Inhalation of radon gas and its decay products ranked as a main source of natural radiation exposure. It is considered as a major cause of lung cancer after cigarette smoking. There are some very important factors that strongly affect the indoor radon levels such as: indoor air-conditions, ventilation systems, and meteorological factors. In this article, we determined the indoor radon concentration and studied the seasonal variations of indoor radon levels to evaluate the annual effective dose received by the inhabitants in Al-Kharj city, Saudi Arabia. For this study, 84 houses were chosen and RAD 7 detector was used to examine cumulative radon exposure concentration from September 2014 to November 2017. Our measurements showed an obvious difference between radon concentration in winter (24.33 ± 11.10 Bq/m³) and in summer (14.54 ± 5.50 Bq/m³), while the average concentration in spring and autumn was very much close to the average annual radon concentration (19.23 ± 8.13 Bq/m³). The average annual effective dose was found to be 0.49 mSv/y. The study revealed that the average indoor radon concentration as well as the annual effective dose rate is found to be less than the action level recommended by ICRP.

Cement, the most widely used binding agent in concrete, is responsible for approximately 5% of global CO ₂ emissions, and cement-based materials are further limited by pronounced brittleness, porosity, and reduced tensile strength and fracture resistance. Over the past decade, this has driven interest in nanoparticle-modified cement, since nanomaterials refine the microstructure, densify the cement paste, and improve the mechanical and transport properties of cementitious composites; however, the high cost of many engineered nanomaterials has created demand for alternative, low-cost, and environmentally friendly substitutes. Eggshell, a calcium carbonate-rich biological waste generated in large volumes worldwide, has emerged as one such candidate: eggshell powder improves the mechanical performance of stabilized soils and bricks, and its incorporation as a partial cement or aggregate replacement in concrete and mortar affects compressive, flexural, and tensile strength depending on the replacement ratio and processing method. This review examines the physical and chemical properties of eggshell powder and the factors governing its contribution to the strength of construction materials. It evaluates recent work on nano-scale eggshell (nano-eggshell) as a further refinement of this approach, including its rheological and mechanical performance in cement mortar. The review further considers the use of eggshell-based composites in radiation shielding, where the same calcium carbonate content relevant to cementitious performance also contributes to gamma-ray attenuation. Through a critical comparative analysis of divergent findings across the literature and integration of the most recent studies (2024–2026), this review identifies the mechanistic basis - a competition between nucleation and dilution effects—that explains the apparently contradictory optimal replacement ratios reported in previous work. Collectively, the evidence indicates that eggshell powder and its nano-scale derivatives represent a viable, safe, and economical additive for sustainable construction and radiation-shielding applications.