Radiation and radioactive isotopes form part of our natural environment. Elevated levels of these radioactive isotopes in the environment can pose a threat to our health. One of naturally occurring radioactive gas is Radon (²²²Rn). Radon is a colorless, odorless, and tasteless gas. ²²²Rn is the second leading cause of lung cancer after smoking, and may pose a significant long-term risk for student, who spend long time in class and halls. In the current work, radon concentrations were measured for one day run in 28 student lecture halls at Umm Al-Qura University - Makkah, Saudi Arabia. Radon was measured with a portable Radon counter (Rad 7 Instrument, Durridge-USA), which was placed on the middle of student halls at a height of about 0.85 m. ²²²Rn gas levels were registered in Becquerels (Bq/m³).

      The measured concentration of radon in the air of halls ranged from 15.6± 8.65 Bq/m³ to 24.8 ±10.4 Bq/m³ with a mean of 19.8 ± 2.919 Bq/m³, while that of the progeny of radon varied from 0.00173 to 0.00276 WLM Y^-1 (average: 0.00213). The mean indoor concentration of radon was considerably less than the lower levels prescribed by EPA (148 Bq/m³), EEC (400 Bq/m³), WHO (100 Bq/m³), ICRP (200–600 Bq/m³) and NRPB (200 Bq/m³). The mean annual effective dose calculated 0.012 mSv y⁻¹, which is less than the UNSCEAR and WHO recommended global lower average dose value of 1 mSv/y. The hazard indices (PAEC, and CPPP) values were lower than the permissibility limit value.

Keywords: indoor radon; Umm Al Qura University; student halls; Rad 7, annual effective dose.