High-pressure pipelines that transport sour natural gas contain high levels of hydrogen sulfide, which is poisonous and has irreparable effects on human health even in low concentrations. These pipes are break valve-assisted and buried underground to minimize gas leakage and protect people nearby. This study examines their leakage through a series of time-dependent three-dimensional CFD simulations. In contradiction of previous works that only considered the above-ground environment, here, for more realism, the computational domain includes the pipeline, trench, covering soil, and above-ground environment. The impact of hole size, leak location on the pipe, wind velocity, atmospheric stability class, time of occurrence (day or night), and the presence of break valves on the dispersion of leaked gas are comprehensively investigated. Results indicate that the effect of hole diameter on hydrogen sulfide concentration in the above-ground environment is dominant to other factors. In addition, the probability of fatality due to gas release and the intensity of the gas leak exposure crisis are studied by combining the dose-response model and CFD simulation results. In this line, LT50, which measures how long it takes for 50% of people in different areas around the pipeline to die from exposure to hydrogen sulfide is calculated. Display omitted •Transient CFD simulation of sour natural gas leak from buried transmission pipelines.•Simulating the leak process considering the effect of closing the line break valves.•Calculating the time required to reach a 50% probability of fatality around the pipe.•The size of leak hole has the most obvious effect on the intensity of gas dispersion.•The intensity of the gas leak exposure crisis was introduced and evaluated.