**1. Introduction**

Indoor air quality (IAQ) has been a well-known problem since the late 1970s. Its significant impact on human health has been addressed several times by the World Health Organization (WHO) in various documents and meetings, and has been carried out at various levels [1–3]. Further, economic studies and researches have highlighted the great importance that IAQ now has in all environments, e.g., houses, schools, banks, post offices, offices, hospitals, and public transport, just to name a few [4]. IAQ also has strong repercussions in the competitiveness of an organization, considering the increase in difficulty in carrying out its job in the best way, its performance, and the social and economic competitiveness between countries, due to the influence on the attention, degree, and number of days lost [5].

Scientific literature contains large documentation in terms of articles, conference papers, reviews, books, editorials, letters, and public articles on chemical contaminants in indoor environments. A search on the Scopus literature database, using the keyword "indoor air quality", led to a total of 7287 publications between 2000 and 2020 in the European Union (EU) (search executed on 19 January 2020), including Norway, Switzerland, and Turkey. According to this search, Italy and the United Kingdom (UK) are major contributors to this total amount of European publications, 12.3% and

10.4% of the total, respectively, followed by France and Germany with 9.5% and 9.1%. Figure 1 shows the relative percentage contribution of each Member State of the European Union (EU) including Norway, Switzerland, Turkey, as well as United Kingdom, which is expected to leave the EU on 31 January 2021. The United Kingdom, Italy, France, and Germany contribute more than 41% of the total amount of publications in the European IAQ field.

**Figure 1.** Percentage of country contributions to the total amount of publications on indoor air quality (IAQ) in Europe from 2000 to 2020. (Source: Scopus, search: 19 January 2020); includes Norway, Switzerland, Turkey, and the UK, which is to be expected to leave UE on 31 January 2021.

This continuous and growing attention on IAQ has evidenced, over time, the need for a profound cultural change, according to WHO indications, in order to develop organic health prevention and promotion actions, and cope with the complexity of such an issue.

Noteworthy, at the European Community level, the resolution of 13 March 2019, defends clean air for everyone and highlights that people spend almost 90% of their time in indoors [6]. In these environments, the air can be significantly more polluted compared to outside [7] and, therefore, considered mandatory to issue indoor air quality certificates for both new and old buildings. From this perspective, it urges member states to adopt and implement measures to combat air pollution at the source.

There are specific cases, such as schools, healthcare, or office environments, where the permanence of workers (e.g., medical, administrative, teaching, and non-teaching staff) is supposed to last for a relatively long period, and where "users", as well, are present (e.g., patients, students, vulnerable and/or fragile subpopulation, some of which with physical and psychological disabilities, etc.). In these situations, it is essential to consider the very close relationships between the various work activities and the quality of the building structure, finish, furnishings, and the degree of crowding of such environments. This includes the presence of technological systems or interventions for energy purposes only, without forgetting the ventilation needs of the environment for aspects related to health, performance, and staff and student performance [8].

The combination of these actions is fundamental for developing and implementing plans for the protection and promotion of health safety for citizens and workers [9]. This represents the priority and the common objectives of both national and European prevention plans (National Prevention Plans (NPPs) and programs from the United Nations (UN) Sustainability Development Agenda).

Nonetheless, several European countries have had to overcome the absence of specific legislation, or legislative acts already developed, due to generic definitions of the characteristics of air quality. For instance, in closed workplaces, such as closed offices where employees have individual working areas that are distinctly divided—either by walls, cubicles, or panels—it is necessary to ensure that workers have healthy air in sufficient quantity, which is also obtained with ventilation systems [10]. It is necessary for updated laws and regulations to be adopted to improve the indoor air quality.

Another fundamental requirement for correct understanding of the air quality pollution phenomena indoors is the availability of reliable (and systematically collected) information, according to well-established protocols, on the quality, quantity, and origin of the pollutants. In this regard, particular attention should be addressed to the activities of the European Committee for Standardization (CEN) and the International Organization for Standardization (ISO), which provide a series of specific indications on the operating procedures with which to carry out the checks.

In recent years, several international organizations, e.g., the European Collaborative Action (ECA), the World Health Organization (WHO), and the International Agency for Research on Cancer (IARC) have produced reference documents, guidelines, agreements, and protocols. For example, the Parma declaration, the Children's Environment and Health Action Plan for Europe CEHAPE), European Union (EU) regulations (e.g., regulation 305/2011, which lays down harmonized conditions for the marketing of construction products); documents, and rules for characterization and determination on many pollutants (e.g., European Standards (EN) ISO 16000—Indoor air quality, European technical specification (CEN/TS) 16516: construction products—determination of emission into indoor air). The purpose of this documentation is to tend to the decrease in the number of pollutants present in indoor environments and to regulate the levels of chemicals that can be emitted from different materials, in order to contain the negative impacts on IAQ. In particular, the activities carried out by the European Committee for Standardization (CEN) and the International Organization for Standardization (ISO) represent important references, because harmonized methods of detection allow for better comparison between the different indoor air quality data produced at the European level. Such methods should be implemented by laboratories that carry out environmental surveys.

Within this context, the aim of this paper is to summarize the entire legislation on IAQ present in the EU (the UK included), in February 2020, along with reference values, guide values, and unitary risks for many kinds of indoor air pollutants present. Particularly, the foundations of the different legislations will be compared for evidencing the main characteristics of each one, and the levels of the main pollutants will be presented and discussed. The focus is to highlight the strengths and weaknesses to deal with this important topic. According to the authors' knowledge, this is the first critical revision of the European legislation. Further, a section will be dedicated to the state-of-the-art research in Italy, from a legislative and scientific point of view. Although there have been many scientific papers and studies performed on IAQ, and a methodic and analytical review of the papers published in the last five years concerning the indoor field will be documented, it will be highlighted that, in Italy, the main problem is the lack of reference standards for residential indoor air quality.
