**1. Introduction**

Indoor Air Quality (IAQ) issues have been known to professionals for a long time but only recently has it found growing attention from European and national legislators. For the World Health Organization (WHO), it represents one of the main public health problems that must be addressed due to the impact it has on the population [1]. Some important milestones have been reached as regards the quality and role of sources, human activities and the often negative effects of the wrong interventions of energy saving measures adopted in buildings. In Europe (EU), in the light of what is historically happening in this period, great attention is paid to the IAQ issue, and the member states of the EU are at the forefront in adopting a series of improvement organic actions on several levels in promotion of prevention in the IAQ field, and training of operators who are responsible for the hygienic and health protection of the displays [2].

### **2. Summary of This Special Issue**

Pending a community framework law for IAQ that considers the WHO guidelines, this Special Issue aims to make a concrete technical contribution to the solution of the various problems related to indoor air pollution. In 11 papers (nine regular papers and two reviews), international scientists report the most recent findings in this field from different points of view, including topics such as IAQ legislation [3], IAQ role in schools [4], hospitals and (micro)environments in general [5-9], the performance of an olfactometer system [10] or the impact of indoor malodor [11], BTEX measures in a fire station [12] and a chemical characterization of e-cigarette (e-cig) refill liquids (e-liq) [13].

IAQ is determined by a set of the presence of important sources of pollution, such as construction and insulation materials, furniture, furnishings, consumer products such as detergents, air fresheners, scented candles, diffuser liquids and electricals including incense sticks, maintenance and cleaning methods, etc. [14–17]. The polluting emissions from the sources depend very much on the poor quality of the raw materials used in the production [18], on the improper or incorrect use, on the state of conservation and maintenance, on the age and on the microclimatic factors [19]. Recently, the European legislators have intensified efforts to reduce and improve the quality of emissions from materials, with the study and development of standardized test methods and with the definition of common criteria for the labeling of emissions of materials.

At the same time, the WHO has put a point of the IAQ guidelines [1], relating to a certain number of pollutants, present in indoor environments, for which the scientific knowledge relating to the effects on humans has been judged robust. For pollutants with carcinogenic action, a unitary risk is defined for the general population associated with their presence in the air. The substances considered are benzene, nitrogen dioxide, polycyclic aromatic hydrocarbons (especially benzo[a]pyrene), naphthalene, carbon monoxide, radon, trichlorethylene and tetrachlorethylene. Alongside these guidelines, we must remember those concerning the risks associated with the presence of humidity and biological agents. The WHO guidelines form a basis for establishing the relevant legislative standards (limits) adopted in the EU by various countries and must also be subjected to periodic review by the competent WHO office. The guide values of pollutants in the air are very low compared to the considerable number of pollutants found in indoor environments.

Within the community, this has also contributed to the implementation of IAQ monitoring campaigns for the purpose of environmental and sanitation assessments as part of specific programs.

In the European context, the activity of the WHO has tried to urge the various European countries with the aim of directing a series of legislative actions; this has led to the issuing of specific acts both on the fundamental role of guiding/reference values for some pollutants of particular hygienic-sanitary interest, and on the mandatory labeling of the emission levels produced by the materials previously with the aim of avoiding risks for health of the general population. Even the EU, while reaffirming the priority of energy efficiency measures, recommends greater healthiness of indoor environments and the development of a specific European strategy on the IAQ topic [20].

In some EU countries, legislative acts on IAQ have been drawn up that, if properly used, can allow a better assessment of the exposure of the general population and the related health risks [3].

#### **3. Conclusions**

Following the trend, now consolidated in several countries, it seems appropriate to encourage the development of reference values or specific action values in order to better manage particularly problematic situations in these environments. In the absence of national references to be used for a comparison, it is possible to use those reported in the legislation of other European countries or, by ad hoc working groups or by analogy, to other standards such as those relating to ambient air.

**Author Contributions:** P.A. and G.S. contributed equally to the concept, creation and editing of this editorial.

**Acknowledgments:** The authors would like to thank all the contributors to this Special Issue, as well as the Editorial team of Atmosphere.

**Conflicts of Interest:** The authors declare no conflict of interest.
