Eight thousand times a year, a car battery explodes in America because someone didn’t observe safety when jump starting a car. The battery releases hydrogen gas, especially when a heavy load is demanded, such as trying to start the car. The trick is to avoid setting the invisible cloud of hydrogen on fire. Since the electrical system negative side (except on some older cars) is grounded to the whole car, connect a jumper cable to the positive terminals first. Then connect the negative cable, not to the battery terminals, but to a substantial piece of metal on each engine away from the batteries. The spark of closing the circuit will happen away from the hydrogen. Wear safety goggles. The car which has a healthy battery should not be running. The change in load can damage its charging system.
Please remember that battery acid can cause skin and eye irritation and destroy clothing.