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What is radon? Should I be concerned about it?

Monday, October 8, 2018

Yes. Concerned, but not alarmed. Although the state of Florida has a lower than average incidence of elevated radon, an area of Alachua County, called the “Hawthorn Formation” by geologists, has a higher likelihood of elevated radon in the home. The Florida Department of Health keeps statistics on testing by licensed radon technicians of homes around Florida, organized by zip code, and there are zip codes in this area that have elevated radon in two out of three homes tested.  

   Here’s an excerpt from “Building Radon Out, a Step-by-Step Guide on How to Build Radon-Resistant Homes,” published by the U.S. EPA, to help you better understand radon, it’s risks, and ways to control them:

   Radon is a radioactive gas. It comes from uranium and radium in soils, which can be found everywhere in the world. Uranium is present in rocks such as granite, shale, phosphate and pitchblende. Uranium breaks down to radium, which then decays into radon. This gas can easily move up through the soil into the atmosphere. Natural deposits of uranium and radium, not man-made sources, produce most of the radon present in the air.

• Radon is in the soil and air everywhere in varying amounts.

• People cannot see, taste, feel or smell radon. There is no way to sense the presence of radon. 

• Radon levels are commonly expressed in picocuries per liter of air (pCi/L), where a picocurie is a measure of radioactivity.

• The national average of indoor radon levels in homes is about 1.3 pCi/L. 

• Radon levels outdoors, where radon is diluted, average about 0.4 pCi/L.

• Radon in the soil can be drawn into a building and can accumulate to high levels. Every building or home has the potential for elevated levels of radon, even those built with radon-resistant features. EPA recommends taking action to reduce indoor radon levels when levels are 4 pCi/L or higher.

Is radon a significant health risk?

   When radon enters a home, it decays into radioactive particles that have a static charge, which attracts them to particles in the air. These particles can get trapped in your lungs when you breathe. As the radioactive particles break down further, they release bursts of energy which can damage the DNA in lung tissue. In some cases, if the lung tissue does not repair the DNA correctly, the damage can lead to lung cancer.

   Not everyone exposed to elevated levels of radon will develop lung cancer, but your risk of getting radon-induced lung cancer increases as your exposure to radon increases (either because the radon levels are higher or you are in the home longer). Smokers who have high radon levels in their homes are at especially high risk for getting radon-induced lung cancer.

   The following is a sample of organizations that state radon is a health threat in homes:

U.S. Surgeon General

American Medical Association

American Lung Association

Centers for Disease Control

National Cancer Institute

National Academy of Sciences

Environmental Protection Agency

How does radon enter a home?

Four main factors drive radon entry into homes. All of these factors exist in most homes throughout the country.

1. Uranium is present in the soil nearly everywhere in the United States.

2. The soil is permeable enough to allow radon to migrate into the home through the slab, basement or crawlspace.

3. There are pathways for the radon to enter the basement, such as small holes, cracks, plumbing penetrations or sumps. All homes have radon entry pathways.

4. An air pressure difference between the basement or crawlspace and the surrounding soil draws radon into the home.

Is there a safe level of radon?

There is no known safe level of radon. As your exposure to radon is increased, so is your risk for developing lung cancer. Even radon levels below 4 pCi/L pose some risk.

How doe air pressure affect radon entry?

   The air pressure in a house is generally lower than in the surrounding air and soil, particularly in the basement and foundation levels. This difference in pressure causes a house to act like a vacuum, drawing air containing radon and other soil gases in through foundation cracks and other openings. Some of the replacement air comes from the underlying soil and can contain radon.

   One reason why this pressure difference occurs is because exhaust fans remove air from inside the house. When this air is exhausted, outside air enters the house to replace it. Another cause for a pressure difference is that warm air rises and will leak from openings in the upper portion of the house when temperatures are higher indoors than outdoors. This condition, known as “stack effect,” causes unconditioned replacement air to enter the lower portion of the house. 

