## 1 Introduction  
Safety can be defined as the state of being free from unacceptable risks which can potentially cause damages to humans, environment or properties [1]. Public safety is very important in industrial environments especially in high-risk fields such as oil and gas, chemical industry or nuclear reactor plants where any event or accident can lead to catastrophic consequences both for humans, i.e., the workers and/or those who are living in the surrounding areas, and for the environment. These kinds of scenarios are typically categorized as Factories at Major Accident Risk (FMAR). A mapping of the most frequent major accidents in this context has been produced in Italy in 2013 by the ISPRA (Istituto superiore per la protezione e la ricerca ambientale--High Institute for Environmental Protection and Research) [2] and they are mainly due to release of dangerous substances (liquid or gas), fires, and explosions. The possible causes are several, such as machinery or piping break up, watertight loss, tanks overfill, mistakes during the handling procedures, submission of material on already filled cans, blending of incompatible materials, valves break up, accidental falls or vehicle collisions, and, more generally, spillover of cisterns. Activities of ensuring compliance with procedures to prevent major accidents are traditionally in charge of humans control, and, as such, they are naturally error prone. On the other hand, with the rapid development of the numerous innovations in manufacturing and in information and wireless communications technologies, we have assisted in the last years to the birth of the new Industry 4.0 era [3]. In this scenario, the Internet of Things (IoT) is a hot topic nowadays either from a research point of view and from an application perspective as well [4]. Needless to say, IoT may provide great support to safety in the considered FMAR scenario in which workers and machines operate in a shared environment and a great amount of heterogeneous information can be collected by sensors and automatically delivered to a Central Controller (CC) allowing a real time control of the whole activity chain (and, as a consequence, of the associated risks). More specifically, the most important sensing technologies in the considered application scenario are represented by sensors for detecting gases with the purpose of avoiding fires and explosions involving flammable gas leakages as well as for controlling the level of oxygen in the atmosphere. In particular, electrochemical and catalytic gas sensors were chosen in this work, whose main focus is the implementation of a reliable limited time delay transmission system using a LoRa radio channel to transmit dangerous gas concentration detected by the sensors. For this purpose, we refer to the wearable device embedding sensing and communications capabilities described in Section 3, which is under development within a collaboration between the University of Siena and INAIL (Istituto nazionale per I'assicurazione contro gli infortuni sul lavoro--Italian National Institute for Insurance against Accidents at Work). Regarding in particular the communication aspects, the considered IoT scenario is characterized by low power devices transmitting infrequent short bursts of data over a low power wide area network. Indeed, the devices cannot have any external power supply (i.e., they run on batteries and they are installed in areas where frequent batteries substitution cannot be always guaranteed). Accordingly, they are expected to be very power efficient. Moreover, most of the detected data are not critical and, as such, can be referred to as Regular Packets (RPs). However, in some cases, i.e., when the concentration of gases crosses a pre-defined threshold, the detected data have to be urgently and reliably delivered and, accordingly, they are referred to as Urgent Packets (UPs).