Humidity itself does not affect radon-detection chemistry, but Cincinnati summer conditions (open windows, lower stack effect, AC running) typically produce lower readings than the same home would show in winter. The closed-house protocol exists to control this.
More detail
Continuous radon monitors and charcoal canisters measure alpha-particle activity from radon decay; the measurement chemistry is insensitive to indoor humidity over the normal residential range (20 to 70 percent RH). What summer Cincinnati conditions affect is the radon equilibrium itself, not the measurement of it. Five summer-condition effects on Cincinnati indoor radon: (1) open windows break the closed-house assumption, allowing fresh-air dilution that drops indoor radon, (2) the stack effect (warm air rising and exiting upper-floor penetrations) weakens or reverses when indoor and outdoor temperatures equalize, reducing the negative pressure that pulls soil gas into the home, (3) HVAC in cooling mode generally has less basement-pressurization impact than heating mode, (4) higher outdoor barometric pressure during summer high-pressure systems reduces soil-gas pressure gradient, and (5) occupant behavior shifts (more time outdoors, more cooking-with-vent-fan, more bathroom fan use) all dilute indoor air. The ANSI/AARST MAH-2023 protocol controls for these by requiring closed-house conditions (windows and exterior doors closed for 12 hours before and during the test, HVAC in normal operating mode, no unusual ventilation). For real-estate testing this is non-negotiable; for informational testing it is recommended. A July test under proper closed-house conditions produces a Cincinnati reading roughly 20 to 40 percent lower than the same home would test in January, which is a real but bounded effect. The closed-house protocol is what makes that result lender-defensible.