## 2 Compatibility between Indoor Radon Exposure and Buildings' Energy Efficiency Improvement
### Energy Efficiency Management vs. Indoor Radon Exposure  
The new energy efficiency challenges arising both from increasingly restrictive regulatory frameworks [14; 15; 16] and a more demanding society seeking more comfortable and less costly buildings, brought profound transformations into the building's design [17]. With the emergence of new construction methods and solutions--high-performance windows, new thermal insulation technologies, high-efficiency air-conditioning systems, and other climatization devices--the paradigm of Nearly Zero-Energy Buildings (NZEB) was built. Therefore, the current tendency of modern times is to reduce energy consumption by improving thermal insulation and investing in the replacement of conventional windows by airtight double-glazed frames, and by reinforcing the performance of heating and cooling systems. Hence, the focus is set on buildings' energy efficiency and thermal comfort reinforcement instead of promoting its balance along with indoor radon exposure and an overall IAQ improvement [18; 19; 20; 21; 22; 23]. The lack of balance between these variables (energy efficiency versus indoor radon exposure) has led to the construction of \"hermetically\" sealed buildings, with very reduced ventilation openings and low air renovation, where automated air conditioning systems has ensured the control of air temperature and relative humidity, however, completely ignoring radon levels and its relation to IAQ improvement [24; 25; 26; 27]. In addition to an increased radon exposure scenario new sources of indoor air contamination have emerged, essentially composed of toxic materials used for finishing materials and furniture, such as paints and varnishes, wallpapers, adhesives, carpets, among others [28]. The progressive reduction in the air exchange rates is associated with the increased concentration of physical, chemical, and biological pollutants in indoor environments. Therefore, to prevent occupants' health problems, it is crucial to increase indoor air renovation [29], promoting the mitigation of radon levels and the elimination visible fungi or mold growth, and keeping the interior room well ventilated, clean, and sanitized. Ventilation stands out as a fundamental factor in radon management, improving overall IAQ, and it can be provided by either natural or mechanical means, but both need a specialized design to avoid malfunctioning [30; 31; 32; 33; 34; 35]. Already in the 1980s, the specialists have recognized the radon problem along with many other air pollutants that affect occupants' health and well-being [36]. In 1982, this phenomenon took huge proportion with the Sick Building Syndrome (SBS), at that time recognized by the World Health Organization (WHO) as a public health problem [37], after 34 confirmed deaths and 182 contaminations by indoor air through Legionella pneumophila, at the Hotel Bellevue-Stratford (Philadelphia, USA) [38; 39]. IAQ assessment is related to the characterization of a set of polluting sources arising from three main origins: physical origin (lighting, noise, electromagnetic fields, temperature, humidity); chemical origin (carbon dioxide, carbon monoxide, VOC, nitrogen dioxide, ozone, formaldehyde, sulfur dioxide, ammonia, radon); and biological origin (bacteria, fungi, viruses, protozoa, arthropods, animal excrement in general) [40; 41]. Thus, a building can be considered \"sick\" when [42]:  
* Around 20% of building occupants exhibit symptoms for more than 2 weeks, such as headaches, eye, nose, and throat irritation, nausea and malaise, skin problems, con-centration difficulties, fatigue, sensitivity to odors, etc. ;
* The cause of the symptoms is not known;
* Complainants' symptom relief is checked shortly after leaving the building. Contamination of the environment can also trigger new disorders, or even aggravate pre-existing diseases (such as rhinitis and asthma) [4].