   Mechanical systems, such as the furnaces or central air conditioners, may also contribute to the difference in air pressure. In areas with very short mild winters, mechanical systems can be the dominant driving force. Air handlers and leaky return ducts can not only draw in radon, they can also distribute it throughout a home.

What can you do to reduce radon in new homes?

   You can easily draw radon away and help prevent radon from entering the home with the following steps.

• Install a sub-slab (or sub-membrane) depressurization system

The objective of these systems is to create a vacuum beneath the foundation which is greater in strength than the vacuum applied to the soil by the house itself. The soil gases that are collected beneath the home are piped to a safe location to be vented directly outside.

• Use mechanical barriers to soil gas entry

Plastic sheeting and foundation sealing and caulking can serve as barriers to radon entry, entry of other soil gases and moisture.

• Reduce stack effect

Sealing and caulking reduce stack effect, and thus reduce the negative pressure in lower levels in the home.

• Install air distribution systems so that soil air is not “mined”

Air-handling units and all ducts in basements and, especially, in crawlspaces should be sealed to prevent air, and radon, from being drawn into the system. Seamless ducts are preferred for runs through crawlspaces or beneath slabs. Any seams or joints in ducts should be sealed.

Does foundation type affect radon entry?

   Because radon can literally be sucked into a home, any home can potentially have a radon problem. All conventional house construction types have been found to have radon levels exceeding the action level of 4 pCi/L.


Radon can enter through floor-to-wall joints and control joints and cracks in the slab.


Radon can enter a  home regardless of whether or not there is a basement. Slabs built on grade can have just as many openings to allow radon to enter as do basements.


The vacuums that exist within a home are exerted on the crawlspaces causing radon and other gases to enter the home from the earthen area below. Even with crawlspace vents, a slight vacuum is still exerted on the crawlspace. Measurements in homes with crawlspaces have shown elevated radon levels.   

Manufactured Homes

Unless these buildings are set up on piers without any skirting placed around them, interior vacuums can cause radon to enter these types of homes as well.

Can we keep radon out by sealing the cracks?

   Sealing large cracks and openings is important to do when you build a 

home, both in the lower portion of the home to reduce radon entry points, and in the upper portion of the home to reduce stack effect. However, field research has shown that attempting to seal all of the openings in a foundation is both impractical and ineffective as a stand-alone technique. Radon can enter through very small cracks and openings. These small cracks and openings are too small to locate and effectively seal. Even if all cracks could be sealed during construction, which would be costly, building settlement may cause new cracks to occur. Therefore, sealing large cracks and openings is one of the key components of radon-resistant construction, but not the only technique that should be employed.

What are the radon-resistant features?

   The techniques may vary for different foundations and site requirements, but the basic elements of the passive sub-slab depressurization system follow. In many parts of the country, the gravel beneath the slab (gas-permeable layer), plastic sheeting and sealing and caulking are already employed for moisture reduction. In these cases, simply adding the vent pipe and junction box is extremely cost-effective for reducing radon, and so cost-effective that even a cost-conscious builder like Habitat for Humanity has been adding these features in many of its homes.

Radon-Resistant Features

Gas-permeable layer

Usually a 4-inch layer of clean, coarse gravel is used beneath the slab to allow the soil gas to move freely underneath the house. Other options are to install a loop of perforated pipe or soil gas collection mat (also known as drainage mat or soil gas matting).

Plastic sheeting

Polyethylene sheeting is placed on top of the gas permeable layer to help prevent the soil gas from entering the home. The sheeting also keeps concrete from clogging the gas-permeable layer when the slab is poured.

Vent pipe

A 3- or 4-inch (recommended) PVC or other gas-tight pipe (commonly used for plumbing) runs from the gas-permeable layer through the house and roof to safely vent radon and other soil gases above the house. Although some builders have used 3-inch pipe, field results have indicated that passive systems tend to function better with 4-inch pipe.

