Radon is a radioactive, non-reactive gas. It exists in nature as unscented and flavorless gas (Cevik et al. 2011). It is found inside Earth's crust in elemental form (Cevik et al. 2011). There are three natural isotopes of radon: radon 222Rn, thoron 220Rn, and actinon 219Rn with half-lives of 3.82 days, 55.6s and 3.6s, respectively belong to the natural decay series of 238U, 232Th and 235U (Cevik et al. 2011; IARC 1988). 222Rn is essential isotope of radon due to their longer half-life; compared to the other two isotopes, it can travel long distances inside the earth in geological formations. Once it comes to the outdoor air environment, it can mount up and ultimately reaches a dangerous level, especially indoor environments. It has been reported that isotope 222Rn (named as radon) has seriously influenced the human health because of its longer half-life and the large quantity of its parent component (238U) in earth crustal materials compare with the other two isotopes. As indicated by the review, radon follows tobacco smoking as a reason for lung cancer (IARC 1988).
In three significant segments, radon levels can be evaluated as to their measurement concerns air, one indoor, the other outdoor, and the third one in the mining of various types. For indoor radon and its daughters, the sources can be inside or outside or both. The inside sources comprise building materials, water, cellar air, soil (Richard and Rebers 1991), while the outside air is just known as the outside source. High indoor radon levels by and large outcome from prominent radon creation and versatility in soils. The pores in building foundations and cracks in floor slabs are also responsible for indoor radon built-up (Nielson et al. 1994). The ventilation rate, temperature and pressure factors inclinations are the center elements liable for changes in indoor radon fixation levels (Rahman et al. 2010; Rafique et al. 2011). The air exchange among indoor and open-air conditions is affected by ventilation rate; the high ventilation rate decreases the indoor radon concentration, and poor ventilation built elevated radon in the inner environment. While the presence of radon in the outside environment depends on time, climatic conditions, and the origin from where the air sample is taken (Ali et al. 2011). Radon can be the measurement in air, both by passive and active techniques. However, the passive/integrated methods are generally considered appropriate for field study and for appraisal of radon focuses throughout extended time scales (Miles 2001; Martinez et al. 2001; Sesana and Begnini 2004; Al-Jarallah et al. 2008). In these techniques, a detector will capitulate a single measurement of the radioactivity of radon in air, averaged over some selected period from a few months to a year or even for a more extended time from which seasonal and annual variations of radon concentrations can be measured. For this purpose, many detectors, be installed at each place exclusive of lost a solo measurement on a day. These detectors can also be helpful for such sites where active techniques either not possible or error in its measurement.
In the Himalayan region of Pakistan, two essential earthquakes occur: in 1965 and October 2005 in Kohistan District and Balakot Region, respectively. Besides that, these earthquakes cause damages, also responsible for the high level of radon emanation from geological faults and fractures (Khan et al. 2010; Khan et al. 2021).
The variation in radioactivity in rocks depends on amount of radioactive elements (Serra 1984; Khan et al. 2021). Different types of rocks, i.e., sedimentary, igneous, and metamorphic, have varying radioactivity values based on the number of radioactive minerals. Clay minerals in mud rocks like shale/clay regulate radioactivity. The radiation levels are high (50%) in mud rocks due to thorium and potassium (Serra 1984). In clastic rocks such as sandstones, uranium is abundant in arkose (> 25 % feldspar), whereas thorium and potassium are abundant in glauconitic form (Serra 1984; Khan et al. 2021). In carbonate rocks such as limestone and dolomite, radioactivity is low (thorium and uranium). Carbonate’s radioactivity is proportional to the amount of argillaceous material present. If the carbonates have a higher argillaceous content, higher will be radioactivity (Serra 1984; Khan et al. 2021). organic sedimentary rocks such as coal, the radioactivity is low. Still, when the organic material is present in shale, i.e., organic shale, radioactivity is high due to clay minerals (Serra 1984; Khan et al. 2021). Radioactivity (potassium) is abundant in salt, gypsum, and anhydrite (together known as evaporites), but other radioactive elements are present in a smaller amount. There are three different types of igneous rocks: acidic, mafic, and ultramafic. From acidic to mafic to ultramafic, the radioactivity (uranium, thorium, and potassium) decreases due to different chemical composition (Serra 1984; Khan et al. 2021).
The Himalayas was affected by a significant earthquake in the last century, for instance, Kangra (1905), Bihar-Nepal (1934), Assam (1950), and Kashmir (2005) earthquake. These earthquakes occurred in Sub-Himalayas (Thakur 2008; Kaneda et al. 2008; Sapkota et al. 2013). In Lesser and Higher Himalayas, several moderate earthquakes occur Nepal (2006), India, Kohistan, and Kashmir (1965) (Kumahara and Nakata 2006; Kaneda et al. 2008). Kashmir seismic tremor happens on eighth October 2005 of size 7.6, and around 80,000 lives were lost (Mona Lisa et al. 2007). Such quakes typically bring about the increased radon radiation from faults and fractures. The fault during this quake lies from the north of Balakot (Tanda Fault) toward the northwest of Bagh (Muzaffarabad Fault) (Mona Lisa et al. 2007) collectively called Balakot-Bagh Fault (B-B Fault).