The air in houses can be affected by bad stuff, called contaminants. Sometimes harmful chemicals enter the air in buildings from nearby contaminated soil and groundwater through cracks or gaps in the foundation – a process known as vapor intrusion. This poses some risk to our health because we spend so much of our time indoors. Currently, it’s difficult and expensive to figure out if vapor intrusion is happening. That’s why we wanted to see if trees can serve as indicators for vapor intrusion. We collected samples from 109 trees in a contaminated area in a Nebraska town and analyzed them for tetrachloroethene (PCE), a chemical used mostly as a cleaner and to make other chemicals. When comparing our results with the data the U.S. Environmental Protection Agency (EPA) had collected, we found that trees are good indicators of vapor intrusion.