutant IAQ, which, in addition to increasing the supply of fresh air, include controlling pollution from emission sources, cleaning the air and improving the efficiency of ventilation [13–15]. Therefore, the indoor air quality (IAQ), especially in indoor residential spaces, has a strong influence on human health; thus, it is essential to design adequate ventilation, which ensures good IAQ since the main purpose of ventilation is to dilute or remove indoor contaminants by providing outdoor air [16–21].

The values set for air renewal to ensure comfort and eliminate odours have been modified over the course of history, according to variations in ventilation theories. Currently, the Basic Document HS 3 for indoor air quality, included in the Technical Building Code [22], provides data on minimum ventilation rates for residential buildings in Spain, depending

on the room of the dwelling. In European countries, in addition to the EN 15251 standard (which should be used if no national standard is available), there are also state regulations that, as in the Spanish standard, provide data on minimum ventilation rates based mainly on body odours (with CO2 as an indicator) and, to some extent, according to primary emissions from some building materials [23,24].

On the other hand, even if the regulations are complied with, indoor air quality may still be inadequate if stagnant air zones are generated, and therefore, health and comfort problems may develop [25–29]. In-depth research in this field is still needed; therefore, the debate on how much ventilation is sufficient to achieve good indoor air quality, capable of preventing both odours and the emergence and spread of diseases, is still ongoing.

Spain is one of the countries most affected by the COVID-19 pandemic. According to data provided by the Spanish Ministry of Health's Centre for the Coordination of Health Alerts and Emergencies [30], the number of deaths as of 16 May 2020 was 27,563, and 230,698 people were infected.

Based on the number of elderly people who have died in nursing homes [31], we find that the most affected autonomous communities (AACCs) were Castilla–La Mancha (14.44%), Extremadura (14.13%), Castilla y León (13.71%) and Aragón (12.90%), compared to the total number of deaths in these communities (elderly and non-elderly). These numbers are very similar to the number of the elderly who died in these communities: Castilla–La Mancha (17.38%), Extremadura (16.80%) and Aragón (15.36%).

According to data provided by the AACCs, the number of elderly people who have died as a result of COVID-19 in the approximately 5417 Spanish homes for the elderly, including public, subsidised and private residences, stands at 18,354 deaths. Only people who died after testing positive for coronavirus have been registered as casualties of COVID-19, i.e., tests have not been performed post-mortem, so those who were not tested are not listed as having died of coronavirus.

It is observed (Figure 1) that the number of private residences in AACCs is greater than the number of public residences in a proportion of 3 to 1, with the exception of the autonomous communities of Extremadura, Castilla–La Mancha and, to a lesser extent, Canarias.

The population pyramid in Spain continues to reflect population ageing, measured by the increase in the proportion of elderly persons, i.e., those aged 65 years old and over. According to the latest statistical data from the Continuous Register of the National Statistics Institute (INE), on 1 January 2019, there were 9,057,193 elderly people, 19.3% of the total population of 47,026,208 [32]. This population of elderly people continues to increase, both in number and proportion. The average age of the population, which is another way to measure this process, is 43.3 years; in 1970, it was 32.7.

Below is a comparative chart of the population over 65 years of age and the number of deaths in that age range for each AACC.

The data show that the communities of Madrid, Extremadura, Castilla–La Mancha and Baleares have been most affected by deceased elderly persons relative to the population of persons over 65 years of age in these communities (see Figures 1 and 2). On the other hand, Ceuta, Melilla, Canarias and Galicia have been less affected.

**Figure 1.** Distribution of the number of public and private nursing homes in the different autonomous communities of Spain.

**Figure 2.** Graph of number of people >65 years old and deaths of people in this range by autonomous community.

The first study carried out in Europe on indoor air quality [33] includes detailed statistical data from the Eurostat Union Statistics on Income and Living Conditions. The data demonstrate a correlation between people's health and the conditions of the buildings in which they live.

In March of this year, the *European Respiratory Journal* published a study that revealed the importance of maintaining adequate indoor air quality conditions in residential buildings. Based on this study, it was deduced that older people are more likely to suffer negative effects on lung health due to indoor air pollution in buildings, compared to younger adults.

It has been shown that city dwellers (especially the elderly and children) spend between 58% and 78% of their time in an indoor environment that is polluted to a greater or lesser degree. This is a problem that has been exacerbated by the construction of buildings that are designed to be increasingly airtight and that recycle air with a smaller proportion of fresh air from outside in order to increase energy efficiency. Pollution of the indoor environments of buildings is the cause of many health problems of various kinds, which can range from simple fatigue or discomfort to symptoms consistent with allergies, infections and cancer, among others.

