A diurnal cycle is any pattern that recurs every 24 hours as a result of one full rotation of the Earth. During this diurnal cycle in which day and night are created, the Earth experiences a thermal effect as it rotates on its axis.
Notice the variability of the concentration. This diurnal cycle has been observed in many buildings throughout the year. The diurnal cycle does not coincide with ventilation rates, as might be expected and at this point, there is no good model to explain this phenomenon.
Figure 3-23 illustrates a measurement conducted during two “complete” diurnal” cycles or 48 hours. Although the radon levels in both illustrations above were well above the 4.0 pCi/L Action Level, had the radon levels been marginal, the inclusion of a partial diurnal cycle could have been the cause of a false-high average. Furthermore, while conducting tests in complete diurnal cycles may not always be feasible it is a best practice to conduct these types of measurements whenever possible.