Typically, when is indoor radon concentration highest?

Indoor radon concentrations are typically highest during the winter months. This phenomenon can be attributed to several factors related to home heating and ventilation practices. During winter, homes are usually closed up tightly to conserve heat, which reduces the natural ventilation that can help mitigate radon levels. As a result, any radon that enters from the ground or other sources becomes trapped indoors, leading to an accumulation of higher concentrations.

Additionally, the colder outdoor temperatures can create a stack effect in buildings, where the warm indoor air rises and creates a negative pressure at lower levels, drawing radon in from the soil. The combination of closed windows, reduced air exchange, and increased radon entry through soil and cracks in the foundation contributes to elevated radon levels in winter.

Summer months tend to have more ventilation due to open windows and air conditioning systems, which can help reduce indoor radon concentrations. Similarly, spring and fall may experience variable ventilation circumstances but generally do not trap radon as effectively as the winter months. Therefore, indoor radon concentrations peak in winter due to a combination of less ventilation and atmospheric conditions that favor radon accumulation.