While there is no firm evidence wearable wireless devices cause cancer, they do operate with wireless frequencies commonly used with bluetooth, Wi-Fi and 3G and 4G signals.  Scientific research indicates biologic harm from wireless devices with long term use, thus precaution is indicated. There are also emerging privacy issues. If you have wearable devices, there are ways reduce exposure by turning the wireless connection off if not needed and avoid wearing the device at night or for long periods.