Junction box

An electrical junction box is wired in case an electrical venting fan is needed later to activate the system.

Sealing and caulking

All openings in the concrete foundation floor are sealed to prevent soil gas from entering the home. Also, sealing and caulking the rest of the building envelope reduces stack effect in the home.

What pulls the soil gas through the pipe?

   If the pipe is routed through warm space (such as an interior wall or the furnace flue chase, following local fire codes), the stack effect can create a 

natural draft in the pipe. Because this method requires no mechanical devices, it is called a passive soil depressurization system.

   If further reduction is necessary to bring radon levels in a home below the action level of 4 pCi/L or even lower, an in-line fan can be installed in the pipe to activate the system. The system is then called an active soil depressurization system.

Is there a way to test the lot before building?

   Soil testing for radon is not recommended for determining whether a house should be built radon-resistant. Although soil testing can be done, it cannot rule out the possibility that radon could be a problem in the house you build on that lot. Even if soil testing reveals low levels of radon gas in the soil, the amount of radon that may enter the finished house cannot be accurately predicted because one cannot predict the impact that the site preparation will have on introducing new radon pathways or the extent to which a vacuum will be produced by the house.

Why not wait to install the features until after the home is completed and a radon test is performed?

It is much easier and far less costly to prepare the sub-grade to improve gas flow before the slab is cast. Also, the pipe itself can be run more easily through the house before it is finished. This significantly improves aesthetics and can reduce subsequent system operating costs by planning to route the pipe through warm space to maximize passive operation of the system.

EPA recommends that all homes built in Zone 1 (high radon potential) areas have radon reduction systems.

   If you are building in a Zone 2 or 3 area, the homes you build could still have high radon levels, particularly if there is a radon “hot spot” in your county. According to an annual survey by the NAHB Research Center, about 60,000 homes in Zone 2 and 3 are built with radon-resistant techniques each year. Contact your state radon office for more information.

To download the complete EPA Radon Guide excerpted above, click on the “Building Radon Out” blue image below.  

   Not sure whether testing for radon is worth the cost? Before you decide, see our blog post How can not testing for radon be an expensive mistake for homebuyers?

    Visit our RADON page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

Here’s links to a collection of our other blog posts about RADON:

 Is a radon mitigation fan required to be on a dedicated circuit breaker/fuse in the electric panel?

What happens to a radon test result if the windows were opened, the test device moved, or there was any other evidence of tampering during the test? 

Where do I find the Florida laws regarding radon?

Can I test vacant land for radon before building a house? 

Is radon mitigation possible for a condominium?

Do radon mitigation systems require maintenance?

What is the average life expectancy of a radon mitigation system?  

Do older houses have higher radon levels than new houses? 

Can the seller tamper with a homebuyer's radon test to change the results?

How long does it take to get the results of a radon test? 

Will opening the windows reduce the radon level in a house?

How was it determined that between 15,000 and 22,000 lung cancer deaths each year are caused by radon?

Can a homebuyer do their own radon test for a real estate transaction with a self-test kit?

What is the danger of radon in well water?

What are the symptoms of radon poisoning? 

Does Florida have radon?

Where does radon come from? 

For how many years does an old radon test result remain valid?

Should homeowners get a pre-listing radon test before selling their home?

What are a homebuyer's options when the radon test comes back high (4.0 pico-curies/liter or more)? 

Do granite countertops emit radon?

How can I tell if a house has a radon mitigation system? 

Can a mobile/manufactured home have a high radon problem?

Do I need to test for radon when buying a condominium? 

Will the radon test come back sky-high in a house that has been empty and closed-up for months? 

Should I buy a house with a high radon level? 

What is the average radon level of indoor and outdoor air in America?

What is the operating cost of a radon mitigation system? 

• Should I buy a house with a radon mitigation system?

• Does the buyer or seller of a home pay for radon mitigation when the radon test comes back high? 

• What is the probability of having high radon in a Florida house?

    Visit our RADON page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.  

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