Toxic chemicals under your home can rise through the same cracks as radon

What’s coming up through your basement floor?

Air. Your house draws it out of soil underneath, day and night, and whatever somebody left down there rides up with it.

Stand on your basement slab and picture what’s directly under your shoes. Concrete, then dirt, then groundwater. Both of those lower two were down there long before your house was, and both belonged to whoever used that ground first, back when nobody was writing down where a drum got emptied. A filling station. A dry cleaner with a machine in back. A shop that degreased engine parts and tipped what was left out by a fence. All of it legal then.

Here’s why it travels toward you instead of away. Warm air rises and leaks out of your upper floors, so something has to replace it, and replacement arrives low down. Cold weather makes that stronger. So does a bathroom fan, a range hood, a clothes dryer venting outdoors, a fireplace, a furnace burning air in a basement. Each one drops pressure indoors another notch.

Soil carries air in gaps between its grains. Lower pressure inside, and that air moves toward you. A hairline crack in a slab. A gap where a pipe passes through. A sump opening, a floor drain, an old sewer line.

Nobody had to spill anything on your property. Nothing has to look wrong. Your yard is fine. Your basement is dry. And a business that closed before you moved in is venting up through your floor while you fold laundry on top of it.

Does a radon test cover this?

No. Same crack in your floor, same route, completely different thing coming up it.

You have probably done radon already. Kit from a hardware store, three days on a shelf downstairs, envelope in the mail, and a number back that looked fine enough to file and forget about.

That kit measured radon, which seeps out of rock, spread around by geology, thick under one county and scarce under another. Solvent came out of one drum, one leaking tank, one careless afternoon, in one spot, decades ago. EPA does not recommend one as a stand-in for the other. A radon result tells you almost nothing about how much solvent is in your air. Same hole in your floor. Two different problems.

There’s a useful half to it, though, and it’s worth knowing. Because both use the same cracks, a high radon result does tell you your slab draws soil gas. It can’t tell you what rides along with it.

And the fix turns out to be the same fix. Suction systems under a slab were invented for radon and have since been installed against chemical vapor too. So a house with a fan already humming on a basement wall has most of this answer bolted to it, which is the one cheerful line we have for you here.

How close does it have to be?

About 100 feet out from the edge of a polluted site. That’s roughly two houses wide.

That surprises people, because most of us picture something underground spreading the way smoke spreads through a room. It doesn’t. Vapor seeps through soil rather than blowing through it, and it thins fast on the way. Folkes and colleagues went back over field cases and found effects rarely turning up more than a house or two past a plume’s edge.

Three situations break that rule, and all three involve something pushing rather than seeping. A landfill making enough methane to shove gas along. A spill inside a building heavy enough to drive vapor down through a floor. A pressurized gas transmission line with a leak, which has pressure behind it by design. Any of those can carry hundreds of feet.

Little and colleagues wrote up one in 1992. A southern California landfill where methane turned up inside nearby homes at close to 1 percent by volume, and chlorinated hydrocarbons reached a house 590 feet away. We read that account in a federal guide’s footnote rather than in their paper.

Fuel gets a kinder rule. Put more than 6 feet of clean, living soil between dissolved gasoline and your building and nobody recommends digging further, because bugs in that soil eat fuel on its way up.

So distance matters. What’s buried down there counts for more. Six feet of live soil handles gasoline. It does nothing at all for dry cleaning solvent.

What happens when somebody tests your house?

A steel ball on your floor for a day, and your own cleaning products out in a shed first.

That ball is a six-liter canister, about the size of a soccer ball, and it arrives under vacuum. Somebody opens a valve and it pulls air in slowly across 24 hours. No pump, no noise, nothing to plug in. Not a sound out of it.

Your own things have to leave before it starts. Cleaners, air fresheners, aerosols, mothballs, scented candles, insect repellant, and whatever is left over from a hobby or an afternoon of car repair. Out of your building 24 to 72 hours ahead, or what you paid to measure is your garage.

One round isn’t enough, and this is where cost meets honesty. Holton and colleagues worked out what a single day can miss: a one-day sample drawn at random had a 40 percent chance of coming in under a house’s true average. Where a house genuinely ran 2 or 5 times over an action level, one day of sampling missed it 20 percent and 6 percent of the time. Those figures reach us through a federal guide’s summary of their work.

Then there’s an empty house in Utah, sitting over a plume of chlorinated solvent, with nobody inside to open a window or run a dryer. Indoor air in that house swung across two to three orders of magnitude.

Weather is doing that to your house. Pressure shifts with season, and a house pulls hardest through heating months, which is why a canister filled in January and one filled in June can tell opposite stories about identical dirt.

