Assessment of radon gas level in underground stations of Tehran-Karaj plain in Iran

The gas radon is one of the most significant elements released when natural uranium and radium decay. The accumulation of gas concentration may be higher in enclosed locations with inadequate and poor air conditioning, especially in underground spaces with special geological structures, and it is vital to measure the amount of radon gas level in dense underground stations. In this experiment, a domestically manufactured environmental radon gas measuring device – electret ionization chamber detector – was utilized to degree the concentration in various underground of Tehran-Karaj plain. During the spring season, all 42 underground stations of Tehran and Karaj cities were measured on different days with a two-day exposure time. The trace of radon gas was observed in all undergrounds, but unfortunately, high radon concentrations were recorded in some locations, such as Ghaem and Tajrish undergrounds in the northern plain by virtue of fault type, water resources, and discrete geological constructions. Based on the US Environmental Protection Agency (EPA) standard, the maximum allowable concentration of radon gas in the air is 148 Bq/m 3 , while the overall average quantity at Tajrish station was 156 ± 11 Bq/m 3 . This amount is above the permissible limit and therefore can be considered a health hazard. The Azadi-Square underground of Line 4 in the center of Tehran also had a near-high concentration of radon because the bedrock was mainly composed of calcite. Accordingly, measures to improve ventilation and facilitate air movement are required in such undergrounds. Where the measurements have been done, this process also needs to be repeated for other seasons due to differences in temperature, humidity, groundwater flow rate and proximity of stations to the faults. • 42 tunnels in Tehran and Karaj cities were scanned for radon gas level. • Locations in northern Tehran had averagely greater radon level by 156 ± 11 Bq/m 3 . • Little evidence was discovered in relationship between radon level and subway depth • Azadi Square station in Tehran showed near high radon level due to calcite bedrock.