There are strict guidelines a radon tester must adhere to when taking a radon sample. In general, the longer the test, the more accurate the results, and we’re talking weeks or months. However, for real estate transactions, certain criteria must be met in order to achieve the best possible results within a very constrained time period, so the EPA and other similar bodies have developed acceptable protocol to be able to give us the best look at an average radon level in a home.
The minimum testing period is 48 hours, and most professional companies use Continuous Radon Monitors (CRM) which sample radon levels every hour. They are also equipped with various anti-tamper indicators to help the tester determine if the test is valid. Items such as temperature, movement, and barometric pressure readings each hour help the tester determine if the machine may have been moved outside (where radon levels are lower) and then moved back in prior to his retrieval of the unit. It sounds crazy, but we have seen many instances of sellers doing things like that, or even opening windows to vent the home and keep radon levels low for the test. Of course this invalidates the test and alerts the buyer and all of the agents involved that the seller is unscrupulous and dishonest, so we always leave a letter with the seller so they are aware of the closed house conditions that must be met. Most sellers honor those criteria.
“Closed house” essentially means the doors and windows must remain closed for the duration of the test, except for normal going and coming. Operating fans that bring in air from outside will also invalidate closed house conditions.
The radon machine must be placed on the lowest livable level of the home (for example, even if a basement is unfinished, if it has the potential to be finished someday, that is the level to place the machine), and it should not be placed near drafts or near heat. The machine should not be placed in kitchens, laundry rooms, bathrooms, crawlspaces, or closets. It should be at least 3 feet from doors and windows to the exterior, and it should be at least 1 foot from any exterior wall. It should also be a minimum of 20 inches off the floor.
Assuming all of those conditions are met, the test should be valid. It surprises some people that the radon level fluctuates throughout the day and is not static, but that is completely normal. A CRM will print out the hourly readings, and they fluctuate, sometimes quite significantly, throughout the duration of the test. One of the reasons for a minimum 48-hour test is to ensure sufficient readings so the average radon level covers a two-day period, giving a better idea of radon levels in the home.