4. Your basement
The soil and rock found under your house could contain radon, which is a radioactive natural gas. After cigarette smoking, radon is actually the second biggest cause of lung cancer in the United States. So, if this type of gas enters your home, you could end up with lung problems over time.
Fortunately, you can test your home for radon and get the gas out for good. “Fans aren’t solutions. When you shut your windows, it would build back up again,” Nolen says. The best thing to do is to buy a ventilation system that you can keep going all year round.