When an employee was working onsite his personal monitor alarmed for hydrogen fluoride (HF). The employee was not wearing respiratory protection at the time of the incident and the reading was 0.7 ppm HF. The employee immediately evacuated the area and contacted emergency services. Once the emergency response team arrived, the employee was provided with a calcium gluconate breathing treatment (nebulizer) at prescription strength (2.5% aqueous calcium gluconate solution.) Emergency responders upon arrival collected HF samples and found 7-8 ppm at the source but not the breathing zone as the worker had already exited the area.