In the last few decades the demand on sensitive and fast gas monitoring systems in industrial and environmental applications is increasing. The main application areas are e.g., the process control and the measurement of the pollution or trace gas constituent of the atmosphere. The monitoring systems for such applications developed by the photoacoustic research group at the University of Szeged are presented. The European Standards rigorously regulate the industrial air pollution. One of the most important pollutants is the hydrogen-sulfide. The main source of the hydrogen-sulfide is the natural gas. Therefore, our research group developed a system for selective and sensitive (1-2 ppm) monitoring of the hydrogensulfide of the natural gas. The corrosion of the pipelines is also a crucial problem for the natural gas industry. It is primarily caused by the hydrogen-sulfide and the water vapor content of the gas. In turn, high concentration of water vapor in the gas can damage some gas preparing process systems, especially in wintertime. We developed therefore a monitoring system, which measures as low as 1 ppm concentration of water vapor in the natural gas. An ozone detector with a sensitivity in the low-ppb range is also presented.