Basement and garden units are the highest-risk spaces for radon, and Cook County sits in a high-radon region. Here is how Chicago landlords test, disclose, and mitigate before it becomes a problem.
Radon is an invisible, odorless radioactive gas that seeps up out of the soil and bedrock as uranium naturally breaks down. You cannot see it, smell it, or taste it, which is exactly why it gets ignored until a tenant or a home inspector asks about it. The U.S. EPA identifies radon as the leading cause of lung cancer among people who do not smoke, and northern Illinois is not a low-risk part of the country. Cook County and the surrounding collar counties sit in a region where elevated indoor readings are common, not rare. For a landlord, this is both a health and a liability issue. A quiet, one-time test tells you where you stand, and the fix is usually straightforward once you know there is a problem. The owners who get caught off guard almost always have occupied space below grade, which describes a huge share of Chicago's rental stock.
Radon enters a building from the ground, so the closer a living space sits to the soil, the higher the potential exposure. Chicago is full of below-grade rentals: garden apartments in Rogers Park two-flats, English basements under Lincoln Park greystones, converted lower levels in Logan Square frame buildings, and basement units in Hyde Park courtyard buildings. These are often the most affordable units in a building, which means they turn over frequently and house a steady stream of tenants. The gas works its way in through cracks in the slab, gaps around plumbing penetrations, unsealed sump pits, floor drains, and the porous block of an old foundation wall. A garden unit with a slab poured decades ago and a sump pit in the corner has multiple entry points at once. Upper-floor apartments in the same building may read perfectly fine while the lower unit tests high, so you cannot judge the basement by what is happening two floors up.
The glacial soils under much of the Chicago area hold enough uranium to produce radon steadily, and our building styles tend to trap it. During a Chicago winter, the stack effect turns a building into a chimney: warm air rises and escapes through the upper floors, and the resulting negative pressure at the bottom actively pulls soil gas in through the foundation. That is why radon readings in a Wicker Park or Andersonville basement are frequently at their highest in the coldest months, when windows stay shut for weeks at a time. Summer is not risk-free either, but it is the ideal season to test and to install a mitigation system while the weather cooperates. Doing the work now, in late July, means a basement unit in Evanston or Ravenswood is protected before the fall lease turnover and long before the building gets sealed up tight for winter.
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