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For a long time, limestone has been massively used in stone building and monuments because of its easy extraction and common presence in the landscape. On ancient monuments, mostly built in urban areas, it is exposed to urban-borne pollutants responsible for specific alteration mechanisms and weathering kinetics. Especially, the dissolution of calcite and the precipitation of new phases will affect the limestone pore network, modify the stones capillary properties, and influence the further alteration. In order to better understand these processes, an altered limestone sample from 'Tribunal Administratif' (TA) in Paris was studied. The main secondary phase was found to be syngenite, which can be explained by the location of the sample close to the soil, a potential source of K (fertilizers). This phase is more soluble than gypsum that is commonly found on altered limestone. In order to assess the reactivity of the system (limestone and new phases), oxygen and hydrogen isotopes were used to trace the transfer of water (D 2 18 O) and identify the location of the reactive areas (susceptible to alteration). For that, TA samples were exposed in a climatic chamber to relative humidity (RH) cycles (25% RH for 2.5 days and 85% RH for 4.5 days) for 2 months with a D 2 18 O vapor to simulate alteration occurring in conditions sheltered from the rain. Results have shown that the water vapor easily circulates deep in the sample and reacts preferentially with syngenite the most reactive phase (compared with calcite and quartz). This phase could evolve in gypsum when exposed to an environment different from the one resulting in its formation.

We present two cases of fatal venous air embolism that occurred in previously healthy adolescents during posterior spine fusion and instrumentation for scoliosis. Previously recorded cases in the literature have been rare and only briefly discussed. Air embolism has been a complication noted in the anesthesia and neurosurgical literature, occurring primarily in total hip patients and those patients undergoing neurosurgical procedures in a sitting position. An examination of the pathophysiology of air embolism in posterior spinal procedures would indicate the use of properly placed central lines and end-expiratory CO2 monitoring as the primary modes of detection and treatment considering the potentially fatal risk factors for spinal patients.