So ask how many rounds a quote covers before you compare two prices. Quotes covering different numbers of rounds aren’t comparable at all, and we would rather you knew that before somebody quotes you cheap.

Does a bad result mean you have to move out?

No. A screening value is a signal to look harder, and it was never built as a pass mark.

That one mix-up causes more panic at more kitchen tables than any laboratory result does, so it’s worth getting straight. These values are set deliberately low. For cancer the target is one extra case per million people across a lifetime, which is about as cautious as our environmental math ever gets.

Here’s how low. Ask the federal calculator about trichloroethylene and the groundwater figure it wants comes out at 0.518 micrograms per liter, against a legal drinking water limit of 5. Legal to drink, and still worth checking for what comes off it into your basement, which is the line in all of this that surprised us most.

Now the half that gets skipped, and it cuts the other way. Screening values are one chemical at a time. Where a few chemicals in a house share one kind of harm, they add up. A real health threat can exist with every one of them sitting under its own value. Passing each isn’t the same as passing.

And the guide carrying all of this opens with a line saying it imposes no requirements on EPA, on states, or on anybody else. Advice. Not law. The record of who built what is older than the hazard, because somebody drew your block by hand a century ago.

So a result over a screening value means keep going. It doesn’t mean pack. Anybody telling you to leave before a second round of sampling has even been booked is running well ahead of anything a number can carry.

Does a closed cleanup site mean the problem is over?

EPA did not add underground vapor to Superfund scoring until a rule took effect on 8 February 2017.

Ahead of that, a site got scored on groundwater, on air, on surface water, and on soil you might touch. What came up through a floor into a bedroom wasn’t on that sheet. Scoring had last been rewritten in December 1990. Nobody was scoring your floor.

So a property cleaned to a standard that never counted vapor could be assessed, signed off and closed, and still have a route nobody had looked at.

Years later somebody reopens it, and by then those houses have changed hands. Resale, foreclosure, a second generation moving in. Owners who arrived after the fact weren’t part of any original resolution, and plenty of them have no idea a system was ever installed under their floor. Neighbors who did go through round one can be reluctant to reopen anything, which we understand. Nobody wants to hear that their street is a case file again.

Federal law does force a look back. Any cleanup that leaves hazardous material in place gets re-examined every five years.

That flips the question for a homeowner, and it is worth putting bluntly. Not whether this site was cleaned up. Ask whether it was ever checked for vapor, and when. A closure letter written before 2017 answers a different question than the one you are asking.

What if a gas station used to be on your corner?

Start with history, not with a test. And start free.

Envirofacts checks a stack of federal databases against one address at once, it costs nothing, and it’s the first thing to open before you pay anybody to sample anything. Search yours, then widen the circle to filling stations, dry cleaners, auto shops and small industry within a couple of hundred feet, rather than only next door.

Then go older than any database, because old paper is worth more here than anything you can type into a search box. Fire insurance maps and city directories catch what federal records never had. A corner station that closed two generations ago may never have made it into a federal record at all.

Scale is our argument for bothering. 52,859 fuel releases were still waiting to be cleaned up as of March 2026, against 534,189 tanks still in the ground and working. Those are everyday addresses on ordinary corners, and some sit a block from a house like yours.

If something turns up close, call your state environmental agency. Two things to ask, in this order. Was that site ever checked for vapor. When.

Does a mitigation fan fix the contamination?

Denver is where this got proved at scale. Folkes and Kurz put suction systems under 337 homes over a plume of 1,1-dichloroethylene, and three years of monitoring across 301 of them showed very substantial drops indoors. About a quarter needed a minor adjustment or an upgrade after installation before they hit the state’s standard. That account reaches us through a federal guide’s footnote rather than from their paper.

Hardware is plain. Holes cut in your slab, a pit dug out underneath, pipes up through them, everything sealed, a fan pushing soil gas outdoors.

Some places put one in before anybody has proved a problem, on the reasoning that a fan is quick and cheap next to a full site investigation, and that the alternative means strangers in your basement for months while you wonder.

Be clear about what you’re buying, though. It’s not an air filter. It works on what arrives from below and does nothing about paint thinner in your garage.

And it cleans nothing up. Where nobody deals with the source, that fan runs indefinitely and becomes part of your house the way a sump pump is. Whoever offers to put one in may pay for that while monthly running cost becomes yours, and if you turn it down, putting one in later may be on you, so get both the offer and your answer in writing.

We’ll leave you with one line rather than a rule. A fan is a fence, not a shovel. It keeps what’s under your floor out of your rooms, and everything that was down there on the day it switched on is still down there, waiting on somebody else entirely.