I totally understood where you were coming from at first with this because I've seen that warning before and was skeptical about buying a belt with that similar warning label. I looked up the Prop 65 info to find out more about it and then I looked at the American Cancer Society site and saw this:

"Scientists consider all of the cancer-related substances to be at least probable carcinogens, meaning that they may possibly cause cancer in some people. But not all of them are known carcinogens (known to cause cancer) by groups and experts outside of the state of California. This means that not every compound on the list as a possible cancer-causing substance has been proven to the worldwide scientific community to actually cause cancer." http://www.cancer.org/cancer/cancercauses/othercarcinogens/athome/cancer-warning-labels-based-on-californias-proposition-65

If you think about it, according to California's Proposition 65 Law, there are a lot of everyday products that contain one or more of these chemicals. More than likely you've already been using something that contains one of these "carcinogens" without realizing it. I really wouldn't worry.