Federal and local officials are carefully monitoring airplanes entering Chicago from Japan.

No aircraft entering the U.S. in recent days has tested positive for harmful levels of radiation. That’s according to a statement from the U.S. Customs and Border Protection.

A spokesperson with Border Protection says radiation occurs naturally in things like marble and granite, even bananas. She says there isn’t always a cause for concern. But with officials in Japan still battling to stabilize nuclear reactors, U.S. border agents have detectors in place at all ports of entry to test for the presence and types of radiation.

Officials say any contaminated cargo will be denied entry. Travelers who show signs of radiation sickness will be referred to health officials for treatment.

Many airlines are adding flights from Japan as people try to return to their countries.