The pollutants present in the indoor air of buildings (residences), whether chemical, physical or biological, vary depending on the activities that take place in such spaces, the health of the occupants, the physical infrastructure of the building and its material assets and the quality of the surrounding air. At present, environmental pollutants, such as environmental tobacco smoke, formaldehyde, radon, mineral fibres, isocyanates and epoxy resins have been identified as some of the main emerging factors that can increase the risk of diseases, such as allergies, asthma, fertility disorders and cancer [34]. Physical factors that influence comfort are mainly related to relative humidity, average air speed, temperature and noise. In addition, there are chemical pollutants, including carbon dioxide (indicative of insufficient indoor air renewal), carbon monoxide, sulphur dioxide, volatile organic compounds, suspended particles, ozone, radon, etc., as well as various pathogens. In short, the population is faced daily with an array of pollutants not only in buildings, but also in the outside air, water, food, consumer products, etc. Determining the pollutant composition, daily exposure dose and interaction with the human body and the environment is quite a complex undertaking [34,35].

Volatile organic compounds (VOCs) play an important role in the process of assessing the IAQ. They negatively affect both the comfort and health of occupants [35,36]. The effects of VOCs in indoor spaces vary from sensory irritation at medium exposure levels to toxic effects at high exposure levels [37]. Formaldehyde (HCHO) is especially important, as it is known to be the most common irritant in indoor air, causing eye, nose and throat irritation (at concentrations above 0.1 mg/m3) and may even cause asthma attacks at high concentrations [38,39].

Previous studies [40,41] provide total VOC concentration data, using the concept of total volatile organic compounds (TVOCs). This parameter is used to allow a simpler and faster assessment than the analysis of a high amount of VOCs, which are usually detected in indoor spaces [42]. The concentration of any pollutant in an indoor space is the result of a balance between the network in that space and what is removed or added by ventilation. Therefore, if the TVOC level is high, it indicates that there are significant sources of contamination or that local ventilation is inadequate. Therefore, it is important to measure TVOCs, as they are an indication of the pollution load in the indoor environment and the sustainability of the ventilation rate [43]. It should be noted that TVOCs do not include all VOCs present in indoor air, as some organic pollutants are excluded, such as low molecular weight aldehydes, e.g., HCHO, because the method of identification differs from that of other VOCs. Therefore, in order to characterise the IAQ and determine the adequate ventilation rate, it is essential to consider the concentration of VOCs and HCHs.

Therefore, it is difficult to assess the health risks (measurement, tolerance level, exposure time, effects, etc.) in the indoor environment; preventive and corrective maintenance in the facilities involved is important in order to promote healthy environments.

In this study, chemical pollutants (such as formaldehyde, nitrogen dioxide, ozone, PM10 and PM2.5 particulate matter and sulphur dioxide), physical agents (such as temperature, humidity, precipitation and hours of sunshine) and socio-demographic variables (such as population density and the capacity of different public or private homes for the elderly) were analysed to understand their association with the indoor air quality of the homes and to determine whether they influence the health of the elderly who live there.

The analysis was carried out in two stages, first taking into account the sizes of different residences and subsequently evaluating the effects of physical agents and pollutants.

For the first stage, the statistical technique of multivariate correlation between the variables was applied in order to determine the relationships between the number of deaths in the residences and the 12 variables based on the size of the residence. Then, a cluster or conglomerate analysis was carried out to determine the relationship between the nursing homes and the autonomous communities under study.

#### **2. Materials and Methods**

In addition to children and pregnan<sup>t</sup> women, other population groups are vulnerable to the effects of air pollution. These include people with respiratory diseases, such as asthma, chronic bronchitis or emphysema, those with cardiovascular diseases or diabetes, and elderly people with chronic diseases, particularly those living in residential homes.

The quality of the air in residences is in the IDA2 category, according to the Regulation of Thermal Installations (RITE) in Spain [24]. In this type of building, it is necessary to have a good quality of air in the indoor environment because the elderly population is more vulnerable to infectious agents.

Age and diseases (cardiovascular, respiratory problems) are factors that make the elderly more likely to be negatively affected by pollutants present in the air of residential environments. Susceptibility to pollutant effects is also influenced by the individual's lifestyle, diet, genetic predisposition, etc.
