Radon is a colourless, odorless and tasteless gas that occurs naturally in rocks and water. It is also a radioactive decay product of radium. Inhaled radon gas is the main contributer to ionizing radiation in the general population. Although it occurs naturally, this differs depending on the make-up of soil while it can also accumulate in buildings (especially in low areas). Epidemiology has shown a clear link between radon gas exposure and lung cancer risk, including in non-smokers, and is considered the second most frequent cause of lung cancer (after cigarette smoking). In this project we, for the first time, will explore whether there is any evidence of epigenetic (DNA methylation) signals as a result of residential exposure to radon that can already be measured at young age (well before the development of lung cancer) and whether any signal could be dose-dependent. If there is evidence of such an association in ALSPAC, and given the ubiquitous exposure but also straightforward methods for interventions to improve the indoor environment, this project may result in (a) updated advice on hazards of radon exposure across the lifecourse and (b) a grant application to investigate this in a much larger cohort, such as the pooled cohorts in the CLOSER consortium), to provide more robust estimates of risk.