A health hazard or health risk was anything which could negatively impact one's health.
In 2370, Dr. Beverly Crusher gave radiation leaks and biochemical contamination as two examples of health hazards she and Lieutenant Worf would be inspecting his Toron class shuttle for. (TNG: "Gambit, Part II")
Also that year, Julian Bashir expressed his belief that a darts rematch with Miles O'Brien would kill the chief engineer. When Jadzia Dax called him on his exaggeration, Bashir admitted he was overstating the potential health risk. (DS9: "Rivals")
In 2376, after seeing Vinka and Azen, two Qomar, become dizzy and disoriented while waiting in line to meet The Doctor, Tom Paris suggested The Doctor might be hazardous to their health. (VOY: "Virtuoso")