The pilot reported that the helicopter was about to take off with a flight nurse, paramedic, and a patient onboard when she noticed a flash and flame reflection on the front left windscreen immediately followed by screams and the cabin filling with smoke. The pilot immediately reduced the throttle, pulled the fuel shutoff valve, and applied full rotor brake. The paramedic on board’s flight suit was on fire. The flight nurse on board managed to open the helicopter’s right-side door, unbuckle the paramedic, and pull her from the helicopter. The flight nurse extinguished the flames, called for additional help, and began cutting away the burned flight jacket and flight suit from the paramedic. In the paramedic’s left chest flight suit pocket were keys and a burnt lithium battery. The nurse reported that when she went to remove the battery to prevent further injury to the paramedic, the battery was extremely hot and smoking, and burned her hand. She threw the battery to the ground and continued to render care to the paramedic. Later two burnt lithium batteries lying on the ground next to the helicopter’s left skid were recovered. The pilot reported no preaccident mechanical failures or malfunctions with the helicopter that would have precluded normal operation. The operator’s parent company issued a safety alert to all of its air and ground ambulance companies warning of the dangers of improperly carrying or storing loose lithium and lithium-ion batteries used in personal electronic devices.