Radon was once thought to always be higher in a building during winter. This is no longer true. Data shows that radon is increasingly even across seasons, and in ¼ of homes can even be at its highest during summer periods.
For most of the 20th century, radon levels in residential buildings were found to have been highest during the winter heating months (October to April in most Northern Hemisphere countries).
The reason for higher winter radon was thought to be due to two factors: (i) reduced dilution of inside air with fresh air when properties are closed up for winter, and/or (ii) forced air ventilation heating, wherein basement or ground level air is heated and distributed throughout a property more so than when the heating is off.
However, in the 21st century, radon levels in North American are observed to be increasingly even across seasons, with many homes even showing higher radon in summer. The reasons for this are still emerging, but one hypothesis is increased use of air conditioning in summer and, as a result, the decreased ventilation of properties in the warmer summer months.
A study of paired winter and summer radon tests (using multiple different radon test technologies) in Canadian homes showed that 47.5% of buildings showed a minimal (<50 Bq/m3) difference, with 24.7% displaying ≥50 Bq/m3 greater radon in winter, and 27.8% displaying ≥50 Bq/m3 greater radon in summer.
Reference: Stanley et al. Radon exposure is rising steadily within the modern North American residential environment, and is increasingly uniform across seasons. In Press, 2019.