*2.1. Comfort*

In indoor environments, the ability to regulate temperature is provided by heating, ventilation and air-conditioning systems. The human body has the ability to regulate its temperature within a range of degrees. Thermal comfort in an indoor environment refers to the general sense of temperature and humidity felt by most people who live there.

Thermal comfort means that a person feels good from the perspective of the surrounding hygrothermal environment. Extreme thermal values can be harmful, even deadly, to human beings. This is because human beings are homeothermic, i.e., they must maintain certain vital parts at an approximately constant temperature. To achieve a feeling of thermal comfort, the most advisable condition is an ambient temperature that is slightly higher than the air temperature and a flow of radiant thermal energy that is the same in all directions and is not excessive above the head. In the UNE EN ISO 7730 standard (thermal comfort in moderate environments) [44], thermal comfort is given by the thermal balance between the body and the environment, i.e., a person will feel comfortable when the internal heat generated and the losses due to evaporation from the body are compensated by the losses or gains due to latent, sensitive or radiant heat with respect to the environment.

The following table (Table 1) shows data on the average temperature and humidity, precipitation, hours of sunshine [45] and the number of elderly people ( ≥65 years) who have died in homes and in total for the different autonomous communities [30].


**Table 1.** Data on deceased elderly people and physical agents in the autonomous communities.

#### *2.2. Air Quality Factors*

The quality of the environment in nursing homes is affected by indoor air pollutants, such as dust, suspended particles, CO2, CO, NOx, VOCs, bacteria, fungi and viruses, as well as pollution from outside. However, nursing homes need special maintenance of the facilities and environment (air renewal) for the following reasons:


#### *2.3. Consequences of Poor Air Quality*

The World Health Organization (WHO) warned that air pollution (outside air) kills about seven million people every year. Indoor pollution levels can be as much as 10–100 times higher than outdoor concentrations, and people (with the elderly being more vulnerable) exposed to poor-quality residential environments can suffer many health problems as a result:


#### *2.4. Chemical Pollutants*

The pollutants studied in this work, as well as their impact on human health, are described below.

#### 2.4.1. Nitrogen Oxides (NOx)

The presence of NOx is related to the burning of fuels, mobile sources (vehicles), industrial processes and some natural processes (lightning, and soil microorganisms). Combustion processes emit a mixture of nitric oxide (90%) and nitrogen dioxide (10%). In turn, nitric oxide reacts with other chemicals in the air to become nitrogen dioxide. In indoor environments, the main sources of NO2 emissions are heating systems and gas stoves, as well as tobacco smoke.

At low concentrations, nitrogen oxides are irritating to the upper respiratory tract and eyes. Prolonged exposure can cause pulmonary oedema. Excessive exposure to nitrogen oxides can cause health effects on the blood, liver, lung and spleen. Nitrogen dioxide is also one of the gases that contribute to acid rain that damages vegetation and buildings and contributes to the acidification of lakes and streams.

#### 2.4.2. Suspended Particles

These particles are usually referred to as total suspended particles (TSP) and include all particles with diameters ranging from less than 0.1 microns to 50 microns, as larger particles are deposited by gravity. TSP is expressed as PM (particulate matter) with a sub-index referring to particle diameter, and the unit is the weight of particles per volume of air (mg/m<sup>3</sup> or μg/m3). The larger the particle size, the shorter the time they remain suspended in the air and the shorter the distances they can travel. Particles larger than 10 microns fall rapidly near the source that produces them; PM10 particles (with a diameter of ≤10 microns) can remain suspended for hours and travel from 100 m to 40 km, while PM2.5 particles (with a diameter of ≤2.5 microns) can remain in the air for weeks and are capable of moving hundreds of kilometres, moving with air currents and penetrating premises through ventilation systems.

The main sources of particulate matter outdoors are road traffic, especially diesel vehicles, industrial processes, incinerators, quarries, mining, stack emissions, coal heating, etc. Other important sources of particulate matter are dust from agricultural work, road construction or vehicle traffic on unpaved roads. On the other hand, particulate matter is present in almost all indoor environments, mainly from combustion appliances and tobacco smoke. It can also have a biological origin, such as pollen, spores, bacteria and fungi. Typically, most particles of anthropogenic origin are in the range of 0.1–10 μ.

The size range that can be considered dangerous in relation to its effects on human health and air quality is between 0.1 microns and 10 microns in diameter since these particles, once inhaled, generally have a greater capacity to penetrate the respiratory system. PM10 particles are deposited in the upper respiratory tract (nose) and in the trachea and bronchi, while PM2.5 particles with a smaller diameter can reach the bronchioles and alveoli of the lungs.
