A recent study published in Nature found there is no significant link between residential radon exposure and COPD as exposure to radon did not appear to have an influence on the clinical characteristics of smokers and ex-smokers with COPD. Radon, which comes from the natural decay of uranium that is found in nearly all soils, typically moves up through the ground to the air above and into home through cracks and other holes in the foundation. The researchers said that they analysed COPD cases drawn from a case–control study conducted in an area of high radon exposure to conclude no differences between radon concentration and sex, age or severity of breathing obstruction as measured by FEV1 percentage. The researchers wrote that  COPD cases with radon concentrations higher than > 600 Bq/m3 exhibited no different characteristics in lung function and that there is no trend towards a worsening of FEV1 percentage with an increase in radon exposure.