CNG has the narrowest flammability range, leading to slim chances of catching fire in the event of a leakage. Simplified, it means that the combustible mixture of natural gas and air does not ignite if the mixture is leaner than 5Percent or richer than 15Percent of the air fuel ratio required for ignition. Thus, this limited inflammability range makes CNG one of the safest fuels in the world. Being lighter than air, in case of a leak, CNG just rises and disperses into thin air.
The CNG conversion kit is definitely safe, with the cylinders specially manufactured for use in vehicles being cast from a single piece of special steel alloy without any welded joints.