Radon levels can be measured using several different technologies. Although there are pros and cons to specific kinds of test types, the most important thing to do is to test a building for a long period of time (~3 months or more).
The amount of radon entering a building can fluctuate up and down quite a lot over a short period of time. From day to day, or even from hour to hour, radon can be very high or very low. This is normal and is influenced by many factors, such as how your home ‘breathes’ outside air, furnace use, the weather and even geologic phenomena.
As a result of these normal fluctuations, measuring radon over a short period (hours or days) will often give a false reading compared to what is the average for a long period (several months). This false reading can be a false positive or a false negative, meaning you might think you have a problem when you don’t, or (worse) you think you’re safe when you are not.
A precise radon test (that gives you a reading = actual amount of radon you are being exposed to over the long term) requires a measurement taken over several months ideally 3 or more.
A study of 776 Canadian ‘closed house’ short (5 day) and long (90+ day) radon tests showed that long term testing was precise >96% of the time. In contrast, short term testing was “wrong” (meaning the short term test failed to predict the long term test outcome) 20-98% of the time depending on season.
Reference: Stanley et al. Radon exposure is rising steadily within the modern North American residential environment, and is increasingly uniform across seasons. In Press, 2019.