According to Fitbit:
"We are aware that some of our customers have reported a skin irritation from wearing their Force device. We conduct testing in order to satisfy a variety of internationally accepted standards relating to the safety of the materials in our devices. Fitbit wristbands are made of an elastomer material similar to many sports watches AND DO NOT CONTAIN LATEX [emphasis mine]. The Fitbit Force clasps and casing are made with a surgical-grade stainless steel, which is commonly used in watches, jewelry and other products in contact with the skin."
http://abcnews.go.com/blogs/headlines/2014/01/fitbit-does-more-than-track-fitness-it-also-leaves-a-rash-for-some/