Ambient gas detection and measurement had become essential in diverse fields and applications, from preventing accidents, avoiding equipment malfunction, to air pollution warnings and granting the correct gas mixture to patients in hospitals. Gas leakage can reach large proportions, affecting entire neighborhoods or even cities, causing enormous environmental impacts. This paper elaborates on a deep review of the state of the art on gas-sensing technologies, analyzing the opportunities and main characteristics of the transducers, as well as towards their integration through the Internet of Things (IoT) paradigm. This should ease the information collecting and sharing processes, granting better experiences to users, and avoiding major losses and expenses. The most promising wireless-based solutions for ambient gas monitoring are analyzed and discussed, open research topics are identified, and lessons learned are shared to conclude the study.Technologies to sense gases and wireless communications support are elaborated in Sections 5 and 6, respectively. The most promising solutions in terms of sensing methods and IoT-enabled solutions are discussed in Section 7 and open research topics are identified. Lessons learned are shared at Section 8 and, finally, Section 9 concludes the study. Background on Environmental GasesSome gases are the key to ensure the functionality of systems and entire industries, as well as the presence of other gases being a problem in other fields, causing the loss of entire production lines, as in the food industry, or even cause the loss of lives and explosions. In this section, the most important gases in terms of pollution monitoring and control, health issues, and accident prevention are listed with their main characteristics.Oxygen (O 2 ) is the most important gas for life, and is crucial in numerous fields. Patients under anesthesia, or recovering from surgery and from certain diseases need controlled O 2 doses to keep them alive and fully recovered. The decrease of oxygen levels in enclosed spaces can be related with other gases leakage, which would lead people inside these spaces to asphyxiate, leading to an unconscious state, or even to death [29]. Numerous industrial processes rely on the correct concentration of this gas to achieve the best results, mainly chemical and combustion, which without the correct percentages will not grant the best performance of systems. Engine control systems depend on the correct mixture to achieve the expected performances, whether it is lower fuel consumption or power and speed [29][30][31][32].Carbon dioxide (CO 2 ) is a colorless, odorless gas, generated by the oxidation and combustion of hydrocarbon, as well as by living beings in the respiration process. It is a key gas for the greenhouse effect, and the increase of its levels, in the presence of other gases, is related with atmospheric pollution. It is also the key gas on the oxygen production by the photosynthesis process. The accumulation of this gas in enclosed spaces can be responsible to s...

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Ambient gas measurement is essential in diverse fields and applications, from preventing accidents, to air pollution warnings, and granting the correct gas mixture to patients in hospitals. Gas leakage can reach large proportions, affecting neighborhoods or even cities, causing enormous environmental impacts.The integration of gas sensors through the Internet of things (IoT) paradigm should ease the information collecting and sharing processes, granting better experiences to users, and avoiding major losses and expenses. With this integration, final users must be capable of operating these gadgets without previous technological knowledge, thus, the proposal of plug-and-play (PnP) sensors and actuators should be considered. This article proposes an IoT-based solution for sensing environmental gases preceding a PnP approach, combining the hardware and software integrations for a better user experience. In this proposal, the PnP interface is defined, using the universal asynchronous receiver/transmitter protocol. The wireless communication is made through the IEEE 802.15.4 and IEEE 802.11 (Wi-Fi) standards. The proposed sensor is capable of detecting smoke, oxygen, hydrogen, carbon monoxide, alcohol vapor, liquefied petroleum gas, benzene, ammonia, methane, and propane. The solution is integrated with a middleware for IoT, called In.IoT, a software that receives the sensed data, stores, and forward them to other smart devices on the same context, allowing an Android-based mobile application to warn users regarding the environmental gases concentrations. The proposed solution is experimented, demonstrated, validated, and it is ready for use. INTRODUCTIONData collection on ambient gases is essential in different fields as environmental monitoring, enclosed spaces supervision, chemical processes, and medicine, where hospitalized patients must receive the correct concentration of oxygen (O 2 ) for a best recovery process. The supervision of mines is essential to avoid explosions due to methane (CH 4 ) escapeTrans Emerging Tel Tech.