Why a Radon Test During Home Inspection Matters for St. Louis Buyers

When you are buying a home in St. Louis, the inspection period is your best chance to uncover hidden problems. You look at the roof, the HVAC system, the foundation. But one issue that is easy to overlook is radon gas. Radon is a radioactive gas that comes from the natural breakdown of uranium in soil. It can seep into any building, and St. Louis real estate sits on geology that makes it more likely to see elevated radon levels. That is why a radon test during home inspection has become a standard step for informed homebuyers in our area.

I have been involved in home inspections for years, and I can tell you that radon is not something to guess about. You cannot see it, smell it, or taste it. The only way to know if a house has a problem is through radon measurement. And the best time to do that measurement is while you still have leverage in the transaction. If you wait until after closing, you own the problem. If you test during the inspection period, you can negotiate or walk away. That is the practical reality of a real estate transaction involving radon.

How Radon Gets Into a Home

Radon gas moves from the soil into the air inside a house through cracks in the foundation, gaps around pipes, sump pumps, and even through the pores of concrete block walls. Basement radon is especially common because basements are in direct contact with the soil. But radon can also enter a home through a crawl space or a slab-on-grade foundation. Once inside, the gas can build up to levels that pose a health risk. The EPA radon guide recommends taking action if levels reach 4 picocuries per liter of air. That is the threshold where the National Radon Program and most health authorities agree that mitigation is warranted.

In the St. Louis area, the underlying bedrock contains granite and shale that release radon as they decay. That means homes here are more likely to have elevated readings than homes in other parts of the country. So a radon test during home inspection is not just a nice extra. It is a necessary part of the home inspection checklist for anyone buying a house in this region.

Short-Term vs. Long-Term Testing

There are two main ways to measure radon: a short-term radon test and a long-term radon test. A short-term test stays in the house for two to seven days. It gives you a quick snapshot of radon levels. A long-term test runs for more than 90 days and gives a more accurate picture of the annual average. For a real estate transaction, you usually have time only for a short-term radon test. The inspection period is typically one to two weeks, so a long-term test is not practical unless the seller agrees to an extended timeline.

That is where the kind of testing equipment matters. Many home inspectors still use passive radon devices, which are small charcoal canisters or liquid scintillation vials that you mail to a lab. They are cheap and easy to deploy, but they have a big weakness: they can be tampered with, and the results take days to come back from the lab. If the test gets bumped or left in a drafty spot, the reading may not represent the actual living conditions.

A better approach for a real estate transaction is to use a continuous radon monitor. This is an electronic device that sits in the lowest livable level of the home and records radon levels every hour or so. It gives a running average and cannot be easily fooled by a window left open or a fan running. Companies like Air Sense Environmental use continuous radon monitors for the homebuyer radon test because the results are available immediately at the end of the test period. That means no waiting for a lab report and no risk of lost samples.

What the Test Tells You

A radon test during home inspection will give you a number. If that number is below 4 pCi/L, the EPA says no action is required, but you might still consider a long-term test later for peace of mind. If the number is above 4 pCi/L, the EPA guide recommends installing a radon mitigation system. The most common system is active soil depressurization, which uses a fan to pull radon from beneath the slab and vent it safely above the roofline. A good radon mitigation contractor can usually install one for a few thousand dollars, depending on the house design.

Some buyers see a high radon reading and panic. They think the house is unlivable or that they need to walk away. That is not usually the case. Radon is a fixable problem. The key is knowing about it before you close. If you have a radon test during home inspection and the levels are high, you can ask the seller to install a mitigation system or to credit you the cost at closing. Many sellers will agree because they know the problem will come up with the next buyer anyway. In some states, radon disclosure is required on the seller disclosure form. Missouri does not have a statewide radon disclosure law, but many St. Louis area sellers still disclose known issues to avoid surprises later.

The Difference Between a Home Inspection and a Radon Test

A general home inspection does not include radon testing. Most home inspectors offer it as an add-on service, but they often use passive radon devices because that is what they have on hand. If you want a more reliable result, hire a specialist who does only radon measurement. That specialist will place the continuous radon monitor in the right spot, make sure the house is kept in closed-house conditions during the test, and give you a printed report at the end.

Closed-house conditions mean keeping windows and exterior doors closed as much as possible, avoiding ceiling fans and whole-house fans, and not running the fireplace or wood stove. These conditions help ensure that the test measures the radon that would be there under normal living conditions, not a diluted sample. A good testing company will explain these rules to the homeowner or the real estate agent before they start the test. Air Sense Environmental, for example, includes clear instructions with every homebuyer radon test so that everyone knows what to do.

When to Test

The best time for a radon test during home inspection is during the cooler months, when houses are closed up more often. Radon levels tend to be higher in winter and lower in summer because of the stack effect and the way air moves through the house. But you cannot always choose your closing date, so test whenever you can. Even a summer test can give you useful information, especially if you run the air conditioning, which keeps the house closed.

If you are selling a home, it is smart to do a radon test before you list. That way you know the radon levels ahead of time and can either address them or disclose them honestly. Some sellers preemptively install a radon mitigation system to make the house more attractive to buyers. That can be a good selling point, especially in areas where radon is known to be an issue.

What to Look for on the Report

The report from a continuous radon monitor will show the average radon level over the test period, along with the hourly readings. Look at the average first. If it is under 4 pCi/L, you are in the clear according to the EPA. If it is between 2 and 4 pCi/L, you might still consider mitigation, especially if someone in the household is a smoker or has respiratory problems. The EPA says there is no safe level of radon, so lower is always better.

If the average is above 4 pCi/L, you need a plan. Talk to a radon mitigation contractor who can inspect the house and give you a quote. Most mitigation systems cost between $1,500 and $3,000, depending on the house size and the type of foundation. Active soil depressurization is the most effective method and works for almost any home. The system runs quietly and continuously, and it reduces radon levels by 90% or more in most cases.

Why St. Louis Homebuyers Should Not Skip It

I have seen too many buyers skip the radon test during home inspection because they think it is an unnecessary expense or because their real estate agent discourages it. That is a mistake. The test costs around $150 to $250, which is tiny compared to the cost of a mitigation system or the health risk of living with elevated radon. And in a seller's market, some buyers worry that asking for a radon test will make their offer look less competitive. But you can request the test as part of the inspection contingency, and the seller can say no. If they say no, you have to decide whether to walk. Most sellers will let the test happen because they do not want to lose a deal over a minor request.

Radon is the second leading cause of lung cancer in the United States, according to the EPA. It is responsible for about 21,000 lung cancer deaths each year. That is a serious risk, but it is also a preventable one. Testing is simple, inexpensive, and reliable. And if you test during the inspection period, you keep control of the situation. You can ask for repairs, negotiate credits, or make an informed decision about whether the house is right for you.

If you are buying a home in St. Louis, put a radon test on your home inspection checklist. It is one of those small steps that can save you a lot of trouble down the road. And if you are selling, consider having a radon test done before you list. It shows buyers that you are transparent and that you care about the health of the people who will live in your home.