When one measurement result is greater than 4 pCi/L and another less than 4 pCi/L, a re-test is needed if the higher result is what?

In the context of radon measurement and interpretation, a retest is required when there is a significant discrepancy between two measurement results to ensure accurate assessment and response to potential radon exposure. The situation described in the question involves one measurement being greater than 4 pCi/L (picocuries per liter) and the other being less than 4 pCi/L.

Re-testing is advised when the higher result is at least twice (or more) the lower result. This threshold is important because it indicates a potentially significant variation in radon levels within the assessed area, pointing to the possibility that the conditions affecting radon concentrations may have changed, or that one of the measurements could be an anomaly.

For instance, if one measurement is 8 pCi/L and another is 3 pCi/L, the higher measurement (8) is more than double the lower measurement (3). This substantial difference justifies a retest to ensure that the radon level is accurately assessed and to determine the necessary actions to mitigate exposure.

The principle behind this guideline is rooted in ensuring that health and safety measures are based on reliable data, given that elevated radon levels can pose serious health risks. The approach helps to confirm the presence of radon and leads to