What causes radon spikes?

Air pressure inside a home is usually lower than in the soil around its foundation, and that difference makes the house act like a vacuum, drawing radon in through foundation cracks and other openings. Ventilation pulls the other way: opening windows, doors and vents on the lower floors increases ventilation, but once they are closed, radon most often returns to previous values within about 12 hours.

Weather is one reason short tests have rules. Do not run a short-term test of just 2 or 3 days during unusually severe storms or periods of unusually high winds. Interference with the test or with closed-house conditions will invalidate the result.

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Test	How long it runs	What it is for
Short-term test	Stays in the home from 2 to 90 days, depending on the device, and for a minimum of 48 hours.	A faster screening result. Because radon levels vary from day to day and season to season, it is less likely than a long-term test to tell you the year-round average.
Long-term test	Stays in the home for more than 90 days.	It is more likely than a short-term test to tell you the home's year-round average radon level.
Continuous monitor	Logs readings for the whole test, run as a short-term or a long-term test under the same durations in EPA guidance.	Logs a graph over time, so you can see trends and spot anomalies rather than a single number.

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A sudden, lasting rise in a home with a mitigation system can mean the system needs attention. Every active system must have a warning device that alerts you if it stops working properly; look at it, usually a manometer, on a regular basis.

Decisions rest on averages. Because of these fluctuations, the annual mean is best estimated with measurements of at least 3 months, and a short-term test is less likely than a long-term test to show your year-round average. If a short-term result is high, take a followup test to be sure.