According to research in The Lancet (2016) and the Journal of the American Medical Association (2016) they appear to be ineffective. Overall health and fitness outcomes were no better after 12 months and weight loss was less in those subjects that wore wearable fitness devices compared to those who did not
According to the National Cancer Institute they probably don’t give us cancer. Cell phone usage has greatly increased over the years and people are concerned because of the radiofrequency energy they emit. This is in the form of non-ionizing radiation. This type of radiation has not been shown to cause cancer.