137Cs in outdoor air due to Chernobyl-contaminated wood combustion for residential heating in Thessaloniki, North Greece

Wood combustion was the key heating source in Greece during the first years at the beginning of the financial crisis. Signals of ¹³⁷Cs were detected in Thessaloniki during the winter of 2013–2014 on weekends and holidays when the residents were at home burning the biggest amount of wood all day. ¹³⁷Cs signals were >6–21 μBq m⁻³ detected using high-volume air filters and γ-spectrometry. No signals have been detected since then, as gas has replaced oil for residential heating, reducing forest wood. Besides, signal <6 μBq m⁻³ is undetectable because this is the minimum detectable activity. ⁴⁰K concentrations were also measured, revealing a constant value of 143 ± 16 μBq m⁻³. The Cs-to-K ratio in air was 0.04–0.14 compared to 0.05 ± 0.01 measured before and after. Higher levels were measured when the air temperature was the lowest, but no correlation was observed with wind or pressure. Simulations using the HYSLIT model were applied on the dates on which the ratio was the highest. The model confirms the experimental results observed. ¹³⁷Cs signals detected and related to the Chernobyl-contaminated biomass used for central heating indicate that contaminated forest ecosystems remain a source of unwanted radioactivity in the environment.