Which type of test interference has the most significant effect on a short-term radon test?

Opening the windows for several hours has the most significant effect on a short-term radon test because it introduces outside air into the testing environment, which can dilute the radon concentration inside the home. This dilution can lead to inaccurately low radon measurements, as the test is designed to capture the level of radon present in a closed and stable environment. Short-term tests are particularly sensitive to changes in air exchange; therefore, allowing outside air in disrupts the conditions necessary for an accurate assessment of radon levels.

While other factors, such as running the furnace fan constantly, turning off the HVAC system, or using air purifiers can affect the distribution of indoor air and potentially impact measurements, they do not have the same immediate, profound effect on air exchange and radon concentration as opening windows does. The integrity of the test relies on maintaining a consistent indoor environment, and opening windows fundamentally alters that balance by introducing fresh air.