why do radon levels fluctuate so much?

We often get asked why radon levels fluctuate, often dramatically, during the 48-hour test period and whether there is cause for concern.  There are many reasons for this and the fluctuations are perfectly normal, but this emphasizes the reason that a long term average gives the best indication of whether radon is truly elevated in the home.

The most common reasons that radon levels fluctuate can be broken down into three basic categories:

1. Air pressure differentials in the home;
2. Use of exhaust devices that vent indoor air to the exterior;
3. Weather.

Typical Radon Testing Chart Showing Fluctuating Radon Levels

Air Pressure Differentials - Induced Soil Suction & Stack Effect from Heating

• Buildings can create vacuums that pull in soil gases/radon from the soil under the building or from the crawl space. These vacuums (referred to as Air Pressure Differentials APD’s) may be very small. But even small APD’s between the house air pressure and the soil gas pressure can greatly affect radon readings inside a home.
• When indoor air is warmer than outside air, it rises up and exits through the upper portion of the house. This air is replaced by soil gases which contain radon. Warmer inside temperatures and colder outside temperatures create a strong stack (chimney) effect drawing radon into the home.
• APD’s and the stack effect are the greatest factors causing radon levels to rise or fall inside a home and the stack effects are usually higher at night and lower during the day.  This results in higher radon levels at night and lower radon levels during the day and creates the diurnal radon fluctuations.

Use of Home Air Exhaust Devices

• When exhaust devices push air out of the home, negative pressure is created which causes radon to be pulled in from the soil under the slab or from the crawl space.
• Common exhaust devices that can increase the negative pressure inside the home leading to higher radon levels include:
  • Gas burning or wood fireplaces;
  • Central vacuum cleaner;
  • Clothes dryer;
  • Bathroom fan;
  • Gas combustion appliances (furnaces, space heaters, ranges, water heaters, etc.);
  • Kitchen range exhaust fan.

​Weather Effects

• Light rain and gentle winds have little effect on indoor radon concentrations.
• Severe storms and high winds can have a great effect on indoor radon levels.
• Rain can block soil pathways and either raise or reduce indoor radon concentrations.
• Indoor radon concentrations are usually higher during heavy rain events and winter seasons.
• High winds can raise or lower indoor radon. When wind blows on the side of house with most door(s)/ window(s), it creates more positive indoor pressure which pushes radon out. When it blows on side with least door(s)/window(s), it creates more negative indoor pressure, which sucks more radon in.
• Barometric pressure changes can change soil gas pressures and amount of radon entering a home.
• Indoor radon levels are usually higher in the wintertime because: 1) the heating system is pulling air up and out of the home (stack effect); and 2) frozen ground/soil saturation caps/blocks soil gas/ radon from normal exiting into the atmosphere from outside soil - causing soil gas/radon pressures to rise.
• Daily radon variation (sometimes 2-3 fold) is usually greater in the summer than in the winter because of more variations in soil surface temperatures during the summer than in the winter.

During a real estate transaction, buyers typically only have a short window of time during their inspection period to complete a radon test, so a short term test with a 48-hour average radon level is typically the standard.  Home owners that have more time to conduct a long term radon test should consider a long term test between 90 days and 1 year. You might also consider a continuous radon monitor that can be plugged into a wall, such as Corentium, that measures the radon level in real time with a digital display and alarm.

Corentium Digital Radon Monitor

Still have radon questions?

Our home inspectors are licensed radon testers and can answer your questions about radon testing in your home.
Contact a radon tester for more information.