MONROE, N.C. — Regina Barrett, a 69-year-old retiree who lives in this small North Carolina city southeast of Charlotte, has not been happy with her tap water for a while. “Our water has been cloudy and bubbly and looks milky,” said Barrett, who blames fluoride, a mineral that communities across the nation have for decades added to the water supply to help prevent cavities and improve dental health. “I don’t want fluoride in my nothing!” said Barrett, echoing a growing number of people who not only doubt the mineral’s effectiveness but also believe it may be harmful despite decades of data poi…