An accidental atomic legacy
The Tashkent Metro, opened in 1977, was the first subway system in Central Asia and a jewel of Soviet engineering. Each of its stations is art, using materials like marble, granite, and engraved metal showing themes from Uzbek history and Soviet achievement. But some of the most bright decorative tiles used in the original Chilonzor line stations hide an atomic secret: their glaze contains uranium.
During the mid-20th century, uranium oxides were commonly used in ceramics to produce brilliant and durable colors. Depending on the concentration and other elements in the glaze, uranium dioxide (UO2) and uranium trioxide (UO3) could create a range of colors, from bright yellow and orange-red to deep greens and blacks. This practice was widespread until the Cold War, when governments began classifying uranium as a strategic material for nuclear programs. For the Tashkent Metro, constructed in the 1970s, this decorative technique was an available, if obscure, method for creating its station art. The uranium itself was likely sourced from mines within Uzbekistan, which was a significant producer of uranium for the Soviet Union.
The tiles themselves emit low levels of beta and gamma radiation, but the primary radiological concern isn't direct exposure. The issue arises from the natural decay of the uranium atoms within the glaze.
The underground radon problem
The uranium isotope used in the glazes, Uranium-238, has a half-life of about 4.5 billion years. As it decays, it transforms through a series of other radioactive elements, one of which is Radium-226. Radium-226, in turn, decays into Radon-222, a colorless, odorless gas.
Because radon is a gas, it can escape from the ceramic glaze and accumulate in enclosed spaces. In the underground environment of the metro stations and tunnels, the gas can concentrate, creating localized hotspots. Radon and its solid, short-lived decay products are hazardous when inhaled. The decay products can lodge in lung tissue, where they emit alpha particles, heavy, high-energy radiation that can damage cellular DNA and increase the long-term risk of lung cancer. Radon is recognized by the World Health Organization as a leading cause of lung cancer after smoking.
While studies have monitored general environmental radioactivity in Tashkent, specific data on radon levels directly attributable to the metro's tiles is not widely published. The health risk to daily commuters is considered low due to the short exposure times and ventilation within the system. However, the presence of these tiles makes the Tashkent Metro an unintentional, long-term experiment in radiological geology and public health.