Low-level ionizing radiation and associated risk in an urban environment: the importance of both paving and building materials

Natural radioactivity depends on primordial radionuclides which decay across a chain of transformations to achieve a stable nuclear state. Transformations involve the emission of particles and photons whose energy can be harmful to organisms even at low-dose. K-40, Th-232 and U-238 are responsible for most of the natural emission of gamma rays from the earth’s crust and volcanic rocks are, in general, the most emissive materials. Volcanic rocks and related volcano-sedimentary lithified deposits have been quarried for construction purposes and for road paving, since the Greek times, in the area where the city Naples is located, halfway between the volcanic districts of Phlegrean Field and Mt. Somma-Vesuvius, respectively. For centuries, lithified pyroclastic products, such as grey or yellow tuffs, have been used mainly for buildings and vertical structures; lava blocks from Phlegrean Fields and, since 18th century, from Vesuvian effusive materials have been historically used to pave the roads of the old town. However, in the last few decades, deteriorated historical paving materials of some roads serving areas undergoing renovation have been partially replaced by volcanic materials of Etnean origin (proceeding from Sicily, indeed) or covered/replaced by non-geologic materials (NGMs) (e.g., asphalt). Considering that 120,000 people live in the old town (over an area of 4 sqkm) being potentially exposed to low-dose ionizing gamma radiations, a survey to estimate the contribution of geological materials to the ambient dose equivalent rate (ADER) was completed. A radiological risk assessment was also completed. Specifically, 2548 measurements of ADER (µSv/h) were made in the open air at 0.2 (ADER0.2) and at 1 m (ADER1) above the ground, respectively, using a handheld gamma-ray spectrometer. Besides, a total of 13 samples of paving materials were collected and analyzed by means of a high purity germanium detector at the Center for Ecological-Noosphere in Armenia. Results revealed a significant activity of all materials, except for NGMs. ADER1 and ADER0.2 values ​​showed a strong dependence on the distance from the ground in the streets paved with geologic materials, while the distance from the ground resulted to be not relevant for ADER in areas paved by NGMs . Based on the ADER1 data, a Monte Carlo simulation was conducted to calculate the outdoor excess lifetime cancer risk (ELCRout) for the population of the study area and for each district belonging to the old town. In one of the districts showing the highest average ELCRout, 51 additional ADER1 measurements were also conducted inside private dwellings to assess the indoor ELCR (ELCRin). Finally, the total excess lifetime cancer risk (ELCRtot) was estimated by summing values of ELCRout to ELCRin. The average ELCRout obtained for the entire study area (1.33E-03) and for individual districts (from 5.20E-04 to 1.44E-03) exceeds the world average reference value (2.9E-04). ELCRin (4.35E-03) and ELCRtot (5.79E-03) are also higher than the average reference values proposed in the literature. This study revealed that low-dose gamma radiations, emitted by paving or building materials of volcanic origin can pose a radiological risk to human health.