Under which condition must retesting for low radon levels be conducted after an initial test?

The correct answer highlights the necessity of retesting for low radon levels particularly when significant changes occur in the building structure or occupancy. If a home undergoes substantial renovations, such as changes in the foundation or significant alterations to the ventilation system, the radon levels may be affected. Additionally, changes in occupancy, such as increased human activity or alterations to how the building is used, can also impact radon levels. Therefore, retesting in these scenarios is crucial to ensure that the indoor air quality remains safe.

Regular retesting schedules, such as the two-year timeframe or long intervals up to ten years mentioned in other potential responses, do not account for immediate changes that might affect radon levels and are more generalized recommendations rather than specific responses to significant alterations or occupancy changes. Likewise, simply retesting upon the sale of a house does not ensure ongoing safety; instead, the focus should be on immediate environmental changes and their impact on radon concentration.